# Tube PA conversion



## Dreaming1 (Mar 29, 2022)

I have a 1954 Rauland-Borg model1916 PA head. Schematic in hand. Looks like an old 50s Gibson octal circuit. The outputs have continuity. I took the tubes out. 5Y3GT rectifier. I will put it back and check voltages at sockets and replace resistors before I put the others back. Then check these caps for AC leaking by. Wax paper from the 50s. Still spec? As long as the bias isn't off too bad, Im going to let it rip. 1 phono channel that is a 6SQ7 going to phase inverter. 2 channels (already tied parallel...) of 6SJ7 going to a 6SC7 mixer. 6SC7 paraphrase circuit phase inverter feeding 2 6V6GT power tubes. Cathode biased, negative feedback loop, 4,8,16Ω impedances. 
Looks fun. Maybe it will sound old. I wish I had a field coil speaker. Looks like it will be dark sounding. May lower the cap values on inputs.
I just got the 3 wire cord in. Solder it up tomorrow and start drilling holes for jacks. 
I am going to add a coupling cap and input resistor to the phono ch. I will play it if it works. Then im liable to try cascading the 6sj7 for a channel and cascade the 6SQ7 and 6SC7 as 2nd channel. Maybe triodes driving pentodes on 2 channels. Maybe try other octals. 
Anyone been here before me? Hip to this trip? Come aboard and watch me lose a finger or find fun in electronics!


The guts. Grab and go action wiring. 

Bottles and cans, bottles and cans, well just clap your hands -Beck

The cypher key


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## Dreaming1 (Apr 26, 2022)

I decided to rip out the bakelite jacks. Fuck 'em. Going to be a working preamp playground, not a museum item. I plated the panel from behind. I will drill these for 1/4" jacks and a switch for 4/8/16Ω speaker impedance. May add one for selectable off/light/heavy negative feedback loop. 
I have all of the electrolytic caps except the first filter cap. I have all of the resistors and the other caps. 
Decided to run a bolt through a hole to use as chassis ground point instead of using the rectifier tube lead. Safety stuff.
I may remove the can cap and both the input socket things. Plate them off and use that area to mount a board holding the power filter stuff. Right by the input jacks though... Better than a pile of these bigger caps though. 
Got a light bulb current limiter set up. Get some shrink wrap tubing and some of the jacks I want to use and I should be into rewire and testing the circuit. Then I will plug all the tubes back in and see if they all work. Then I may get to play it.


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## weedstoner420 (Apr 27, 2022)

Greetings fellow tube amp nerd, that's a nice find! Looks to be in good shape inside too. So many possibilities!


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## Dreaming1 (Apr 27, 2022)

weedstoner420 said:


> Greetings fellow tube amp nerd, that's a nice find! Looks to be in good shape inside too. So many possibilities!


hey! You deep into this stuff? I have some questions about cascading preamp stages. I know impedance matching becomes a thing. Found rob robinettes page and uncle Doug on youtube for tech advice. Have a decent grasp of ohms law and the math to figure out bias and resistor values type stuff. But not a lot of experience. I bet redesigning these pre stages is going to be good experience and hopefully a good time that sounds good.


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## weedstoner420 (Apr 28, 2022)

I mean I know a bit... As far as impedance matching when cascading gain stages, just make sure the resistance after the coupling cap (including volume pot and/or grid leak resistor of the next stage) is much higher than the preceding stage's output impedance. Usually between 500k and a couple meg is fine. Not sure how much it actually matters but that seems to be how most amps are designed...

Also if you're cascading multiple gain stages, you may have to attenuate the signal pretty heavily between stages so it doesn't turn into a square wave too fast. That can be done with just a voltage divider - 2 resistors equivalent to a volume pot turned down very low.

When in doubt, start from a known good circuit/schematic and adjust component values to taste. All those octal tubes probably have 9-pin equivalents that may be used in other circuits you can use for reference (i.e. 6SJ7 is close to a 5879).

I hadn't heard of Rob Robinette but his page looks pretty cool! Unfortunately I don't have the attention span for long YouTube videos like uncle Doug... My main reference for stuff is Merlin Blencowe's books and articles (aka the Valve Wizard), so check him out too if you haven't already.

I'm following along and look forward to seeing where this thing goes!


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## Dreaming1 (Apr 28, 2022)

Thanks. I have looked through several preamp schematics for these 6.3v tubes. And checking the tube reference material. Looking at the curves. I will do some math, but also plan on using resistor/capacitor substitution boxes to change the circuit while I monitor the pins for current and voltage. And use my ears too. 
I have the basic working knowledge of circuit design, but no insight or experience directly with amps. This will all be fun and or a learning curve fight.


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## weedstoner420 (Apr 28, 2022)

Dreaming1 said:


> Thanks. I have looked through several preamp schematics for these 6.3v tubes. And checking the tube reference material. Looking at the curves. I will do some math, but also plan on using resistor/capacitor substitution boxes to change the circuit while I monitor the pins for current and voltage. And use my ears too.
> I have the basic working knowledge of circuit design, but no insight or experience directly with amps. This will all be fun and or a learning curve fight.


Nice, I hope it's a fun learning experience! Like I said, a known amp schematic from like Marshall/Fender/etc is probably a good starting point, at least as far as the overall design, and tweak component values from there.

