Peavey Session 400 Headroom

[Pryde]

I am working to fully revive a Session 400 LTD but have one "possible" issue remaining. Had a low volume distortion issue before and found it was the faulty dual-diode in the power section (thanks to Enzo)

I put possible in quotations because this may or may not be an issue but the amp is so loud and clear up to a volume of 5-6 and then there is some distortion produced only with bass notes (E,F,F#,G or so) being picked/attacked hard (using a strat). It is difficult to say if this is a problem because the amp/speaker is being driven very hard at this point and it is damn LOUD. The speaker is a 15" black widow. The owner says it can be produced on another cab/speaker as well but I have not heard it.

I have:
completed full cap job (all electrolytics in the circuit)
replaced power dual-diode
checked EVERY solder joint and tested continuity
diode-tested all power transistors and pre-drivers
removed speaker magnet and cleaned voice-coil and gap

So at what point should this amp start overdriving the big widow? It seams to have clean headroom up to ~6on the volume knob then gets into some breakup on bass notes. Normal or not? Thanks for any thoughts on this.

[g1]

That amp should be 200watts into 4 ohms. Is the black widow 4 ohms? If someone put an 8ohm in, you won't get the full 200watts.
The number on the volume control where you start to distort completely depends on the level of the signal going into the input jack.

[Enzo]

Isolate the problem.

Turn up the amp and ball up your fist and whack the op hard. Does the amp react? Could be the speaker vibrations shaking a bas connection in the chassis.

Disconnect the combo speakers and run the amp output over to some other cab. Still act that way?

Disconnect speakers so it is silent. Make up a signal tracer and see what the signal sounds like going into the power amp. And trace signal through the preamp stages too.


If we didn;t discuss them before, you have a bunch of little electrolytics through the preamp: 2uf 35v ones, and they most likely all need replacing. As they get old and dry out, they can cause this sort of thing.

[Pryde]

Thanks.
Yes the speaker is a 4 ohm Black Widow
All electrolytics have been replaced including the small 2uf's (used film caps)
As said, I chop-sticked the circuit and whacked the amp like crazy and can't ID a loose joint

Unfortunately I don't have another 4 ohm speaker/cab to try. The owner says it did produce distortion on another speaker though.

I will signal probe the pre and power again but could not detect anything before

[Enzo]

That amp will never stress a Black Widow.

Up front, have YOU heard this or is it just a report from customer? When you say you could not detect anything, I have to ask.


And you may simply be clipping the amp. Picking hard sends spikes down the signal path. If they exceed the limits of any stage, they will clip.

[Pryde]

No I can produce it here but as I said only with very high volume and attacking bass notes. That's why I am wondering if it is just a headroom issue because otherwise it sounds great unless REALLY pushed

[Pryde]

OK one more development. After more extensive chopsticking of the preamp I notice 1 of the larger .02 ceramic disc caps appears to be microphonic when tapping it. Just really echoes loud through the speaker. I don't hear any crackling but it is very microphonic. Likely culprit?

[Paleo Pete]

If I'm not mistaken this is a solid state amp? I haven't played one in a long time and then only briefly, but it seems I remember if you crank it the thing would start to develop a bit of clipping above about 5 or so on the volume knob. And especially on bass notes. As Enzo said, bass notes played hard can clip at almost any volume level, but it becomes a lot more noticeable when you start pushing it. I've noticed this with a lot of solid state amps. ( I also hear things like this a lot easier than many people, the military hearing checker guy told me I was hearing frequencies the human ear shouldn't be able to hear.) I never did like the 15" Black Widow for a guitar speaker, and this is one of the reasons. Great for bass, but a bit too much chance of blasting or distorting bass notes for guitar.

Also try a different guitar, or lower the pickups slightly. There is a chance the pickups are a bit close to the strings, that can easily send a strong enough signal to the amp to create pickup clipping. (probably not the proper term, but as close as I can describe it). Solid state amps will pick this up same as tube amps, but it usually sounds nastier. My Squier Strat is doing that right now, I'm going to tweak the pickups a bit before band practice Sunday and see if I can tone it down a bit. I'm running it through a Fender Champ, way different story, but I can hear the difference between the normal amp distortion I usually get and the "pickup clipping" I'll call it, that I'm hearing now. (amp volume at 8, Bass at 10, treble at 4, guitar volume maxed, both tone controls full treble, nothing but a guitar cable between it and the amp..) I tried adjusting the pickups recently, it wasn't as loud as my other guitars, and I think I got them a bit too high...

[km6xz]

Good catch. Disc caps are microphonic and nonlinear with voltage across them due to piezo effect but none should be audibly microphonic. A speaker close to the cap, vibrating the whole pc board could sure cause that distortion. Replace it and verify that it was the cause.
I have to disagree with the suggestion to replace the small electrolytics, they are not being stressed with voltage or heat, the two factors that shorten life of electrolytic caps so they will probably outlive all of us. I suggest specifically not to change parts without evidence of it needing to be replaced. The reason is that any work on a circuit increases the odds of tech induced defects, poor solder joints, solder bridges, miswiring or component polarity or value mistakes. AFTER repairing the amp to proper verified operation, if you want to restore it by renewing all the parts, that is up to you but until it is working fully, any changes introduces ambiguities that make a diagnosis less accurate.

[Pryde]

Thanks for the info km6xz. I actually replaced every electrolytic cap on the board, including the small 2ufs with film caps.

I ended up replacing 2 of the .02 ceramic discs in the preamp with 100v film caps as they both sounded microphonic with tapping and what do you know... the distortion is cleared

Man this amp has fought tooth and nail but I think it is finally getting the hint that I'm not giving up.