Tube PA conversion

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...

Dreaming1 · Feb 26, 2023

weedstoner420 said:
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...
View attachment 5209287

I have only seen 1 old sizzle tone. It sounded great. Idk where they all went. I don't hardly see them "around these parts."
I tried to wire up 6sc7's in series. Did a thing where I divided the voltage instead of sharing it. Got distracted by life, and got pulled into drum/percussion land. Looked at it again last week and figured it out. Was quiet and very clean. So, I reduced cathode resistance and boosted plate volts on one tube and it got heavy. I'm playing around with it now. Once I get it to where I like it, I will add the 6sl7 in and set it up.
Looks like Im having 3 inputs and going to set up a threeway switch to route preamps to PI. Probably be a bit confusing in the future. Will have to plug in for which pre wanted, and have switch set to corresponding position for that pre to be sent to PI. Trying to keep all the pre's isolated from each other. Not even sure it matters, but I like the idea..? End up with 3 preamps with individual volumes, and a tone knob. Might look at push/pull or concentric potentiometer and do a bass/treble cut, bandpass thing.

weedstoner420 · Feb 28, 2023

Dreaming1 said:
I have only seen 1 old sizzle tone. It sounded great. Idk where they all went. I don't hardly see them "around these parts."
I tried to wire up 6sc7's in series. Did a thing where I divided the voltage instead of sharing it. Got distracted by life, and got pulled into drum/percussion land. Looked at it again last week and figured it out. Was quiet and very clean. So, I reduced cathode resistance and boosted plate volts on one tube and it got heavy. I'm playing around with it now. Once I get it to where I like it, I will add the 6sl7 in and set it up.
Looks like Im having 3 inputs and going to set up a threeway switch to route preamps to PI. Probably be a bit confusing in the future. Will have to plug in for which pre wanted, and have switch set to corresponding position for that pre to be sent to PI. Trying to keep all the pre's isolated from each other. Not even sure it matters, but I like the idea..? End up with 3 preamps with individual volumes, and a tone knob. Might look at push/pull or concentric potentiometer and do a bass/treble cut, bandpass thing.

I have seen a couple but this is the first one I've played/opened up. I can definitely tell this was a "budget" amp - not particularly sturdy or serviceable, cheap parts/small transformers, etc. I wonder if the reason you don't see many around is cuz they just didn't last all that long...

Plus some annoying design idiosyncracies that make it hard to debug... It's got two output transformers (one for each pair of output tubes) meaning you need two dummy loads to run it quietly for testing. And the first two preamp tubes have their filaments wired in series, with the filaments themselves serving as part of the cathode resistance for the power tubes. So if you remove the power tubes, the preamp tubes won't light up, and if you remove the preamp tubes, the power tubes run super cold. Then when the power tubes start to get old/not conduct as well, the preamp tubes' filament voltage goes down... All in all, not an amp I would look forward to troubleshooting in the future.

I like your idea about keeping the preamps of your amp separate as much as possible. Instead of a switch you could try doing it the way Fender did on their 2-channel amps, with 220k mixing resistors. Something like preamp -> coupling cap -> 220k resistor for each channel, then the three 220k resistors all join together at the input to the PI (see twin reverb schematic attached). Would simplify operation at least, and you could run multiple channels at once. Not sure how much of an impact it would have on the overall sound though, separating them with a high resistance instead of just an open circuit...

Dreaming1 · Feb 28, 2023

Yikes. That design does sound like a bad scene. I didn't think about the budget aspect of the sears amps. But, it makes sense.
I thought about mixing across resistors. Still might. I will try both ways. Idk if it matters sound wise. Just thinking cross talk and possible negative interaction. If using mixing resistors, I could use a switch and a single input to desired pre amp. Then, I would have an unused place for another switch. Switchable NFB loop, tone before/after pre amp or bypassed, cathode bypass cap selector, or even a standby switch.
I have tried a few different set ups on these cascading 6sc7's. I have some real creamy compressed stuff happening. Sort of fuzz territory. But way to much gain. I will try to tame it with some grid stop resistors to attenuate signal before each tube. May have to reduce plate V on first one or both.
I will try to figure it out and get something to show this week. This is the fun part for me. I need resistor and capacitor substitution boxes. Flip switches and listen to what happens while watching some meters.

weedstoner420 · Feb 28, 2023

Dreaming1 said:
Yikes. That design does sound like a bad scene. I didn't think about the budget aspect of the sears amps. But, it makes sense.
I thought about mixing across resistors. Still might. I will try both ways. Idk if it matters sound wise. Just thinking cross talk and possible negative interaction. If using mixing resistors, I could use a switch and a single input to desired pre amp. Then, I would have an unused place for another switch. Switchable NFB loop, tone before/after pre amp or bypassed, cathode bypass cap selector, or even a standby switch.
I have tried a few different set ups on these cascading 6sc7's. I have some real creamy compressed stuff happening. Sort of fuzz territory. But way to much gain. I will try to tame it with some grid stop resistors to attenuate signal before each tube. May have to reduce plate V on first one or both.
I will try to figure it out and get something to show this week. This is the fun part for me. I need resistor and capacitor substitution boxes. Flip switches and listen to what happens while watching some meters.

