Peavey Session 400 repair

[George Biner]

I repair amps as a hobby. I recently repaired a 1978 Peavey Session 400 amp (no cab, just the chassis). I wanted to post to record my observations.

Being from Southern Cal, the home of Fender, I will say that for rock musicians around here, Peavey has never had a very stellar name for one reason or another. But I get it that they do well in the pedal steel area and especially in the south. And I have an open mind.

First the raves: this thing has an awesome amount of power, the components are not exotic, the build quality is good, you can easily get to everything, you can easily separate the preamp board from the power amp board, and once I got it working, it sounded great after 55 years.

Now the rants:
1. There are no component names / reference designators on the official schematic or on the layout drawing: this is a drag because it hugely increases the time it takes to find a given component on the actual board. I really, really don't get why any electronics mfr company would do this, I mean, how do you even discuss a certain component? (Peavey amp: "It's the resistor connected to the emitter of the first input stage transistor." Fender amp: "R2".) I had to create my own designations. Still scratching my head on that one.

2. There are two volume controls, input volume and master volume ("sensitivity") and they both are really arcane the way they are designed, seemingly needlessly complicated, not referenced to ground as normal -- if anyone has any insight as to why these are designed this way let me know, I am curious.

3. Peavey schematics like to run ground and power supply rails all over the page, this makes it hard to read -- Fender puts all the supply V circuitry in a corner of the schematic, out of the way, and keeps ground lines to a minimum.

4. There is a fair amount of local feedback used in circuits that makes it difficult to informally analyze them. My impression is that Peavey had a very clever designer who knew certain circuits that do the job but are not very simple or well known. So the amp works, but no mortals understand it.

This particular amp had two wrong coupling caps installed, a missing transistor and two bad transistors, which is a lot of baggage and why it took me forever to fix it (sorry, Mr. Customer). Usually, when you discover something broken, it fixes the amp -- not this here. I also didn't have much track record in audio discrete solid state circuits, so this was a good learning experience for me.

Well, that's my post and I hope I can contribute to the future well-being of any Session 400 out there now that I know more.

note: schematics are here: https://www.audioservicemanuals.com/p/p ... schematics

[George Biner]

I also recorded all the DC bias voltages for all the transistors (except the power TO-3's) and I plan on adding those to the schematic at some point as a public service. If anybody has a good way of getting these onto the PDF schematic, let me know, (I don't have a way of modifying pdf's). We can add in reference designators as well. This will be a great resource in the future.

I hope Peavey fans haven't put out a contract on me or anything

[Larry Dering]

George, thanks for sharing your experience and thoughts. Always a good read on repairs.

[Ken Fox]

Hope these help. Robert Lawrence did these some years ago. Note diodes are shown in wrong polarity on power amp schematic

[James Collett]

George, thanks for getting my amp working! I'm glad I wasn't crazy when I had difficulty troubleshooting the amp (or should I say double-trouble-shooting), but I'm grateful you were able to figure it out. I'll let George decide if he wants the headache of another one, but he did a great job!

On a related note, it sounds great, and I have found that it takes dirt pedals better than any solid state amp I have tried. Even on its own, dialing in the EQ and adding a touch of the spring reverb takes you right to that 70s steel sound. I get why so many people swear by this amp model.

[Larry Dering]

George, please share what the final repairs involved. So many of these posts stop short of answering the final problem. I was a long time subscriber to the music electronics forum. Enzo was one of the main contributions and since his retirement the site has really fallen out.

[George Biner]

Larry,

Well, as I said, the is particular amp had two wrong coupling caps installed, a missing transistor and two bad transistors. It took me a lot of time to understand and troubleshoot it due to: 1. I don't do a lot of solid state 2. I only work on amps occasionally so I'm rusty 3. the documentation wasn't the best (the layout was close but not exact and the schematic differed a bit as well (it didn't have the footswitch) -- so with the actual hardware, I had three sources of info that didn't always agree) and 4. some of the circuits were foreign to me. The lack of reference designators in the schematic was a bit aggravating as well. And it's always stressful working on an amp because if you screw up, you can blow something up and then have a whole 'nother repair to do, which you want to avoid so you have to be careful.

I will try to post images of some of the circuit sections to show how unconventional they are in my opinion.

The amp was making horrible sounds -- I started at the input and went section by section, writing down voltages, desoldering and testing suspected parts when I had enough evidence to suspect them. There are two main boards, the preamp board and the power amp. One great thing is that you can run each by itself pretty easily -- most of the time, I had the preamp board out running off a bench 50 V supply.

Previous work done on the amp was horrendous -- the PCB in places was delaminating and burnt, so I had to fix all that.

At one point, I was inputting a sine wave from my signal generator, and the signal looked very distorted in the circuit -- I traced it back and it was coming out that way from the sig gen (a very old Heathkit) -- oy -- so I got a new old sig gen from eBay, and that arrived damaged -- oy -- so I finally got one that works. Luckily, the owner James, was chill on the time-frame.

I love fixing things and saving them from the trash heap and these old amps remind me of when I was a teenager and just getting into electric rock music. I had a career as an electrical engineer, but not in music/audio (the pay is too low), so I enjoy trying to pit my wiles against the ratty old Peaveys and Fenders and Marshalls.

I will check out the music electronics forum.

[Larry Dering]

George, thanks for the feedback.