Marshall DSL tubby sound

[julianbr]

Hello everybody! I'm have 2 Marshall DSL amps, DSL50 and DSL100. I really like the amps and I've never had a problem with them. Tracked with them last month, sounded great. I usually run the preamp gain on both channels on 5 and use a pedal (OCD, Plextortion, Brownie) to drive the preamp harder. Started recording last week and now the amps really sound unfocused, especially on low notes. It sounds like a distortion pedal when the batteries run down. It sounds that way with no pedals plugged in. Checked everything, cables, new batteries in pedals, speaker cab ect... I find it strange that both would start acting up at the same time. The only difference was this time I plugged them into a Furman power conditioner strip and then they started sounding like crap.
Just ordered some new tubes and will re-bias the amps. Any other things I should be looking out for? Thanks for any advice you can offer.

[booj]

I'd get a meter and check the voltage from the Furman. Also, I'd check the connector AC plug for the Furman, as well as checking the amps in a regular wall socket.

[J M Fahey]

Agree with Booj and would go a step further, just plug them straight "on the wall"

[Gibsonman63]

Ok, here's a silly question. Are you running speaker cable from head to the cabinet or instrument cable?

[julianbr]

Thanks for all the replies so far. Yes, I went back to plugging straight in the wall. I will test the furman with a meter but I'm not going to use it again.

Yes, using speaker cable from head to cab. Tried three different speaker cables, same problem. Tried 4 different guitar cables going straight in to head, same thing.

I've been dealing with this problem for over a week and have tried about every configuration possible. I hope it is just the tubes. Should have them on Friday. I just find it so strange it started happening to both heads at the same time.

Once again, thanks for all the helpful suggestions so far. I really appreciate it.

[julianbr]

Just re-tubed the DSL50 with JJ EL34's and JJ12ax7's (bias 43 mv) sounding great for about two hours, now it's sounding like crap again.
Please help, this is driving me nuts.

[J M Fahey]

Really, the correct way to diagnose it is to feed it, say, 50mV @ 1KHz, set all controls as you use them, and slowly rise the volume control , scoping the output, until it starts clipping and then , say, a 10% more, so I have a sine wave with a partial flat top, and take a "mental picture" of that, which is my reference waveform: amp working properly, just turned on.
Then reproduce the failure, playing for 2 hours, or whatever's necessary, then play, say, 15 extra minutes to make sure it crapped for good and re-inject the same test signal, with all controls as previously set.
Compare this "fail" waveform with the previous, "good" one.
You will see either fuzzies or superimposed "grass" (oscillations), or unsymettrically clipped waveforms or excessive crossover or much earlier clipping on one or two halves of the wave, whatever, which each in turn means something and has a specific cause.
Otherwise, hunting for the vague and undefined problem called "sounds like crap" is akin to "hunting the snark" where you are chasing an animal you wouldn't recognize even if it were in front of you.
That's what scopes were invented for.
The alternative is to draw a schematic, write all "good" DC voltages and recheck them when the amp misbehaves, trying to find an abnormal one.
Problem is, that is "static" testing (at idle) and you need "dynamic" testing (full throttle).
Otherwise you can spend ages testing.