Looking at the schematic, I'd be tempted to just hook a guitar up to one of the mic inputs and see how it sounds. It's probably decently rockin' as-is (once you confirm it's electrically safe and operational of course).


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## Dreaming1 (Jun 1, 2022)

Im still at it. Had to focus energy in other areas of life.

so here is where and how Im building the filter cap section. 
Maybe  I should lay some non-conductive sheeting over it before I layer the input section over it. The ground side will be by the input jacks. Im shrink wrapping all the wire connections too.
Switches will be for 4,8,16 Ω speaker impedance and the NFB will be switchable ( on 8 Ω right now) between off and 2 resistor values for more or less NFB.
I am just recapping it first. Then get it running with tubes and I will play it as is for a bit. Switch between .022 and .047 coupling caps at input to see which I prefer.
Going to make ch 2 be the phono tube and use 1meg resistor (on hand) and .047 cap for input on 6sq7. The schematic has 10meg and I have seen fender using 5meg. Should I use higher resistor value?
After I get it going and check it out, I will start with pairing the 6sj7 tubes in parallel as ch 1.


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## weedstoner420 (Jun 2, 2022)

Dreaming1 said:


> Im still at it. Had to focus energy in other areas of life.
> View attachment 5142749View attachment 5142750View attachment 5142751
> so here is where and how Im building the filter cap section.
> Maybe  I should lay some non-conductive sheeting over it before I layer the input section over it. The ground side will be by the input jacks. Im shrink wrapping all the wire connections too.
> ...


Those 6SJ7's are using grid-leak bias, which is more common for small-signal inputs like a microphone, not so much for guitar (maybe older guitars with low output pickups). The cathode is connected right to ground, and you need that super high value resistor (5-10 meg) to generate bias voltage on the grid. If you use the usual 1 meg in that spot, I don't think it would hurt anything, but it would be biased very warm and probably have very little headroom.

I think a more "contemporary" design would be a 1 meg grid leak resistor, and a resistor in the 1k - 2.2k range from the cathode (pin 5) to ground. The 6SQ7 for the phono input is already set up that way - the 0.5 meg pot acts as the grid leak resistor, and the 470 ohm above it (in the schematic) is the cathode resistor (shared with the 6SC7).


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## Dreaming1 (Jun 2, 2022)

weedstoner420 said:


> Those 6SJ7's are using grid-leak bias, which is more common for small-signal inputs like a microphone, not so much for guitar (maybe older guitars with low output pickups). The cathode is connected right to ground, and you need that super high value resistor (5-10 meg) to generate bias voltage on the grid. If you use the usual 1 meg in that spot, I don't think it would hurt anything, but it would be biased very warm and probably have very little headroom.
> 
> I think a more "contemporary" design would be a 1 meg grid leak resistor, and a resistor in the 1k - 2.2k range from the cathode (pin 5) to ground. The 6SQ7 for the phono input is already set up that way - the 0.5 meg pot acts as the grid leak resistor, and the 470 ohm above it is the cathode resistor (shared with the 6SC7).


I meant what you said about more modern input, not just replacing the 10meg with 1meg. Maybe leave one and alter the other one for now. I just got the filter cap section done. Added another terminal strip to hold unused output transformer leads. If the tubes work, I might get to try it out over the weekend.


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## weedstoner420 (Jun 2, 2022)

Dreaming1 said:


> I meant what you said about more modern input, not just replacing the 10meg with 1meg. Maybe leave one and alter the other one for now. I just got the filter cap section done. Added another terminal strip to hold unused output transformer leads. If the tubes work, I might get to try it out over the weekend.


Nice! (Edit: dang it, devil horns emoji doesn't translate from my phone...rock on!)


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## Dreaming1 (Jun 4, 2022)

I think I'm wired for sound. .047 cap on 1st ch and .022 cap on 2nd ch. I threw a .022 coupling capacitor before the .5meg pot. Is that right? I like the idea of a capacitor between my guitar and any DC volts. Just parked the other speaker outs and wired the 8Ω with the NFB to the output jack. I have to check my speaker collection. I think all my 12" ones are 16Ω, but I have a 4x12 cab that only has a pair in it. I can wire it for 8 or 16. 
The old solder...what is up with that? Tough. And everything smells so different from modern toxic fumes. Extra lead? Maybe I can skip my paint chips today. 
I will start with rectifier tube in and check some voltages. Then pop the rest of the tubes in and give it a go. 
Need to grab some of those tube sockets with the leads on them and an analog ammeter. Then I can mess with bias on the power tubes if needed. 
Maybe tomorrow, but probably mon or tues. If it works, I will record some and throw it up on yutube.


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## weedstoner420 (Jun 5, 2022)

Dreaming1 said:


> View attachment 5144558
> I think I'm wired for sound. .047 cap on 1st ch and .022 cap on 2nd ch. I threw a .022 coupling capacitor before the .5meg pot. Is that right? I like the idea of a capacitor between my guitar and any DC volts. Just parked the other speaker outs and wired the 8Ω with the NFB to the output jack. I have to check my speaker collection. I think all my 12" ones are 16Ω, but I have a 4x12 cab that only has a pair in it. I can wire it for 8 or 16.
> The old solder...what is up with that? Tough. And everything smells so different from modern toxic fumes. Extra lead? Maybe I can skip my paint chips today.
> I will start with rectifier tube in and check some voltages. Then pop the rest of the tubes in and give it a go.
> ...


