Peavey LTD 400 low volume distortion

[Pryde]

I decided to start a new thread as my original issue was resolved as I discovered my bonehead move that was causing blown fuses.

OK this may or may not be an issue as I am not familiar with these LTD 400 amps but when the volume is turned low (say 1-3) there is a noticeable distortion when playing guitar. Almost as if I have a light overdrive pedal on.

However, when the amp is turned up from 3 to full blast the distortion actually clears up producing a loud, clean tone that actually sounds very good. Is this normal for the LTD? I would assume there should be NO evidence of distortion even at very low volume settings?

Anything I should suspect or check with this issue?

Thanks

[Enzo]

Have we already swapped out those little 2uf caps?

[Jazz P Bass]

It wouldn't be the speaker, would it?

[Pryde]

@Enzo:
Yes the 2uf couplers were swapped out for 2.2uf poly films

@Jazz P Bass
It very well could be the speaker as I don't know its history/abuse. It is very strange but even played loud as the notes decay you can hear very low-level distortion in the background. As long as you are playing/attacking strings it is loud and clean. Maybe I am just being to picky?

Not getting any DC on the speaker and everything seams to be ok except for this low, background distortion. Not sure if this is something others have seen with this amp and could offer any insight but I am not going to sweat it if not

[Enzo]

Assuming no distortion at low levels is assuming distortion only comes from overdrive.

You don;t have to know the history of the speaker, just play the amp chassis through a different cab and see. A rubbing voice coil is more noticable at low volumes, played louder, you don;t notice it. And a related idea, you could have crossover distortion in the power amp, say one of the bias diodes shorted. More effect at low volumes.

The Session/LTD schematic lacks test point voltages, but the similar circuits of the Mace A-series and others do have voltages. See if yours are off anywhere, a DC level out of place will affect the operation. You could have an iffy transistor.

Make a signal tracer from another amp and probe stage by stage to hear where it comes from.

[Pryde]

THanks Enzo,

I have a signal probe and will trace through the audio path and see if I notice anything.

Again I want to thank everyone for sharing your skills and knowledge, really top notch people here.

[Bruce / Mission Amps]

@Enzo:
Yes the 2uf couplers were swapped out for 2.2uf poly films

...

How about replacing the cheaper ceramic caps with film caps as recommended by others?

[Pryde]

Damn the speaker is not the issue with the distortion. Tried a known good cab and it is still as present as ever.

Enzo,
you mentioned possible power amp diode short. Would I be able to hear it with an audio probe or would I need to remove it and test it?

I guess I can start probing at the input and make my way through the schematic and see what turns up.

Again it is most noticeable on any note decay or with the volume low (either amp or guitar volume). Playing fast and steady sounds loud and clear.

[Enzo]

I would just check the bias diodes in the power amp for voltage drop. One of them is a dual drop diode though

[Mick Bailey]

I've had Trace Elliots with similar problems. Some use three biasing diodes and due to drift the amps end up with a little crossover distortion which gives a slight ghosted distortion at low volume, particularly noticeable as the note decays. The diodes read fine for voltage drop, but it isn't enough to get rid of the 'notch'. A scope would be ideal for checking your output to see if it's crossover distortion. You need to check for this before swapping out components.

[ReadyTeddy]

I've had Trace Elliots with similar problems. Some use three biasing diodes and due to drift the amps end up with a little crossover distortion which gives a slight ghosted distortion at low volume, particularly noticeable as the note decays. The diodes read fine for voltage drop, but it isn't enough to get rid of the 'notch'. A scope would be ideal for checking your output to see if it's crossover distortion. You need to check for this before swapping out components.

The symptom sure does sound like crossover distortion.

[Pryde]

Hello All,
I took a break but am back at repairing the amp. So I checked power amp bias diodes and found that the dual diode is open (removed it completely and checked diode with DMM and got continuity both directions) I believe the part is a 13886 diode. right now I am assuming this is my culprit for the crossover distortion I am hearing.

It appears that right now the other diodes are ok. Before I order the dual diode from Peavey (no equivalent that I can find) is there any other rare components that I should also order for this amp while I am at it?

[Enzo]

Wait. How can the diode be open if it shows continuity both ways? Open means no continuity either way, open is like sticking your probes in the air. Bidirectional continuity sounds more like a shorted diode to me.

[Pryde]

Yes sorry that's correct. It would be shorted as I am getting continuity in both directions. Also the amp is not blowing fuses it just has the crossover distortion.

I also priced the dual diode from peavey and they want $11. A bit steep but I don't know where else to source them from.

[Enzo]

If those bias diodes short, it won;t blow fuses, all it will do is increase crossover distortion, so it won't hurt the amp.

I see these guys sell them for $3, see if they still have them.

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

Install one 1N4007 in place of the dual and see if it reduces the crossover distortion.