Yeah it's definitely easy to overdo it when cascading preamp stages. A couple in a row without much attenuation turns into a square-wave mess that is not pleasing to the ear at all.

I'm actually just finishing up a long-term project that's a clone of the SLO 100 amp, the OG high-gain design that inspired the 5150 and others. It uses a lot of voltage dividers for attenuation between stages, adding gain a little bit at a time across 4-5 stages instead of 2 or 3. And another famous high-gain circuit, the Dumble ODS, is basically 4 cascaded gain stages in a row, but with a 20:1 voltage divider between the 2nd and 3rd. Long story short, don't be afraid to use a voltage divider to attenuate between cascaded gain stages.

Dreaming1 · Jul 15, 2023

I got my 4x12 cabinet. Celestion swamp thang and the tonker. I have been playing this head with it. I figured out that the caps I put in replacing the .047uf are .47uf. I will adjust those and see what happens. I don't want to neuter this too much. I did get a 6sj7gt and it is a world better. Smoother and a bit lowered gain maybe, which was better in this situation. And not microphonic at all.
I have not had success cascading the two 6sc7's. I am about to start digging around since I got some fresh caps.

weedstoner420 · Jul 25, 2023

Dreaming1 said:
I got my 4x12 cabinet. Celestion swamp thang and the tonker. I have been playing this head with it. I figured out that the caps I put in replacing the .047uf are .47uf. I will adjust those and see what happens. I don't want to neuter this too much. I did get a 6sj7gt and it is a world better. Smoother and a bit lowered gain maybe, which was better in this situation. And not microphonic at all.
I have not had success cascading the two 6sc7's. I am about to start digging around since I got some fresh caps.

Good catch, if you're talking about the coupling caps I think .047 would still give you a cutoff frequency below the guitar's range. I don't think you'd lose any low end until you get to like .005uf...

And how are you cascading the 6SC7s? And what's the issue you're having with it?

Dreaming1 · Aug 13, 2023

weedstoner420 said:
Good catch, if you're talking about the coupling caps I think .047 would still give you a cutoff frequency below the guitar's range. I don't think you'd lose any low end until you get to like .005uf...

And how are you cascading the 6SC7s? And what's the issue you're having with it?

I meant every .047uf cap got replaced with .47uf caps. LOL.
So, I just now replaced those caps with .1uf caps. I left the .47uf input cap and the .47uf cap on the power switch circuit. I got improved high end clarity, lost the slight throbbing effect with the volume dimed, and gained a hair of headroom. It is still very ballsy. I have some .068uf and .047uf that I can mess with too. Then I'm going to play with plate and grid voltages. I would like a touch more headroom, so I will raise plate voltage.
I need to get my meter on there and see what my voltages are in the 6sc7 circuit. I think that's going to be the deal. I had a single tube working. Wired up 2nd one without testing it, then wired them into each other. I got nothing out.
I have been rolling tubes in my Fender TRRI. 6L6's quit, so I got a new set of TAD. Mmmmm... tasty. Then I got some NOS 50's and 60's 12ax7 and 12at7 tubes. RCA and GE long plates, Grey and black. Short plates too. Older, bigger plate versions have softer attack, rounder eq, and more full,3D sound. Later years seem more focused and sharper attack. Then I got some Wurlitzer branded 12ax7's. Damn! Silent and beautiful. I know branding is just stamps, but I read that the electric organ companies selected for lowest noise. I found a Telefunken made in Germany for a "decent" price. But, swapping tubes in the Vox ac15c1 was less dramatic. I have a full set of JJ's in that one. I am getting some 5751's for it. See if I get any extra headroom or clarity, or just a shift in where the volume knob is for same sonic territory.

weedstoner420 · Aug 13, 2023

Dreaming1 said:
I need to get my meter on there and see what my voltages are in the 6sc7 circuit. I think that's going to be the deal. I had a single tube working. Wired up 2nd one without testing it, then wired them into each other. I got nothing out.

Ah yeah could be just a missed connection somewhere. Are there still two 6SC7's in the circuit (the one where the two halves were originally in parallel for the two mic channels, and the one for the PI)? Or is there a third one now?