Nice! I think the cap on the input jack is only needed if using grid-leak bias, since the grid is at a small negative voltage, but it doesn't hurt to have one in there either way.

The power tubes are cathode biased so measuring bias current is easy, just measure voltage from pin 8 to ground and divide by 250 (or whatever that resistor measures now). Then to get the idle dissipation (watts) per tube, divide that number by 2 and multiply it by (pin 3 voltage - pin 8 voltage). (Apologies in advance if you already knew that...)

I usually adjust bias by starting with a high-ish value cathode resistor (like 470 or 1k ohms) and clipping different resistors in parallel with it till you find the right value, then replace that with a single 5 watt resistor of the same value.

And yeah, I know that old electronics smell...I feel like it's extra rosin on the wires and joints or something...? When it's hard to melt, I'll add some fresh solder to the joint first, then once it mixes with the old it all becomes easier to work with.

Look forward to hearing the end result!


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## Dreaming1 (Jun 6, 2022)

weedstoner420 said:


> The power tubes are cathode biased so measuring bias current is easy, just measure voltage from pin 8 to ground and divide by 250 (or whatever that resistor measures now). Then to get the idle dissipation (watts) per tube, divide that number by 2 and multiply it by (pin 3 voltage - pin 8 voltage). (Apologies in advance if you already knew that...)
> 
> I usually adjust bias by starting with a high-ish value cathode resistor (like 470 or 1k ohms) and clipping different resistors in parallel with it till you find the right value, then replace that with a single 5 watt resistor of the same value.
> 
> Look forward to hearing the end result!


No need for apologies. I enjoy refresher courses. Puts the fun in fundamental. Lets me check my info and I could always learn something. 
Well...it is awesome. I was expecting something very clean. But, this thing is giving me growl and scream. Very rock-n-roll. Rolling back the guitar volume cleans up, but gets darker too. The phono channel is the cleanest. 
I ran it through an old peavey amps speaker. Chassis says 35w 8Ω. I will run it through the 2x12 at 8Ω too. There is a problem, of course. The tone knob is giving squeals when the ch2 knob is all of the way down. Idk why. And ch1 was throbbing (or maybe that's farting out?) When cranked up. 
Ch1 sounds older and cooler. Maybe the .047 cap, maybe the circuit parts. Ch2 has more bite. Makes more modern sounding drive. 
Give me a day or so, I'll get some audio and video together, edit it, and post the link here. Wish I knew a better player than me. You will too.


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## weedstoner420 (Jun 6, 2022)

Dreaming1 said:


> No need for apologies. I enjoy refresher courses. Puts the fun in fundamental. Lets me check my info and I could always learn something.
> Well...it is awesome. I was expecting something very clean. But, this thing is giving me growl and scream. Very rock-n-roll. Rolling back the guitar volume cleans up, but gets darker too. The phono channel is the cleanest.
> I ran it through an old peavey amps speaker. Chassis says 35w 8Ω. I will run it through the 2x12 at 8Ω too. There is a problem, of course. The tone knob is giving squeals when the ch2 knob is all of the way down. Idk why. And ch1 was throbbing (or maybe that's farting out?) When cranked up.
> Ch1 sounds older and cooler. Maybe the .047 cap, maybe the circuit parts. Ch2 has more bite. Makes more modern sounding drive.
> Give me a day or so, I'll get some audio and video together, edit it, and post the link here. Wish I knew a better player than me. You will too.


Yooooooo that's awesome! Yeah it doesn't take much to get a decent sound out of some tubes, that circuit is about as simple as it gets.

I will not judge you by your chops, my repair and design skills surpassed my playing skills long ago lol...


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## Dreaming1 (Jun 6, 2022)

I only recapped it. Now I can play it for awhile and then change around the circuit a bit. I will try swapping the tubes to see if they make any difference. Then I want to maybe start combining tubes or cathode bias the pre. 
The phono ch doesn't do anything till around halfway up. May need better pot.


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## weedstoner420 (Jun 6, 2022)

Dreaming1 said:


> I only recapped it. Now I can play it for awhile and then change around the circuit a bit. I will try swapping the tubes to see if they make any difference. Then I want to maybe start combining tubes or cathode bias the pre.
> The phono ch doesn't do anything till around halfway up. May need better pot.


Yeah only having one triode before the phase inverter isn't going to give you much gain... If it were me I'd be thinking about changing the 6SQ7 to a dual triode type like 6SL7 or 6SN7, so I could add a gain stage before that volume control...


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## Dreaming1 (Jun 6, 2022)

weedstoner420 said:


> Yeah only having one triode before the phase inverter isn't going to give you much gain... If it were me I'd be thinking about changing the 6SQ7 to a dual triode type like 6SL7 or 6SN7, so I could add a gain stage before that volume control...


Guitar into 1st half, then volume, then 2nd half to PI?
What about. Remove ch2 from mixing tube, and then run the 6sq7 into 6sc7 on pin 4? Gives 2 stages of triodes.
I have a lot of gain on tap with the 6sj7 channels. Surprising amount. I like the phono ch being cleaner.
This is going to be a lot of fun for me. Thanks for your help.


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## weedstoner420 (Jun 7, 2022)

Dreaming1 said:


> What about. Remove ch2 from mixing tube, and then run the 6sq7 into 6sc7 on pin 4? Gives 2 stages of triodes.


That would also work. If you're just using it for guitar I guess there's no need to have two almost-identical channels.

