Peavey LTD 400 low volume distortion

[J M Fahey]

Or, if you have the space for it, 2 x 1N400x in series.
You should thermally couple them to the heatsink.
Worst case, degrease a small area of it swabbing with some solvent, let it dry, scratch surface slightly with enery paper to make it rougher, degrease again and epoxy diodes there.

[Pryde]

Thanks everyone
I have plenty of 1N4007's so I will solder a couple in series for testing. Maybe I can just clamp them to the heatsink/rear panel temporarily for the test.

I am not sure if the Vibroworld site is still operational so I will likely need to get it from peavey

[Enzo]

Seriously, try ONE 1N4007 first. It may sound acceptible and in which case, the amp will run cooler in the long run.

Aren't there two more of them on the amp? I see a second one in the power amp, lower left, below the diffy pair. It biases the VA transistor. But my prime candidate is in the preamp, it is the bias element for the reverb driver. I'd steal that one for the power amp bias and mount my one or two 1N4007 there.

After trying one 1N4007 as power amp bias, if it works out, leave it. But if you need to use the dual part, then steal the reverb one, and you won;t have to order anything.

[pontiacpete]

I was able get to Vibroworld, looks like they might have the part

Peavey Parts@Vibroworld.com The Source for Discount Tube Guitar Amp parts.

[Pryde]

Ah I see the other one in the preamp section hooked to the base of the 430/431 transistors.

OK I will try the single 4007 in the power amp first. If it clears the distortion I will just leave it. If not, would you then suggest to steal the dual one from the preamp and try it in the power amp or maybe just look for other problems?

[Pryde]

Hmm,

I installed the 4007 diode in the power amp (in place of the dual-diode) and still have the exact same distortion as before. So the diode maybe wasn't the issue although it was indeed shorted per the DMM test?

What next? Should I try swapping the preamp dual diode to the power amp? Not sure it will be helpful or worth doing?

[Enzo]

Well, it was a gamble. Try two just to see. A shorted bias diode definitely will cause crossover distortion, regardless of whatever else might be wrong.

The simple thing is to scope the signal to see exactly what is happening.

[Pryde]

Alright. I put another 4007 in series and I think that did it! The distortion seams to be pretty much gone at all volume levels.

So, do I leave the series 4007's in place or should I order a new Peavey dual-diode and replace the original? I decided to just leave the preamp dual diode alone and get a new one for the power amp if it is necessary

Thank you to everyone for all the help, especially Enzo!

[Jazz P Bass]

Get the correct part.
If nothing else, because of the way it mounts to the heat sink.

[J M Fahey]

Cool!!
But just to play it safe, check that it's not overbiased now.
2 x 1n4007 might be different to the original 2 diode pack.
Measure voltage across emitter ballast resistors and post it here.

By the way, I'm posting suggestions "blind", can somebody post or link to the LTD400 schematic?

[Enzo]

OK, not that we know it is the issue, steal the good one from the reverb for this purpose. That will ensure the thermal tracking and all is correct. The parts PV sells are selected for voltage characteristic, they are not just generic parts. Then replace the reverb driver part with your plain diodes.

Two 1N4007 might be more or less equivalent to the dual diode, but in the bias circuit of the power amp, it needs to do thermal compensation, so I'd want to stick to the original as much as possible.


But certainly nothing wrong with just ordering the right part, of course.

[Enzo]

See if this works...

[Pryde]

Thanks all.
I am going to go ahead and order up a new one from PV on Monday. Looks like there is an ebay merchant selling them for 99 cents but its out of Mexico so I don't think I will hassle with it

[NZaudio]

The best you could and really should do is getting a scope, from a friend or somewhere. If there is a signal distortion, you can see it. It might well be in the pre-amp part or even RF on it. You cannot "hear that" with a tracer. You really need to see what's going on. Feed a sinewave signal in the input, approx 300mVpp at 1Khz.
About these diodes.... they are used for biasing the poweramp and should be in thermal contact with the heatsink.(Otherwise the design is no good)
You can also replace these with a transistor circuit and a trimpot where you can actually "set" the bias current yourself. Iknow that many many Chinese poweramps have that problem and do not work properly.

[Pryde]

Yes a scope would be nice to have for sure. I am primarily a pedal builder but I have built and worked on tube amps and have always managed without a scope. That being said, I realize the benefit of one. I need to look into a low-cost but "decent" scope.

This old solid state PV was a first for me but a good learning experience.