Tube PA conversion

[Dreaming1]

Personally I would want that first grid stopper to be at least 10k, not 10 ohms (unless that's a typo)...


Nope. Ohms. I started slowly increasing it. Makes the tube microphonic at upper range. Could be parasitic osc.
Will try what you describe.
I am using cathode bias, but I saw some grid leak biased 40's-50's octal circuits using 47k input resistors. And a post where a guy suggested the designers loaded down the signal with impedance to an input level the grid leak bias design could handle. And then the older pickups had lower outputs too.
So that's why I started using the grid resistors. I thought, I would try to tame it before it went to grid.

You could also put a voltage divider like 470k/470k between the two 6sc7's...


Will get back to multi stages. Not wrapping my head around single triode just yet.

I'd save that black beauty cap for a hi-fi amp myself...


Ahhh...not my thing. I appreciate the sound, but don't want the equipment.
I have "several" C-64 C=,gameboys, and ataris video game consoles for music prurposes, pedal boards, and "a lot" of old CRT tv's... Once I get locked in to a collection...

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.


Excellent. Thank you. This amp seems to want to be a 3 ch amp with each ch having a different octal flavor, so an FX loop seems like a good idea for that spare socket.

 

[weedstoner420]

Nope. Ohms. I started slowly increasing it. Makes the tube microphonic at upper range. Could be parasitic osc.
Will try what you describe.

Huh, the grid stopper is supposed to make it less noisy, not more. If you're missing the 1 meg (or some high value like 100k+) from input jack tip to ground, maybe that could be why...?

Excellent. Thank you. This amp seems to want to be a 3 ch amp with each ch having a different octal flavor, so an FX loop seems like a good idea for that spare socket.

I looked up Merlin's loop, and there is a turret strip layout someone has posted online, but I couldn't find the actual schematic. Here's that in case you're curious (it's from his book "Designing Tube Preamps for Guitar and Bass" which I highly recommend). The schematic doesn't show it, but if you want the loop to be totally transparent, you'll have to put a voltage divider right before it, with a ratio that's the inverse of the tube's gain factor (so for a 6sl7 with a gain of 100, something like 1 meg in series with the grid and 10k from grid to ground). Otherwise it'll add more gain, which you may or may not want.

Good luck!

 

[Dreaming1]

Huh, the grid stopper is supposed to make it less noisy, not more. If you're missing the 1 meg (or some high value like 100k+) from input jack tip to ground, maybe that could be why...?


I looked up Merlin's loop, and there is a turret strip layout someone has posted online, but I couldn't find the actual schematic. Here's that in case you're curious (it's from his book "Designing Tube Preamps for Guitar and Bass" which I highly recommend). The schematic doesn't show it, but if you want the loop to be totally transparent, you'll have to put a voltage divider right before it, with a ratio that's the inverse of the tube's gain factor (so for a 6sl7 with a gain of 100, something like 1 meg in series with the grid and 10k from grid to ground). Otherwise it'll add more gain, which you may or may not want.

Good luck!

Yeah, I saw somewhere about whining tube could be fixed with cathode bypass cap and/or grid resistor. My issue hopefully is related to the resistor to ground. Or perhaps just these tubes. A lot of microphonics talk happens around octals in the forum posts.I will get back to it and see what happens.
I have read up on some loop circuits. Lower the gain going in, and recover stage amplifies back up to unity gain. Maybe even a pot to adjust boost level. And also about the loop circuit placement in the amp circuit.

 

[Dreaming1]

I took liberties with eq, compression, delay, and reverb. But here is the amp set to 3 to 4 on rhythm part and set to 6 to 7 on lead part. Adding 1M resistor on input tip to ground didn't fix it. Everything is good until the amp volume hits 7.5 when the high pitched whining begins. Guitar volume turned down and all is silent. It would be my luck that my junk strat was part of the issue. I guess I should give the other ones some love and see. Everything is connected. It is all one AC circuit.

 

[weedstoner420]

I took liberties with eq, compression, delay, and reverb. But here is the amp set to 3 to 4 on rhythm part and set to 6 to 7 on lead part. Adding 1M resistor on input tip to ground didn't fix it. Everything is good until the amp volume hits 7.5 when the high pitched whining begins. Guitar volume turned down and all is silent. It would be my luck that my junk strat was part of the issue. I guess I should give the other ones some love and see. Everything is connected. It is all one AC circuit.

Dang, sounding pretty good! Some trippy riffs there man... @.@

 

[Dreaming1]

Thanks, man.
I've been reading about grid resistors. Going to throw much higher values in there and see how it goes.
Later: It didn't do anything. I have a 1M resistor to ground on jack, and a 68k grid resistor.
I started reducing plate resistor. No reduction of anything happened down to 22k. Then I went to 1k. Sounded very quiet and very clean. Just barely to the edge of crunchy rhythm when maxed out. Probably what a 40s-50s guitar amp was like.
So, I'm back to the 68k plate resistor and am bringing up the cathode resistor tomorrow.