Simplest way is probably to break the connections to the second 6SJ7's plate, and redirect the 6SQ7's plate to that .047 coupling cap instead of the one after the mixer tube (see attached drawing, hopefully it makes sense). Then the "mic 2" volume control will work too, and if you want you can replace the "phono" volume control with a 1 meg to ground and 10k grid stopper, unless you want two volume controls on one channel.


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## weedstoner420 (Jun 7, 2022)

weedstoner420 said:


> That would also work. If you're just using it for guitar I guess there's no need to have two almost-identical channels.
> 
> Simplest way is probably to break the connections to the second 6SJ7's plate, and redirect the 6SQ7's plate to that .047 coupling cap instead of the one after the mixer tube (see attached drawing, hopefully it makes sense). Then the "mic 2" volume control will work too, and if you want you can replace the "phono" volume control with a 1 meg to ground and 10k grid stopper, unless you want two volume controls on one channel.View attachment 5145769


Now that I think about it, you'd probably run into feedback/oscillation issues doing it that way - having two series gain stages from the same channel sharing a cathode resistor (the 6SQ7 and both halves of the 6SC7 all share that one 470 ohm cathode resistor)...so maybe do as above, but also disconnect the 6SQ7 cathode from that 470 ohm and give it its own dedicated 1.5k cathode resistor...could also change the 470 to a 680 so the bias points of the 6SC7 stages stay roughly the same...


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## weedstoner420 (Jun 7, 2022)

weedstoner420 said:


> Now that I think about it, you'd probably run into feedback/oscillation issues doing it that way - having two series gain stages from the same channel sharing a cathode resistor (the 6SQ7 and both halves of the 6SC7 all share that one 470 ohm cathode resistor)...so maybe do as above, but also disconnect the 6SQ7 cathode from that 470 ohm and give it its own dedicated 1.5k cathode resistor...could also change the 470 to a 680 so the bias points of the 6SC7 stages stay roughly the same...


Lol, sorry for the multiple posts. I just realized the 68k feeding the 6SQ7 plate also goes to the 6SC7 plates...so the 6SQ7 will also need it's own dedicated plate resistor (100k should be fine). Here's my final draft, again hopefully you can make sense of it...


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## Dreaming1 (Jun 8, 2022)

No problem. Circuits are funny like that. Everything connected. Makes me think I'm smart, then makes me think I'm dumb, then makes me question if I know what is happening at all. I got your drawing. 
I am going to hang out with this amp as it is for awhile. But, I will change it in near future. 
2x12 had 8Ω speakers, so I will hook up the amp to switch 4&8Ω outputs. Try to tame the gain a bit with a pair of speakers. They are eminence swamp thangs. Low end heavy voicing. I was going to have a pair of brighter ones to complete the cab. So, I will probably use speakers in AC15 and twin reverb for comparison too. 
I got it all together, now I just have to get back in there and record some so I can post a video.


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## weedstoner420 (Jun 8, 2022)

Dreaming1 said:


> No problem. Circuits are funny like that. Everything connected. Makes me think I'm smart, then makes me think I'm dumb, then makes me question if I know what is happening at all.


Yes, 100%. And super rewarding when you actually get it working!


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## Dorian2 (Jun 8, 2022)

Great thread. Good luck with the amp!


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## Dreaming1 (Jun 9, 2022)

Cheap yamaha strat copy. Audix d6 dynamic jammed into the cone of an eminence swamp thang. LOUD! Too loud. Makes this 150w speaker squeal. Need to check the speaker leads again. Pull off the NFB and check that out. 
62j7 channels are dirty. 1st ch makes throbbing sound if knob is turned up much past halfway. Ch 2 has a weird interaction with tone knob. Almost more gain, but could just be highs getting in? And when ch 2 knob is turned down all the way, there are pops and squeals. 
The tone knob sucks. Doesn't do a lot, and doesn't sweep sound smoothly. Some parts I hear, some parts of the sweep I don't. 
The phono ch is clean. It doesn't do anything until turned up past halfway. It also did not enjoy a digitech looper pedal into it. Cut off when I started loop, and came back on with a... slow rise in volume when I ended loop. 
Both channels liked all my other pedals. Using a digitech mojo overdrive as a treble booster worked great! They both loved fuzzface and big muff type fuzz. Delay was cool. Makes me wish I had a spring reverb unit to run into it. 
I'm playing it, cause it is so damn gnarly, but Im going to open it back up soon. I need to make a wooden brace to hold the chassis while it's on. And I have a twin reverb that probably blew some power tubes and a resistor to fix. So on to pcb stuff again. This was cool as hell.


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## weedstoner420 (Jun 9, 2022)

Dreaming1 said:


> Cheap yamaha strat copy. Audix d6 dynamic jammed into the cone of an eminence swamp thang. LOUD! Too loud. Makes this 150w speaker squeal. Need to check the speaker leads again. Pull off the NFB and check that out.
> 62j7 channels are dirty. 1st ch makes throbbing sound if knob is turned up much past halfway. Ch 2 has a weird interaction with tone knob. Almost more gain, but could just be highs getting in? And when ch 2 knob is turned down all the way, there are pops and squeals.
> The tone knob sucks. Doesn't do a lot, and doesn't sweep sound smoothly. Some parts I hear, some parts of the sweep I don't.
> The phono ch is clean. It doesn't do anything until turned up past halfway. It also did not enjoy a digitech looper pedal into it. Cut off when I started loop, and came back on with a... slow rise in volume when I ended loop.
> ...