Dreaming1 said:
I have been rolling tubes in my Fender TRRI. 6L6's quit, so I got a new set of TAD. Mmmmm... tasty. Then I got some NOS 50's and 60's 12ax7 and 12at7 tubes. RCA and GE long plates, Grey and black. Short plates too. Older, bigger plate versions have softer attack, rounder eq, and more full,3D sound. Later years seem more focused and sharper attack. Then I got some Wurlitzer branded 12ax7's. Damn! Silent and beautiful. I know branding is just stamps, but I read that the electric organ companies selected for lowest noise. I found a Telefunken made in Germany for a "decent" price. But, swapping tubes in the Vox ac15c1 was less dramatic. I have a full set of JJ's in that one. I am getting some 5751's for it. See if I get any extra headroom or clarity, or just a shift in where the volume knob is for same sonic territory.

Some nice selections in there. I've been messing around with 7025's in high-gain circuits for the lower noise, and new production ones are almost the same price as a 12AX7

Dreaming1 · Aug 13, 2023

weedstoner420 said:
Ah yeah could be just a missed connection somewhere. Are there still two 6SC7's in the circuit (the one where the two halves were originally in parallel for the two mic channels, and the one for the PI)? Or is there a third one now?

There is a 3rd 6sc7 now where the 6sq7 was. I wired the inputs together on first tube. Then from the 1st into the 2nd with its inputs tied together, and on to PI. I will dig into it this week. Got it back where I can work on it. Will make sure both parts work and then put them together again.

I see people say NOS tubes are too expensive. Probably depends on the tube. But decent new production tubes aren't really cheap either.

weedstoner420 · Aug 14, 2023

Dreaming1 said:
There is a 3rd 6sc7 now where the 6sq7 was. I wired the inputs together on first tube. Then from the 1st into the 2nd with its inputs tied together, and on to PI. I will dig into it this week. Got it back where I can work on it. Will make sure both parts work and then put them together again.

I see people say NOS tubes are too expensive. Probably depends on the tube. But decent new production tubes aren't really cheap either.

Nice, hopefully it's something simple. In my experience, "no sound" is usually easier to diagnose than "bad sound."

Tubes have definitely gone up in price in recent years. I wanna say like 1.5x for new production, not sure about NOS. The old stock of guitar amp tubes like 6L6, 12AX7, etc are definitely gone be the priciest, but the off-the-beaten-path types like octal preamp tubes, compactrons, stuff that's "electrically equivalent but different pinout," etc are still pretty cheap, you just gotta know where and how to use them...

Dreaming1 · Aug 19, 2023

Unfortunate news, I may be losing my mind. I was turning a volume knob wired to nothing. Fisher Price amp for this big boy.
I have sound. It is shit. Too much signal. The amp takes a long warm up time to give sound, then with volume low, the sound sputters out like a voltage starved transistor fuzz pedal doing the "ripping velcro." High volume settings and it just fades away. I have to play lightly and the signal comes back. Im going to mess with the cathode and plate resistors relationship. And will reduce signal into 2nd tube.
I may add a small input resistor. Turning volume down on guitar helped not overload things.

Dreaming1 · Aug 19, 2023

I got a sexy shoulders 5y3. No tone, but it looks cool.

Dreaming1 · Aug 20, 2023

Had a minute between duties today, so I added a couple of 200Ω grid stoppers. It's getting better. Nice cleans for a second, but mostly raunchy filth. At the higher volume settings I have a low toned oscillation. Too much still. I will start adjusting resistors. Brush up on load lines and bias points too. My knowledge has large blank areas here.

weedstoner420 · Aug 20, 2023

Dreaming1 said:
Had a minute between duties today, so I added a couple of 200Ω grid stoppers. It's getting better. Nice cleans for a second, but mostly raunchy filth. At the higher volume settings I have a low toned oscillation. Too much still. I will start adjusting resistors. Brush up on load lines and bias points too. My knowledge has large blank areas here.

Do you have any voltage dividers between those stages? That might help clean it up. Maybe start with a 1 meg volume knob wired between them and see if you can find a good setting, then measure it and replace with resistors...

Dreaming1 · Aug 20, 2023

I do now. Not any cleaner or more headroom before grind, but the distortion tightened up nicely. The oscillation at highest volume setting changed too. More of a throb than a doppler effect tone. I will keep at it. I need some cleans from it, and I can always lower the input signal, but having all of these crunchy and creamy dirt flavors is rounding out my amp situation.

weedstoner420 · Aug 20, 2023

Dreaming1 said:
I do now. Not any cleaner or more headroom before grind, but the distortion tightened up nicely. The oscillation at highest volume setting changed too. More of a throb than a doppler effect tone. I will keep at it. I need some cleans from it, and I can always lower the input signal, but having all of these crunchy and creamy dirt flavors is rounding out my amp situation.

Huh, is the volume knob in front of the first tube? Or between the first two? It's pretty hard to overdrive a tube with just the guitar input, but two triodes in a row with no attenuation between could be too much...I dunno, just a guess. Good luck and good job sticking with it!

