Peavey Session 400 repair
Posted: 6 Sep 2023 2:55 pm
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
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