Dang that sounds pretty good! I love the sound and feel of a simple no-frills tube amp. Glad it worked out for ya!


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## Dreaming1 (Jun 9, 2022)

This would have blown minds in 1954. Speakers and eardrums too. I ran it through a cheap peavey practice amps speaker first. It sounded cool and seemed to be less bothered, but I cant imagine that speaker was anywhere near 150w. I don't get it. A pair of 6v6 tubes and it is as loud as my twin reverb. My wife agrees and suggests it is too loud. I say "what?" 
The gain on the mic pre's is too much and the phono ch is too low. Classic Goldilocks problem. I need to open it back up, but it is good fun for now. I will make a video of the sound problems when I get around to it. See if I can't figure some things out.


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## weedstoner420 (Jun 10, 2022)

Dreaming1 said:


> This would have blown minds in 1954. Speakers and eardrums too. I ran it through a cheap peavey practice amps speaker first. It sounded cool and seemed to be less bothered, but I cant imagine that speaker was anywhere near 150w. I don't get it. A pair of 6v6 tubes and it is as loud as my twin reverb. My wife agrees and suggests it is too loud. I say "what?"
> The gain on the mic pre's is too much and the phono ch is too low. Classic Goldilocks problem. I need to open it back up, but it is good fun for now. I will make a video of the sound problems when I get around to it. See if I can't figure some things out.


Haha yup, 20 tube watts is indeed quite loud! I've built a few music-listening amps using a circuit very similar to your phono channel - headphone jack from phone or laptop -> 6J5 gain stage -> 6SL7 PI -> 2x 6V6 power amp -> 8" full range speaker - and yeah they are plenty loud. Also the typical fender tone stack has a big mid-scoop that makes it "sound" quieter than something with more mid-range or a flat EQ.


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## Dreaming1 (Jun 21, 2022)

I'm about to open it up again. Put in the ohm selector switch and check my OT leads again. Wiring the switch gave me problems for a bit.
Im thinking about trying to reduce gain before I start redesigning too much. Maybe put in two 68k resistors as voltage divider before the coupling capacitor.
If that isn't it, I will remove it and start playing with resistor values. I saw someone say to remove the screen bypass cap and that would lower gain.
I read someone describe guitars in grid leak biased circuits as splatty. That's exactly what I get.
Here is a circuit I found that is another version of what I have...

After exploring this, I will cathode bias a channel and start learning some new stuff.
The adventure continues...


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## Dreaming1 (Jun 21, 2022)

Then I saw this which is a Gibson GA-5. A copy of a tweed champ. It only has an input resistor, so I guess I will give that a go too. Then play with resistor values.


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## weedstoner420 (Jun 21, 2022)

Hmmm, I can't comment on the overall sound of grid leak bias for guitar because I've never tried it. To me, "splatty" = too much gain across too few gain stages. 

I wouldn't hesitate to change that 10M grid-leak resistor to 1M and put a 1k between the cathode and ground (basically changing it to cathode bias), and see what it does to the sound.

And re: putting in a voltage divider to attenuate signal, I'd put it after the coupling cap. If it's before the coupling cap it will affect the DC bias of that tube...


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## Dreaming1 (Jun 21, 2022)

weedstoner420 said:


> Hmmm, I can't comment on the overall sound of grid leak bias for guitar because I've never tried it. To me, "splatty" = too much gain across too few gain stages.
> 
> *seems that way. Attack clips hard and then the decay cuts off sharply. Like a transistor fuzz box doing the voltage starved velcro tearing sound.*
> 
> ...


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## Dreaming1 (Jun 21, 2022)

What about in that circuit above? The signal is input resistor-capacitor-resistor to ground-tube grid. And they are saying it is voltage divider function. With a capacitor inside it? How does the cap function in that with relation to the tube?


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## weedstoner420 (Jun 21, 2022)

Dreaming1 said:


> What about in that circuit above? The signal is input resistor-capacitor-resistor to ground-tube grid. And they are saying it is voltage divider function. With a capacitor inside it? How does the cap function in that with relation to the tube?


The cap is just there to block the DC voltage on the tube grid from getting to your guitar. The grid voltage is always gonna be negative with respect to the cathode - with grid leak bias, the cathode is grounded (0V) and the grid is at some small negative DC voltage (they call it -0.8V in that one schematic), and thus the cap is required. In cathode bias, however, the cathode is at a small _positive_ voltage, and the grid is at 0VDC - still negative with respect to the cathode, but then the cap is not required to block DC.

The cap and 10M resistor to ground technically form a high-pass filter, but the resistance is so high that even with a .0047uF like cap in the first pic, the cutoff frequency is still way below 20 Hz (f=1/(2*pi*R*C))...

So the cap blocks DC but is basically invisible to audio frequencies, and then you're left with the 2 resistors forming a voltage divider. But it's not even that much of a voltage divider, like even with the 75k//5M combo, you only lose like 1-2% of the input voltage...so I'm not really sure why the smaller resistor is there...maybe it has some interaction with the impedance of the guitar pickups...? That's probably something where it would be good to experiment with different values for that first resistor (say 0 up to 100k) and see if any of them make a difference.


