Classic 50 2X12 Conversion Questions

[Wittgenstein]

I was given an old Peavey Classic 50 2 X 12 that was not working and had been left in a garage for a few years. The PCB board was fried in the preamp section causing it smoke whenever plugged in. If it was hand wired ptp I could have removed the bad capacitor and been done with it, but to buy a new PCB board for the pre-amp section is too much money in my opinion for a Peavy amp. I removed the chassis to reuse the cab, and cannibalized what I could for parts.

50 Watt RMS into 8 or 16 ohms
4 x EL84
3 x 12ax7
Reverb Tank
2 x 12" Blue Marvel 8Ohm Speakers


My question is what amp to build so I can reuse the transformers and tubes. The peavey has a ss rectifier so there is no 5.3V heater tap. I will have to either go with SS rectification again, or add a 5.3VAC transformer in the new build. The first circuit to catch my eye is the Hoffman style AC30 which does not include tremolo. It also uses 4 x EL84's and 3 x 12ax7 in the preamp section. Any thoughts on how feasible this would be? Is going from a 50Watts to 30Watts too big a drop? Is there another circuit that would be a better fit?

[Chuck H]

First, the Classic 50 isn't a 50 watt amp. Second, It's a good sounding amp with the right speakers. If I were reusing the orignal transformers I might consider building the original circuit too because I know it sounds good with those transformers. Now...

What kind of amp do you want? The 50 watt (actually more like 36) vs. 30 watt thing is really not a big deal. Bias condition can change that for you. I can't speak for the suitability of the Peavey OT for class A PP operation or the B+ voltages in that amp though. Do you want a high gainer or just a basic amp that you need to crank up for distortion?

[Wittgenstein]

I suppose that I could rewire the classic 50 into a handwired version, and the Hoffman AC30 is a simpler circuit. Does the AC30 not run the OT for class A push pull operation? I cant remember the voltage on the secondaries of the transformer, but I have it written down, and I can post it when I get home. Ideally I would make the amp as close to the AC30's sound as possible, but I'm not too concerned if it comes out otherwise, either high gain or low breakup. I just have the bug to start another amp project, and I figure I can put these transformers and tubes to use to save a few bucks.

[Chuck H]

If you want an AC30, build an AC30. The B+ is high for the Peavey PT (about 400 IIRC) The AC30 typically uses a rectifier tube, biases at or over 100% dissapation and is cathode biased. If you add a "sag" resistor (68 to 100 ohms to simulate the rectifier tube sag and voltage drop) and bias to 100% dissapation I think your B+ will be about 360. If you cathode bias you can then subtract the cathode voltage (let's call it 10 volts) so your plates effectively have 350Vp (IIRC the AC30 is around 340Vp). You'll probably be fine cathode biased to 100%. So I think the iron you have would be fine for an AC30 project.

IMHO the Peavey speakers sound like doo doo and could make any amp sound harsh and thin. I had a pair of Emminence Private Jacks (that I didn't like with a different amp) and when a friend asked me what he could do about the ice picky thin tone of his Classic 50 I figured I would try the Private Jacks in it before moving to any mods. It was a totally new amp. The Private Jacks are the exact right speaker for that amp. It's like wherever the Private Jacks are lacking the Classic 50 has too much and whatever the Classic 50 lacks the Private Jacks offer a bump. Just a great match up and made all the difference in the world. I mention it because once an amp is overdriving most of the EQ comes from the OT and speakers. So, FWIW, Private Jacks seem to mate up with the Peavey OT very well. So, if you can swing the $$$, a pair of Private Jacks could be a nice addition.

[olddawg]

So the board was unrepairable because? If it isn't totally carbonized I don't see why it can't be repaired.

[Wittgenstein]

So the board was unrepairable because? If it isn't totally carbonized I don't see why it can't be repaired.

The board was fried in several places (needed all new PCB boards), and it was left outside in a wet garage for many years. It was going to be trashed, so I figured I would see what I could do with it. It looked like someone had already messed with it trying to get it working. After looking over all the damaged parts and calling Peavey for replacement part costs, I realized that it would cost more to repair the amp then I think it was worth. I totally agree that the Blue Marvel speakers are not very good. I do know that they sound a little better with my 5E3. I plan on making my new build into a head and purchasing a nice set of speakers.

Thanks Chuck for the suggestions. I am still learning as I go and I figured this would be a good educational project. I will have to read up some more on adding a "sag" resistor to a SS rectifier, as well as how the AC30 operates. I'm starting to feel pretty comfortable with Fender circuits, but I know very little of other designs.