Session 400 repair

[Greg Crawford]

I recently bought a Session 400 from a musician who got it out of a barn somewhere. He said he spent a lot of time cleaning it up, but it has a long way to go. Serial number dates to 1976. He had stacks of Peaveys, and said he would not get around to fixing this one. He plugged a guitar into it, and it played, tremelo and reverb was working, with a lot of scratchy static. He made full disclosure that two resistors were loose on the board when he opened it up.

I got it home and took the chassis out. I haven't traced the schematic yet, but a .33 ohm and a 1 ohm ceramic resistor were barely attached or loose from the amp board. I also found a 47 uf cap dangling by one lead on the other board. I don't know how this could have happened, but there it was. And it amazingly made noise! Any parts of the amp that were sheltered from mice and dirt appear pretty clean. The power transistors were numbered by sharpie, and I saw some previous solder repairs in a couple of places. The top of the reverb tank was nasty and rusted, but looked like new inside.

I am not a tech, but it appears that maybe the power transistors that are in the affected circuits are in parallel, which would explain the amp still working to an extent.

The amp is a twin speaker model, with two 12" speakers. The previous owner said one speaker was blown, so he replaced it with a Celestion G12 he had laying around. So it has definitely mismatched speakers. I read somewhere that the solid state amp was pretty forgiving with speaker loads. I was thinking about installing some switches so that I could switch from speaker A to speaker B to A and B for comparison listening, since it has two different speakers. I know you wouldn't want to switch them with the power on.

Is this a stupid idea?

I am making a shopping list for parts. What should I consider replacing while I am in there? As I said, I am not a tech, so I figured I would fix all the obvious issues, clean the pots and jacks, and see what I have at that point.

Favorite component suppliers?

Thanks in advance for any responses.

[TRAP TRULY]

Those Session 400s are great old amps and worth the repair expense in my opinion. I'm working on a couple myself. I called Peavey to inquire about parts (electrolytic caps) and they still had everything I needed in stock.
The guys in the service department are very knowledgeable and helpful and said the service department is open if I needed to ship the amp to them.

[George Biner]

I spent a fair amount of time recently repairing a Session 400 so I'm interested in these amps and their sheer vast power levels.

Yes, the power amp stage has a couple legs of power transistors in parallel, so if the current path is broken on one leg, the others are still there and will drive the speaker. Although, you will get different drive strengths on the positive going signal vs. the negative going which I would imagine would sound funny but maybe not noticeable. It would be nice, after you fix what's obviously broken, to make sure that each leg is contributing current and you don't have a broken power transistor device.

The 47 uF cap is probably a filter cap on one of the internal voltage supply rails -- I see 50 uF caps on the schematic and 47 is a more common value. Again, it would work without it, but might be noisy.

Solid state amps are forgiving with loads in that they can drive an open circuit no problem, unlike a tube amp. I don't see a problem putting in a speaker switch to AB the speakers.

[Greg Crawford]

Thank you for the reply. I have components ordered. I also found a .02 disc cap flopping back and forth on the power amp board. I had an orange drop of the correct value and replaced that one. So two power resistors, one electrolytic, and now one disc cap, all loose. On inspection of the solder side of the board, I noticed 50 percent of the components still had the hole through the board visible. I thought you generally filled that hole with solder? Maybe that's why those parts had broken leads.

I removed the speakers. The original one had a couple of small tears that I repaired with glue and coffee filter paper. The original speaker reads about 4.3 ohms, the Celestion reads 3.8.

I scrubbed and cleaned decades of cigarette? smoke off the tolex (amazingly, it doesn't smell bad), buffed the corner guards, cleaned the casters, buffed and clear lacquered the Peavey emblem (hey, it was cast aluminum, not plastic!) and generally just cleaned/polished/treated all of it. I used a scotchbrite pad on the rusty transformer and painted it a nice satin black. I repainted the black inside the cabinet while I had the speaker baffle off of it. I sanded the rusty mouse peed top of the reverb tank and painted it to prevent further corrosion.

Half of the power transistor plastic covers were missing. I drew one up in Tinkercad, and 3d printed one. It fits great, I'll have to see if the transistor gets hotter than the PLA's melting point. If it does, I'll have to print it in ABS.

It's all back together with the exception of the chassis, waiting for components to show up. I'm anxious to get it working. I've heard these things have massive power.

Again, thanks for the replies. I certainly appreciate the knowledge/experience you guys bring to the forum.

[George Biner]

Wow, sounds like you need to re-solder every single component hole or else you will have them constantly failing. Can you take a picture of that? I wonder if that was a bad solder job in the manufacturing.

[Greg Crawford]

George, I exaggerated the number of visible holes. There were probably 10 or so. I reflowed the ones that didn't have the broken or loose components, so the only ones visible now are waiting on new parts.

But I just wasn't accustomed to seeing any on the few circuit boards I've messed with previously.