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## Dreaming1 (Jun 21, 2022)

weedstoner420 said:


> The cap is just there to block the DC voltage on the tube grid from getting to your guitar. The grid voltage is always gonna be negative with respect to the cathode - with grid leak bias, the cathode is grounded (0V) and the grid is at some small negative DC voltage (they call it -0.8V in that one schematic), and thus the cap is required. In cathode bias, however, the cathode is at a small _positive_ voltage, and the grid is at 0VDC - still negative with respect to the cathode, but then the cap is not required to block DC.
> 
> The cap and 10M resistor to ground technically form a high-pass filter, but the resistance is so high that even with a .0047uF like cap in the first pic, the cutoff frequency is still way below 20 Hz (f=1/(2*pi*R*C))...
> 
> So the cap blocks DC but is basically invisible to audio frequencies, and then you're left with the 2 resistors forming a voltage divider. But it's not even that much of a voltage divider, like even with the 75k//5M combo, you only lose like 1-2% of the input voltage...so I'm not really sure why the smaller resistor is there...maybe it has some interaction with the impedance of the guitar pickups...? That's probably something where it would be good to experiment with different values for that first resistor (say 0 up to 100k) and see if any of them make a difference.


"You know a little."  Thank you.


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## weedstoner420 (Jun 22, 2022)

Dreaming1 said:


> "You know a little."  Thank you.


Hah, I do like to get down into the nitty-gritty...just don't give me anything with transistors in it, those things still make my head spin.


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## Dreaming1 (Jul 8, 2022)

I haven't changed anything yet. Just exploring. Hooked up to a pair of 12" speakers with 4Ω impedance. Running this way altered the amp channels. Changing the amps impedance setting had more effect with the 8Ω speaker load than the pair. 
I run a lot of pedals in front of it too. Makes the channels better. So I think an input resistor might calm the gain down. 
I bought a nos (if anyone can be trusted with money) 6sj7's and a 6sc7. Sylvanias and a Raytheon. I am about to get set up to test it while live. Then I can check voltages and current and start playing with resistor values. 
Saw some info about screen coupling caps should be tied to cathode instead of ground. I will check that out too.


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## weedstoner420 (Jul 9, 2022)

Dreaming1 said:


> Saw some info about screen coupling caps should be tied to cathode instead of ground. I will check that out too.


I just googled this and the first result was from Merlin Blencowe's page. Designing Tube Preamps for Guitar and Bass is definitely one of my primary sources for info on this subject...


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## Dreaming1 (Jul 12, 2022)

Got some tubes in yesterday. Pair of 6sj7's and I grabbed a 6SL7. Maybe the 6sc7 will arrive today. Going to enjoy some tube rolling and then have spares. 

Update: The original tubes test "good" by operating at all. These new 6sj7 are probably stronger. Ch 1 cleaned up some. Ch2 didn't.


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## Dreaming1 (Aug 24, 2022)

Life has kept me busier lately. I got back into the guts of this amp. Peeled back to a single 6sj7 into the phase inverter. A lot nicer. Still getting crunchy around 3 o'clock on the volume knob. Backing volume down on guitar is cool. Might explore a tone bleed on my strat. 
Im going to try reducing input signal. Then I will mess with the screen and plate values. I may leave it as is for ch1. Will set up ch 2 as cathode biased and compare. It sounds old. I dig it. 
Then I have the 6sl7,6sq7,and 6sc7 tubes to play with. Getting interesting now. I will post some a/v later.


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## Dorian2 (Aug 24, 2022)

I use a tone (treble) bleed circuit on my LP @Dreaming1 . Well worth it and simple to construct, apply, and easily reversible. You should go for it.


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## Dreaming1 (Aug 24, 2022)

Here's some sounds. Cleaned up some without the 6sc7 mixer adding gain. Still no headroom. Clean up to 2-3, then crunch zone. Pretty cool. I got an eminence the tonker, so I will mix that with the swamp thang and see what happens. 1 more and my 4x12 will be complete.


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## weedstoner420 (Aug 25, 2022)

Dreaming1 said:


> Here's some sounds. Cleaned up some without the 6sc7 mixer adding gain. Still no headroom. Clean up to 2-3, then crunch zone. Pretty cool. I got an eminence the tonker, so I will mix that with the swamp thang and see what happens. 1 more and my 4x12 will be complete.


Whoa! Sounding real good now, definitely a nice range of sounds from just one volume knob. Great job!


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## Dreaming1 (Aug 29, 2022)

I am moving on down the line one tube at a time. I wired up just the 6sc7 into .047 cap, into volume knob, into PI. It sucks. The phono knob is doing the same thing with this as with 6sq7. It acts like a switch. Nothing up to halfway, then sound comes through speaker. I will try the tone pot.
I will mess with plate voltage, try a cathode bypass cap. If I get it quiet, and less gain, I will bridge the two grids with input signal. 
Noisy. Microphonic a bit. Fuzzy gain structure makes a splatty attack, and the decay on note fall apart.


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## weedstoner420 (Aug 29, 2022)

Dreaming1 said:


> I am moving on down the line one tube at a time. I wired up just the 6sc7 into .047 cap, into volume knob, into PI. It sucks. The phono knob is doing the same thing with this as with 6sq7. It acts like a switch. Nothing up to halfway, then sound comes through speaker. I will try the tone pot.
> I will mess with plate voltage, try a cathode bypass cap. If I get it quiet, and less gain, I will bridge the two grids with input signal.
> Noisy. Microphonic a bit. Fuzzy gain structure makes a splatty attack, and the decay on note fall apart.