 

[Dreaming1]

I'm getting real close to getting rid of the squeal. 10k cathode resistor gets up to 9 on the dial before it starts. And there is still some good crunch on tap. 22k gets a very polite,clean amp that is on the verge of crunch when dimed.
Get this amp hooked up with 12k or so on cathode and I should be good. I may go back later and try some higher plate resistors and adjust the cathode resistor again to hear the spectrum of the tubes response at different settings. But, I'm immediately adding another 6sc7 gain stage and start playing around.

 

[Dreaming1]

I had to go to a 20k cathode resistor to get the squeal to stop. Or at least go beyond my hearing range. No oscilloscope to check if it still there.
So, it neutered the amp. Real calm and clean. Just a touch of crunch. Like an old, little amp. So, I wired up a second stage just like the 1st. 20k on cathode, 68k on plate, no grid resistor, just a coupling cap of .068uf between stages. The balls are back. Clean until halfway up, where it crunches on digging in hard. Edge of breakup sound. Crank the volume, and this amp is touch sensitive. The chefs kiss. Mmmmwha! Very rock'n roll when on 10.
I'm wiring up a 3rd 6sc7 gain stage. I need to check the 6sl7 out next. Then I may need to order more of them. Could head in the 8 gain stage direction.

 

[weedstoner420]

I had to go to a 20k cathode resistor to get the squeal to stop. Or at least go beyond my hearing range. No oscilloscope to check if it still there.
So, it neutered the amp. Real calm and clean. Just a touch of crunch. Like an old, little amp. So, I wired up a second stage just like the 1st. 20k on cathode, 68k on plate, no grid resistor, just a coupling cap of .068uf between stages. The balls are back. Clean until halfway up, where it crunches on digging in hard. Edge of breakup sound. Crank the volume, and this amp is touch sensitive. The chefs kiss. Mmmmwha! Very rock'n roll when on 10.
I'm wiring up a 3rd 6sc7 gain stage. I need to check the 6sl7 out next. Then I may need to order more of them. Could head in the 8 gain stage direction.

Nice!

I'm currently working on my 2nd build of the SLO 100 circuit (the "OG" high-gain shredder amp), and one of principles of that circuit is adding a small amount of gain at a time, with attenuation between every stage. It ends up being more musical and responsive than trying to slam one or two tubes really hard.

Also, if you've still got both sides of that tube paralleled, you could undo that and make your two gain stages from one tube... wiring the two halves in parallel doesn't change a whole lot for preamp tubes

 

[Dreaming1]

Nice!

I'm currently working on my 2nd build of the SLO 100 circuit (the "OG" high-gain shredder amp), and one of principles of that circuit is adding a small amount of gain at a time, with attenuation between every stage. It ends up being more musical and responsive than trying to slam one or two tubes really hard.

Also, if you've still got both sides of that tube paralleled, you could undo that and make your two gain stages from one tube... wiring the two halves in parallel doesn't change a whole lot for preamp tubes

I could see that being better. I also want to play around with "almost too much" on the 1st stage and using the 2nd stage to choke it way down. I ordered another 6SL7.
They are still; the grids and plates are tied together respectively, and they already share a common cathode. Just 1 big triode. I did it for ease of wiring. Just put the things where they were closest. I wondered if that was making anything wonky. Will using both sides individually and them still sharing the cathode not have an effect? Like localized feedback loop between stages or something?
I ordered this: https://www.mojotone.com/Mojotone-Discrete-Hi-Voltage-Series-Effects-Loop-for-Vacuum-Tube-Amplifiers
Thinking about mounting it on tube side of chasis on standoffs, where the canned cap and mic transformers were.

 

[weedstoner420]

I could see that being better. I also want to play around with "almost too much" on the 1st stage and using the 2nd stage to choke it way down. I ordered another 6SL7.
They are still; the grids and plates are tied together respectively, and they already share a common cathode. Just 1 big triode. I did it for ease of wiring. Just put the things where they were closest. I wondered if that was making anything wonky. Will using both sides individually and them still sharing the cathode not have an effect? Like localized feedback loop between stages or something?
I ordered this: https://www.mojotone.com/Mojotone-Discrete-Hi-Voltage-Series-Effects-Loop-for-Vacuum-Tube-Amplifiers
Thinking about mounting it on tube side of chasis on standoffs, where the canned cap and mic transformers were.

Ahhh, I forgot about the shared cathode on that tube...disregard that suggestion lol. It's fine for two unrelated signals, like the how they originally used it for the two mic channels, but cascading the two sides would probably be very unstable.

That Mojotone FX loop is a pretty cool piece of kit, and over top of the old mic transformer sockets is probably a perfect place for it...

 

[Dreaming1]

I got the fx loop pcb, waiting on some stand offs. I wired the 6sj7 back in. Messed around with raising plate voltage a hair. Now adding some more input impedance.
I am about to wire up the next slot for cathode biased 6sj7. I will play with that. Decide if I want to keep the grid leak biased circuit or not. Then on to 2 stages of 6sj7.
If I end up with a 2 channel amp, it will have channels that are in phase. That could lead to some bassman style ch jumpering. I had planned on keeping them separated. But...
I have the 6SL7 to try, and am waiting on a 2nd in the mail. Will most likely replace the 6SC7's with these. More gain stages.
Thanks for your time and help.