Dreaming1 · Aug 20, 2023

weedstoner420 said:
Huh, is the volume knob in front of the first tube? Or between the first two? It's pretty hard to overdrive a tube with just the guitar input, but two triodes in a row with no attenuation between could be too much...I dunno, just a guess. Good luck and good job sticking with it!

The volume knob is behind both tubes before PI. I will draw out what I have going on for a circuit. Not that it matters. Things are in constant flux right now.

Dreaming1 · Sep 8, 2023

I must have had a bad solder joint somewhere. I unhooked the 6sc7's and seperately,both work. I am having no problem overdriving this amp. I may need to get my meter in there. I should check output tubes plate disapation. And I have 2 taps on input transformer. 115v and 130v. It is set on 130v tap, but my wall outlet is supplying117v. Am I doing the EVH Brown sound approach with variac lowered AC? I should at least swap the wires and listen.
For the single 6sc7 tube circuit, I have a 680 ohm cathode resistor, 68k ohm plate resistor, and a 10 ohm grid stopper resistor between guitar and grid. Both plates are tied together, and both grids are tied together. Pretty good stuff. Real clean until about halfway up. Sounds old to me. Bass forward. It gets plenty of grainy dirt when dimed.
I will mess with these 6sc7 for awhile. Then will try one driving a 6sj7. Then run a pair of 6sj7 in series. Then figure out what strapped triode configuration is.
I saw a post where a guy explains a circuit for switchable grid leak bias and cathode bias for 6sj7. Sounds like a cool option. If this amp ends up as a 3 channel single octal tube preamp flavor thing, that spare socket makes me want to try tube fx loop send and recover with a 6sl7 or other split cathode dual triode. :confused:

I got gifted a NOS .1uf 600v black beauty capacitor. It measures good. Should I just admire it, or install it? Magical or hype? have a .1uf orange drop as coupling cap on the 6sj7 at the moment. That is at least in the signal path.
Later on.
A few minutes later, I tried 1k cathode resistor on 6sc7. It cleaned way up. Somehow focused the sound. Pretty nice clean sound. But, after 7 on the volume, tube becomes very microphonic. I guess the reduced differential between cathode and plate voltages is doing that..? Might be nice clean with more input impedance.

weedstoner420 · Sep 10, 2023

Dreaming1 said:
I must have had a bad solder joint somewhere. I unhooked the 6sc7's and seperately,both work. I am having no problem overdriving this amp. I may need to get my meter in there. I should check output tubes plate disapation. And I have 2 taps on input transformer. 115v and 130v. It is set on 130v tap, but my wall outlet is supplying117v. Am I doing the EVH Brown sound approach with variac lowered AC? I should at least swap the wires and listen.
For the single 6sc7 tube circuit, I have a 680 ohm cathode resistor, 68k ohm plate resistor, and a 10 ohm grid stopper resistor between guitar and grid. Both plates are tied together, and both grids are tied together. Pretty good stuff. Real clean until about halfway up. Sounds old to me. Bass forward. It gets plenty of grainy dirt when dimed.
I will mess with these 6sc7 for awhile. Then will try one driving a 6sj7. Then run a pair of 6sj7 in series. Then figure out what strapped triode configuration is.
I saw a post where a guy explains a circuit for switchable grid leak bias and cathode bias for 6sj7. Sounds like a cool option. If this amp ends up as a 3 channel single octal tube preamp flavor thing, that spare socket makes me want to try tube fx loop send and recover with a 6sl7 or other split cathode dual triode.
I got gifted a NOS .1uf 600v black beauty capacitor. It measures good. Should I just admire it, or install it? Magical or hype? have a .1uf orange drop as coupling cap on the 6sj7 at the moment. That is at least in the signal path.
Later on.
A few minutes later, I tried 1k cathode resistor on 6sc7. It cleaned way up. Somehow focused the sound. Pretty nice clean sound. But, after 7 on the volume, tube becomes very microphonic. I guess the reduced differential between cathode and plate voltages is doing that..? Might be nice clean with more input impedance.

Personally I would want that first grid stopper to be at least 10k, not 10 ohms (unless that's a typo). Typical values for guitar amps are in the 22k - 68k range, with a 1 meg strapped across the input jack from tip to ground (unless of course you're doing grid-leak bias).

You could also put a voltage divider like 470k/470k between the two 6sc7's to control the gain, or even a second volume knob (or use a dual-section pot in place of the single volume knob).

I'd save that black beauty cap for a hi-fi amp myself. .022 uf is plenty for guitar without letting too much low end through.

Oh yeah, and if you're looking for a single-tube FX loop, you might want to check out "Merlin's practical serial effects loop." I've stuck that into a few amps and it's as simple and transparent as they come. You can use just about any dual triode for it

Tube PA conversion

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