Could just be a bad volume pot the way it cuts in and out all of a sudden like that. Especially like around the 2:00 mark in the vid. That and maybe a noisy plate resistor as well? (Dunno if you have changed out either one of those already...)


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## Dreaming1 (Aug 29, 2022)

I left all components as I found it, just moved the input to the 6sc7s' first grid. I wired it up backwards on the volume pot the first time. Had to look at again the next day with fresh eyes. I will mess with this circuit for awhile. See what happens before I decide what to do with 6sc7. Then I will move on to the 6sq7. Then try a cathode biased 6sj7, and finally the 6sl7. 
I feel like this amp might end up with 2 channels and have the phono and tone knob be for bass and treble controls. But I have eyeballed a tube driven FX loop...an lot of different EQ boxes could go there.


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## weedstoner420 (Aug 30, 2022)

Dreaming1 said:


> I left all components as I found it, just moved the input to the 6sc7s' first grid. I wired it up backwards on the volume pot the first time. Had to look at again the next day with fresh eyes. I will mess with this circuit for awhile. See what happens before I decide what to do with 6sc7. Then I will move on to the 6sq7. Then try a cathode biased 6sj7, and finally the 6sl7.
> I feel like this amp might end up with 2 channels and have the phono and tone knob be for bass and treble controls. But I have eyeballed a tube driven FX loop...an lot of different EQ boxes could go there.


Dang, sounds like you've got your work cut out for you  hope you land on something you enjoy playing!

For tube driven FX loops, I'd highly recommend looking up Merlin's Practical Serial Effects Loop... it's simple, effective, and can be built on just a spare octal tube socket and a 5-position terminal strip if you're clever with the layout.


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## Dreaming1 (Aug 30, 2022)

Fun work. I am leaving the 6sj7 as is. I dig that a lot. If I end up with 3 or 4 channels, what is the best way to do that? Individual inputs and just jam all of the tubes outputs together into the phase lnverter? Or do I need some sort of switching to isolate each ch from the PI, one at a time scenario?
I unhooked the tied plates on 6sc7. So I am only using first half of tube. I swapped the pots out. Got a real nice clean with volume knob just right, at points it cuts out and above half it is hairy and scary. 
Here is demo. At 35 seconds the volume is going to attack you, so be mindful. From clean to nasty. And at the end you can hear the pulse of the circuit. I will keep messing with this part. The cleans are nice and I can hear character changes. The 6sc7 is still an option to me.


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## weedstoner420 (Sep 1, 2022)

Dreaming1 said:


> Fun work. I am leaving the 6sj7 as is. I dig that a lot. If I end up with 3 or 4 channels, what is the best way to do that? Individual inputs and just jam all of the tubes outputs together into the phase lnverter? Or do I need some sort of switching to isolate each ch from the PI, one at a time scenario?
> I unhooked the tied plates on 6sc7. So I am only using first half of tube. I swapped the pots out. Got a real nice clean with volume knob just right, at points it cuts out and above half it is hairy and scary.
> Here is demo. At 35 seconds the volume is going to attack you, so be mindful. From clean to nasty. And at the end you can hear the pulse of the circuit. I will keep messing with this part. The cleans are nice and I can hear character changes. The 6sc7 is still an option to me.


Dang, that is a weird one... Maybe a cold solder joint or loose tube socket pin...? Does the sound change if you tap around on stuff with a chopstick (or other plastic/insulated object) or wiggle either of the 6SC7's? And does it do it at about the same spot on the knob when you tried different pots?

Not sure if this has anything to do with it, but is there anything connected to the first 6SC7's grid besides your guitar? If not, I'd stick a 1 Meg resistor across the input jack from tip to ground, and a 10k in series between the input and the grid, with one leg of the resistor mounted directly on the grid pin. I'm not sure if it will make a difference sound-wise, but it's good practice to prevent oscillation.

For mixing the channels, it looks like the original designer was comfortable tying both 6SC7 plates AND the 6SQ7 plate to a single 68k plate resistor and .047 coupling cap... Personally that would sketch me out, if it were me I'd go with separate plate resistors and coupling caps for each channel. Then after each coupling cap have either a volume knob or a 100k or 220k resistor in series, then join all those at a 1M resistor to ground, then on to the PI. Hopefully that makes sense. Probably overkill but I'm just a diy'er and not concerned about part counts...


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## Dreaming1 (Sep 1, 2022)

Bad solder joint or tube socket wouldn't surprise me. Bad connections or junky ground reference is always a thing. Tapping the tubes just shows the 6sc7 as v1 to be a bit microphonic. 
I am only connecting my guitar to 1 preamp tube at a time and it goes to the phase inverter. All other pre tubes are out of the amp. Im keeping it simple, to prevent any circuit interactions as I try to figure out this stuff.
The 1st volume pot seems jacked up. Resistance on it wasn't measuring right when moving wiper. This 2nd pot measure good, but similar behavior to where clean cut out and where hairy gain starts. 
The last paragraph is exactly what I was thinking, except the 1M resistor to ground. I didn't know about that. I have seen audio circuits combined on a mixing resistor though.


