Whatizitman
Poster Extraordinaire
I recently came to the conclusion that I'm just way too cheap to buy or build my dream amp (2203/2204). So I'm modding a Fender PA 100. I picked it up a few months ago for $125. I had $100 leftover from a recent sale. So, really only $25 out of pocket. Brought it home and it was DOA. No biggie, as I was happy to have something to work and learn on. And just bringing an old amp back to life is rewarding enough in that regard. I'm still very new to tube amp build world. So from the getgo this amp has been a fun adventure.
The first big finds were that the 6L6 bias circuit was open, and at least one of the radioshack 6L6s was toast. No other signs of blown/arced tubes, though. I had a quad of new 6L6s I got with a JCM900 head trade (more on that later), so that came in handy. Replaced rec diode and cap and bias was within reason with the new tubes. But the amp still worked intermittently and blew fuses. After staring and poking at the thing for a few days, I came across a broken chassis ground wire that was very hidden. Bingo. New filter and pre bias caps, and Bob's was my uncle. I love Bob. Really, it seemed like the Radioshack 6L6s were the only thing that were weren't from the factory on the amp. And they were old.
Reverb working ok. Not TR. But ok. With pedals the amp is very workable for guitar. Even better for bass. Even betterer with some external EQ and/or mid-hump pedals. A DOD250? Nice. Rat? Hell yeah. Bridge some channels and it gets a little louder and fuller. It's not a cascade, though. But it's nice.
For those of you unfamiliar with the PA100, it's more or less a standard 100 watt Silverface tube power amp. It has four input channels with Z and tone stacks (~James) suitable for mics. All are shared cathode, each with one triode between two 7025s. Their summed in with a 2nd triode stage. The reverb circuit differs from the AB763 in ways I don't entirely comprehend. Seems like there's an extra triode buffer(?). Anyway, suffice to say that if one were to mod a PA 100, they would find plenty of space (they are huge) and pre-amp tubes to do so. Check out what Fuchs does with them, for example.
This is the closest schem to mine. Some of the tone stack values were... different. Everything seemed original, though.
I could have left it at that and been happy enough for my daily noodling. I then turned my attention to 2204 build ideas, and put my also fairly recently acquired JCM 900 4500 up for sale/trade, in hopes of procuring some dough toward that 2204 build. No bites. There are DRs polluting Facebook Marketplace everywhere, just sitting. There's a 50watt combo near me for $550 that's been sitting for a month now. Nothing 900 is moving. I took it off market and started getting together plans for modding the 900 to an 800. Short story is that it's just not worth it, as I'm sure y'all familiar with the dual reverbs can attest to. It is a good amp for a working pro. Just not good for my tinker and noodle avocation.
I needed to work on something. So back to the PA100. I figured I could cut my teeth on some simple things first, and see how it goes. The first mod was to change the grid leaks and stoppers for standard values. Not my best lead dress work. But it did work. Better response for sure. Changed the bias resistor and cap to a more typical fender value (split four ways). That helped, too. By this point, including the amp, I'd probably spent around $150-160ish. Not bad for a working vintage 100watt Fender beast.
After some digging, I concluded the tone stacks were James style, and set about to figure if I could mess with the mids.
And it's all gone downhill from there... I need to stop writing here, but will post some pics and 2203 molestation status later today for anyone along with me for this ride. I took a break from TDPRI posting for a while. Happy to be with y'all again!
The first big finds were that the 6L6 bias circuit was open, and at least one of the radioshack 6L6s was toast. No other signs of blown/arced tubes, though. I had a quad of new 6L6s I got with a JCM900 head trade (more on that later), so that came in handy. Replaced rec diode and cap and bias was within reason with the new tubes. But the amp still worked intermittently and blew fuses. After staring and poking at the thing for a few days, I came across a broken chassis ground wire that was very hidden. Bingo. New filter and pre bias caps, and Bob's was my uncle. I love Bob. Really, it seemed like the Radioshack 6L6s were the only thing that were weren't from the factory on the amp. And they were old.
Reverb working ok. Not TR. But ok. With pedals the amp is very workable for guitar. Even better for bass. Even betterer with some external EQ and/or mid-hump pedals. A DOD250? Nice. Rat? Hell yeah. Bridge some channels and it gets a little louder and fuller. It's not a cascade, though. But it's nice.
For those of you unfamiliar with the PA100, it's more or less a standard 100 watt Silverface tube power amp. It has four input channels with Z and tone stacks (~James) suitable for mics. All are shared cathode, each with one triode between two 7025s. Their summed in with a 2nd triode stage. The reverb circuit differs from the AB763 in ways I don't entirely comprehend. Seems like there's an extra triode buffer(?). Anyway, suffice to say that if one were to mod a PA 100, they would find plenty of space (they are huge) and pre-amp tubes to do so. Check out what Fuchs does with them, for example.
This is the closest schem to mine. Some of the tone stack values were... different. Everything seemed original, though.
I could have left it at that and been happy enough for my daily noodling. I then turned my attention to 2204 build ideas, and put my also fairly recently acquired JCM 900 4500 up for sale/trade, in hopes of procuring some dough toward that 2204 build. No bites. There are DRs polluting Facebook Marketplace everywhere, just sitting. There's a 50watt combo near me for $550 that's been sitting for a month now. Nothing 900 is moving. I took it off market and started getting together plans for modding the 900 to an 800. Short story is that it's just not worth it, as I'm sure y'all familiar with the dual reverbs can attest to. It is a good amp for a working pro. Just not good for my tinker and noodle avocation.
I needed to work on something. So back to the PA100. I figured I could cut my teeth on some simple things first, and see how it goes. The first mod was to change the grid leaks and stoppers for standard values. Not my best lead dress work. But it did work. Better response for sure. Changed the bias resistor and cap to a more typical fender value (split four ways). That helped, too. By this point, including the amp, I'd probably spent around $150-160ish. Not bad for a working vintage 100watt Fender beast.
After some digging, I concluded the tone stacks were James style, and set about to figure if I could mess with the mids.
And it's all gone downhill from there... I need to stop writing here, but will post some pics and 2203 molestation status later today for anyone along with me for this ride. I took a break from TDPRI posting for a while. Happy to be with y'all again!
