Packard Bell PA-20

[Waxxer69]

Hello,
I'm a complete novice here although I do have a little electrical knowledge. I recently acquired a Packard Bell PA-20 amplifier that I'd like to use as a guitar amp. The amp works fine now and sounds great, but as it was made NOT to distort it creates a little problem in the guitar amp application. I've had some electronics techs at work say I replace certain resistors with pots and I'd get distortion, but I'm sure it can't be that simple. I'm also not interested in frying myself or this amp. Can I add a distortion circuit somewhere in this amp? And if so where?

btw, this is a great forum and I hope to eventually build an amp rather than modifying an existing one.

[ampgeek]

Welcome Waxx!

Very cool amp you have there! Some thoughts on how I would approach it would include:

Build a lightbulb limiter before plugging it in (unless you already know it is OK). Old electrolytic caps don't last forever

Convert the mono input into a guitar jack.

Swap out V1 for a 12AX7

Fire it up, plug in an axe and see what it sounds like.

That will provide grist for mill on what to do next. You might consider diode or mosfet clippers to add some dirt. And, yes, dinking around with resistors and caps can get you over the top on distortion. But...I always like to sneak up on it to avoid missing a sweat spot along the way.

http://www.muzique.com/lab/zenmos.htm


Good luck!

Dave O.

[menger66]

Looks like an excellent amp to covert to something for guitar, if it's got a decent sized output transformer you could try a 18watt marshall or matchless spitfire, modding it first to something useable too get your feet wet then maybe plan a total rebuild, good luck mark

[Phil_S]

Try a 12AT7 or 12AX7 in place of the 12AU7. This will increase first stage gain. It might be all that's needed.

[M Fowler]

Also, relocating the 1m volume pot after the tone stack.

RCA mono input jack converted to 1/4 guitar jack with 1m across it and 68k resistor to V1 grid.

[Waxxer69]

Thanks for the welcome guys! I actually have installed a 12ax7 in the v1 position. It got louder but but really just barely starts to distort when turned all the way up.

Thanks for all the advice. I really appreciate it.

[M Fowler]

Solder a 25uf/25v electrolytic bypass cap parallel to R4 1800 Ohm (1k8)
another bypass cap .68 parallel R24 1k8. Positive to tube pins.

Wire the input jack so it goes to both the mono input and stereo input.

Adjust the two individual volume pots as mixers to mix high and low, should get some dirt.

[Waxxer69]

I kinda tried your last suggestion M Fowler (i don't have the exact caps yet). But I was happy to get some distortion. I only have one volume pot though. Will that matter? I do plan to try out the other suggestion when I get a chance. I have an unexpected work trip in a couple days, but as soon as I get back....

[M Fowler]

I kinda tried your last suggestion M Fowler (i don't have the exact caps yet). But I was happy to get some distortion. I only have one volume pot though. Will that matter? I do plan to try out the other suggestion when I get a chance. I have an unexpected work trip in a couple days, but as soon as I get back....

I would just play around with it and if you can find another pot to experiment further. Having the two pots to mix the two circuits makes a really sweet sounding amp that cleans up but with the pots about 3/4 gets some nice dirt. Not too aggressive just usable distortion with tone.

You can use other cap values too from .47 up to 250 mfd make each triode a different bypass cap value. Use what you can find.

[ampfab]

I would try disconnecting the negative feedback circuit. temporarily disconnect the feedback circuit at the output transformer, (wire that connects the green output transformer wire to R19 and C9), see how that sounds. sometimes that seems to make the amp more lively.
I would also rebuild the input to a more typical guitar type input and move the volume pot to after the tone stack like others suggested.
cool project!
in my experience it's important to try one thing at a time.

[Mant]

And then maybe add bypass cap on cathode of V1.
But definitely do one thing at a time and then listen.
If you do too many thing at once and you dont like it or it stops working
You dont know which one of tuose changes was the culprit......