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## weedstoner420 (Sep 1, 2022)

Dreaming1 said:


> Bad solder joint or tube socket wouldn't surprise me. Bad connections or junky ground reference is always a thing. Tapping the tubes just shows the 6sc7 as v1 to be a bit microphonic.
> I am only connecting my guitar to 1 preamp tube at a time and it goes to the phase inverter. All other pre tubes are out of the amp. Im keeping it simple, to prevent any circuit interactions as I try to figure out this stuff.
> The 1st volume pot seems jacked up. Resistance on it wasn't measuring right when moving wiper. This 2nd pot measure good, but similar behavior to where clean cut out and where hairy gain starts.
> The last paragraph is exactly what I was thinking, except the 1M resistor to ground. I didn't know about that. I have seen audio circuits combined on a mixing resistor though.


The 1M to ground would just be to give a ground reference to the PI's grid. Right now your volume (formerly tone) pot is doing that duty. Might not be needed if you have separate volume controls for each channel right before the PI, but in that case you'd still probably want mixing resistors to minimize interactions between volume controls.

Are you using a brand-new volume pot? Or swapping the original pots around? If original, I'd try a brand-new one to eliminate that as a possibility. You might just be swapping a bad one for a differently-bad one


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## Dreaming1 (Sep 1, 2022)

Yeah, I am just swapping the two pots that are on it. I desoldered them and checked resistance on them. One was bad. I will try some new ones. In a dark room I can see flashes of blue gas in the 6v6 pair. First one then the other, but farther up the volume knob, they both glow a bit at the bottom. I will check the socket and my wiring.


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## weedstoner420 (Sep 1, 2022)

Dreaming1 said:


> Yeah, I am just swapping the two pots that are on it. I desoldered them and checked resistance on them. One was bad. I will try some new ones. In a dark room I can see flashes of blue gas in the 6v6 pair. First one then the other, but farther up the volume knob, they both glow a bit at the bottom. I will check the socket and my wiring.


Blue glow won't hurt anything, some power tubes do it more than others. I think it's just electrons that don't know where they're going and hit the glass instead of the plate. I never noticed any correlation between the blue glow and the quality of the tube, but the ones that do it always do it more when they're being pushed hard.

If it jumps up and down in sync with your playing then it's totally fine. If it goes up and down randomly or when you're not playing, it could mean you've got subsonic or ultrasonic oscillation going on, like the tubes are conducting something but it's outside your range of hearing.


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## Dreaming1 (Sep 8, 2022)

Oh man...Im an idiot. I was running the wire leading to volume pot to a grounded lug on the terminal strip. I got it going nicely with a volume pot. The break up is gritty/grainy. I will play around with some values for this before I move on.


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## Dreaming1 (Sep 9, 2022)

Not a fan of this as the pre amp circuit. It does have an old time quality to it though. The stratish guitar sounds real grainy. The tele sounded more normal, but a little darker and low end heavy. Im going to mess around with plate and cathode voltages and see what happens.


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## weedstoner420 (Sep 9, 2022)

Dreaming1 said:


> Not a fan of this as the pre amp circuit. It does have an old time quality to it though. The stratish guitar sounds real grainy. The tele sounded more normal, but a little darker and low end heavy. Im going to mess around with plate and cathode voltages and see what happens.


Yah I agree, sounds pretty primitive. The middle setting has some nice depth to it though. And I'm glad you got the wiring sorted.

Maybe adding a smallish cathode bypass cap on the first stage, like between 0.47 and 2.2uF/25V, or lowering the first coupling cap from 0.047 to something between 0.001 and 0.01, would help to bring out the high end and tighten up the low mids...


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## Dreaming1 (Sep 12, 2022)

I tied both inputs and both plates together of the 6sc7. It lowered the noise floor. I had them disconnected completely. Must be driving slightly harder, the break up is different in a way that I cant quite put words to. It makes the tubes glow blue with the gain cranked. 
I have a twin reverb reissue and a vox ac15c1, so a higher gain amp with a 4x12 appeals to me, but these clean to crunchy channels aren't hurting my feelings because they sound so different than these other amps. I was already running the twin clean and the Vox kind of clean together.


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## Dreaming1 (Sep 13, 2022)

I reduced the cathode bypass cap on the 6v6's to 33uf. I thought maybe it was responsible for the throbbing. It may have been. It is gone now. I added 47k resistor to input of 6sc7, and reduced the coupling cap to PI to .022uf. Less bassy. Makes the distortion sound probably better for a mix. I will record a bit later. Then I will try cathode bypass cap on the 6sc7. May rush through checking the 6sq7 out and wire up a pair of 6sc7 and try series and parallel with that.


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## Dreaming1 (Sep 13, 2022)

Added 33uf (smallest value I had on hand for electrolytics) cathode bypass to 6sc7. Moved into more modern sounding territory. Lost the tweediness and black tolex has replaced it. Pretty cool though. Cleans are sparkly and the grind is thick. Not chuggy, but palm muting gets close. It's wild what a well placed capacitor can do.


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## Dreaming1 (Sep 14, 2022)

The 6sq7 is very microphonic. Cleans are nice. Halfway up on the volume knob and it starts singing a high tone that just gets worse. Break up sounds mushy and terrible. This tube is getting evicted. So next up is a pair of 6sc7's


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## weedstoner420 (Oct 7, 2022)

Not trying to derail, but possibly of interest - a Silvertone 1474 I've been working on for a friend. Looks like it was recapped at some point, I replaced a few noisy resistors, leaky and shorted caps, etc. It sounds pretty good with everything turned to 10...


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