Tube PA conversion

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.

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.

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.

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

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.

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.

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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...)

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.

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.

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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.

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.

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.

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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...

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.

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.

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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

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.

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.

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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.

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.

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.

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.

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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...

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.

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.

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.

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