PA amp into a guitar tube amp? Need advice!

[Eldin]

Hey everyone!

I posted about the amp I picked up a while back, but I didn't have the time to work on it so it was left on the side. Turns out this whole virus thing actually had a positive!

My main question is, how do I go about getting the amp to go into more distortion?

It's also fairly bass heavy, so I reduced the 10uf capacitor on pin 4 of the 7C7 to a .47uf thinking that would help, but it still has way too much bass. The tone adjustment knob is more or less not there.

I think lowering the 600ohm resistor on pin 7 of the 7C7 would allow for more overdrive in the tube, but I'm not sure what levels are safe (or if that's even right)!

Also added a 10k grid resistor to the mic input to ground.

Attaching a pic of the schematic!

 

[cademan]

I don't play guitar but if you go to youtube you can find all sorts of people who have converted PA amps for guitar use especially the Bogen Challenger series.

 

[gadget73]

if you want to drop the LF response, making the .01 between 3 and 5 of the 7F7 say a .001 will do it.

Put a cap across the 2K (listed as 2M, old schematics used M for thousand) on pin 2 of the 7F7 to bump the gain. If you're not looking for any bottom end, maybe a 0.47 would be about right. Tweak as you see fit.

 

[Patrice B]

I've bought a Traynor YVM-1 before XMas and decided that it was fine... for bass. I know there are some mods to make it a little more dirty. As for dirtiness, I made a clone of an Alembic bass preamp... Best of both worlds in my case: stock PA (recapped electrolytics) and a separate preamp to add some grunt when needed.

 

[whoaru99]

What I have done with a couple PA amps in the past is pretty much gutted the front end circuitry and rebuilt them using plans from AX84. More or less left the output stages as they were.

 

[Eldin]

Actually that did bring the bass down a bit, but it's still a small bit boomy.

I put the cap across that pin, and it didn't really seem to do much of anything? Even went up to a 1uf and nothing much changed.

I wonder if I'm just going to have to swap out the 7c7 and 7f7 for something else?

 

[gadget73]

7f7 is basically a 6SL7, which is not horrible different from a 12ax7. Lower gain, but not completely different. There really isn't much that will give more gain in the same socket form. I'm surprised a cathode cap there didn't bump the gain though.

7c7 is a high gain pentode, Not much that will give more gain either.

Just noticed you said you had a 10k from grid to ground on the mic input. Thats probably too low. The original was 500k, which I expect is closer to right for a guitar pickup. 10k probably will load it down too much.

somthing else that should brighten it up some, try about a .0047uf (prob have to experiment with this value) across the 250k resistor that goes from the center wiper of the level pot to pin 4 of the 7F7

 

[Eldin]

I'll try removing the 10k later today and see how it goes thanks!

I've been reading some more too and it seems old fender amps had a switch which seems to have done exactly that. Increase the brightness. I'll try it thank you

I went to a friends house and that has a lot of guitar pedals and a 16ohm speaker cabinet. The amp sounds really REALLY good surprisingly! I was afraid that increasing the gain would cause it to go into oscillation but it didn't.



whoaru99, I don't know if I even could do that with this because the tubes run on 7 volts.

 

[whoaru99]

I didn't look at the characteristics of the 7-series tubes but I was thinking by "gutting" more the circuits not the tubes. If the tubes have similar characteristics to 12AX7 then the AX84 circuits might still largely apply.

 

[Eldin]

Ohh I see what you mean. I'm still fairly new to tube audio so I'm trying to learn how and where to make various adjustments and what they all do!

I have some old telefunken 12ax7s that I'll try to make a diy amp out of eventually

Thanks for the link to AX84, that looks like a fun thing to check out