Peavey Classic 50

[mac dillard]

My Son brought me a Peavey Classic 50 to repair today. We bought this amp from a pawn shop some 12 years ago. Son has had it loaned out to a buddy for the last few years. As the old cowboys say, this amp has been road hard and put up wet. Fast check shows that F1 the 3 amp mains fuse is blown, F3 and F4 are blown and best I can tell C10 the bias circuit cap is open. All of the diodes check good. All of the axial electro. caps test good. I have no exprience working on PCB's and in the process of getting ready to ship this old work horse back to Peavey for repair. Thought I would ask if anyone has a suggestion of what I could check before I send it back. Thanks.

[Enzo]

Hi, several thoughts.

F1 is the high voltage fuse, not the mains fuse. THE most common reason for high voltage fuses to blow is bad power tubes.

F3,4 are low voltage supply fuses. Those supplies mainly power the preamp heaters, the solid state circuits, and the relay.

What comes to mind is the power tubes failed and shorted the high voltage to the low, and all those blew. Might be something totally different, but that is my working theory.

You lytic caps probably are good, but you cannot test them with a meter. All that does is measure their value. A cap that is fine at 2 volts from a meter may leak like a screen door at working voltage.

An open C10 would be a pretty odd failure. Even shorted or leaking would be unusual, but open? What leads you to think open?


But if you don't intend to work on it yourself, then why bother? By replacing a few parts, you are not "helping" a repair tech. He/we would much rather see the amp AS FAILED.

The service department at Peavey is fine, good guys. But this is just a guitar amp, ANY competent amp repair tech can do the work. Peavey has a network of authorized repair shops all over the country, I am one of them. One is probably not from from you. Shipping an amp is not cheap. But even them, amps are amps, and tubes are tubes, they all work the same way. There is no special training needed to work on PV amps rather than Fender or Marshall. SO a local shop ought to be able to help you.

[mac dillard]

Thanks for the reply Enzo. As for shipping the amp to Peavey, I live in an area where there is no amp tech close. There are no Peavey service centers within 100 miles so I will have ship the amp somewhere. I decided to go ahead and tackle the amp myself. F3 and F4 were blown. The other blown fuse was the 120 volt ac power in fuse which does not appear to have a number. I pulled the power tubes, hooked up my light bulb current limiter and turned the amp on. bulb stayed dim. Took it off of stand by and the bulb stayed dim. Left on on for a short time. Then plugged the amp into the 120 volt ac and turned it on. Saw smoke and quickly turned it off. Inspection showed that the smoke was coming from R61 and R62. They were and the board there was burned. I pulled the boards and started looking. R61 parallel with R62 measured 8.2 ohm. Measuring from out going side of R61 and R62 to the ground pad on the board gave me 85 ohms. Looking at the back side of the board , it looks ok. I checked ohms between the pins of the op amp U1 and got 45 ohms between pins 1 and 5. I have ordered half a dozen each of U1 and the 22 ohm resistors for R61 and R62. If the op amp is not the problem then I don't know where else to look. Any direction would be greatly appreciated. There are some advantages to living rural. Getting parts is not one of them. $1.89 in parts will cost me 4 times that to get them shipped.

[Enzo]

You cannot count on a hand meter. It reveals many things, but will tell you nothing about unwanted voltages coming from somewhere, like through a bad tube or through a leaky cap. SO replace the parts and be ready for them to either work or reveal deeper problems.


I must say between pins 1 and 5 on the op amp is an odd pair of pins to compare.

[mac dillard]

I was just checking between all of the pins to see if there was anything. Parts should be here in a week or so. Then we will smoke test it again. I am at a loss as to why the power up with the light bulb limiter did not indicate a short. Did a little digging on the net and it would appear that smoking R61 and R62 relates back to a bad 4558 a lot of the time. Mostly I see where you have offered sound advice to folks with these amps and would seem to be the resident expert on them. I really thank you for chiming in on my post. I am going to retube it when the patient is well. What is the recommendation for the V1 position tube.

[Enzo]

Bulb. You are making the same faulty assumption that people make when they wonder why a fuse didn;t blow when some resistor burnt up.

If you make a 1/2w resistor dissipate 2 watts, it will burn up. But that is only 2 watts. Your mains fuse is 3A. That means with 120v mains, 360 watts must pass through before it even thinks about blowing. Your bulb limiter is not going to go brite because the amp uses 2 watts more than idle normal does.

The bulb limiter is a great tool, but it is a gross indication of major current flow. It is not a magic fault detector that somehow knows when any little part is bad.

[mac dillard]

Follow up to the Peavey Classic 50. Turns out that the problem was failure of U1 the 4558 op amp. I replaced the op amp, R61, R62 and retubed the amp. It is ready to go back to work..

[Amp Kat]

Follow up to the Peavey Classic 50. Turns out that the problem was failure of U1 the 4558 op amp. I replaced the op amp, R61, R62 and retubed the amp. It is ready to go back to work..

Glad you got the amp going but I'm having a hard time figuring out how a damaged op-amp blew up a 3 amp fuse. Guess I've seen stranger.

[mac dillard]

Me too on the 3 amp mains fuse. I have had it running for about 4 hours with a walkman plugged into it and have not had any problems. Possibly a power tube problem.

[nashvillebill]

Isn't U1 the reverb driver? Dunn why that would blow fuses.

[Enzo]

It runs on the -27 rail. If it shorts between power pins it is then a dead short across the 27v supply, with only 11 ohms in series to hold it back.

[Chuck H]

I would say that for someone who was ready to send the amp back to the MFG mac practiced some mad trouble shooting skills to narrow this problem so fast. I would actually have liked to read more about the process. Glad the amp is working. Nice job mac.

[mac dillard]

Thanks for the attaboy Chuck. Wish it had been all my skills but I will have to give the credit to Enzo. From the blown fuses F3 and F4 and the smoked resistors R61 and R62 I knew that the -14volt and -27 volt rail was at least part of the problem. A quick search of R61 and R62 failure turned up an old thread about them where Enzo thought that the op amp was the problem. Sure enough it was. Credit where credit is due. Thanks Enzo for shearing your knowledge.

[Enzo]

Sharing was a sheer pleasure.