Valve guitar Amp Fading in and out

[sohosteve]

Strange fault developed with my year old Marshall valve combo.
I recently installed two brand new matched EL34 power tubes and had it re-biased by a soundman at one of our shows who had a full electronic workshop in the gig's basement.
Sounded absolutely superb. Louder, tighter and punchier than I'd ever heard it before.
Anyway, that was about fifteen gigs and two months ago but at a festival gig a couple of days ago it appears to have developed a very strange fault which I'll try to describe.
The first note I play is silent for the first quarter second or so and then rapidly fades in.
Signal then remains until I stop playing, however briefly (dead stop power chord followed by another) results in the first sound after silence fading in rather than coming in hard and fast.
eg any signal after silence has a very slow attack.
As long as signal (eg playing) is constant this doesn't appear to be happening.
I say 'doesn't appear to be happening' because this fault only manifested towards the end of a festival gig where the switch overs between bands was very quick and the amp has gone straight into storage.
So I've had no opportunity to give it a proper test in isolation.
Consequently I also don't know if the constant working sound was good or overly distorted - most of the last songs in the set feature very overdriven fuzz (MK 1 Tonebender) that, intentionally, makes the amp sound like it's dying anyway!

Any clues or suggestions would be hugely appreciated before I head the several hundred miles out of London to troubleshoot it.
I have a whole collection of pre-amp tubes I can take with me. And assume (or at least hope) that the power amp tubes (with less than 20 gigs worth of use) are still fine....

Thoughts please, oh wise ones...?

Oh yeah, it's DSL40C

[pdf64]

The 1st thing to rule out is a bad switching contact on the fx return socket (assuming that you weren’t using the fx loop).

[sohosteve]

The 1st thing to rule out is a bad switching contact on the fx return socket (assuming that you weren’t using the fx loop).

OK, I'll try that.
So far I haven't used the FX send/return at all, but I will try looking into that.
Could that actually make the first signal the amp (after silence) receives have such a slow attack though?
I have to admit that I am not an electronic engineer, but it's almost like a component (capacitor?) needs to be 'charged' up with some signal before it allows any more through.
I'm probably talking out of my hat there!
As they say, 'a little knowledge can be a dangerous thing'!

[eschertron]

That's the issue that brought me to MEF in the first place: Dreaded Switching Jack Problem

Also make sure there's no weird auto noise gate settings or programmed envelope settings on the amp (assuming it's one of those amps that have such controls/effects).

[sohosteve]

That's the issue that brought me to MEF in the first place: Dreaded Switching Jack Problem

Also make sure there's no weird auto noise gate settings or programmed envelope settings on the amp (assuming it's one of those amps that have such controls/effects).

The fault descriptions in the FX Loop problem don't really resemble the problem I'm getting.
eg A very slow envelope on the first signal it receives any time I stopped played for a brief moment - one or two beats in length for a momentary dead stop, followed by a big power chord - that faded rather than crashed in due to the technical issue.

No built in ADSL style effects - fairly basic valve amp with channel switching and reverb.

[Jazz P Bass]

I would put the old set of tubes back in and try it. (if you still have them)

[pdf64]

The fault descriptions in the FX Loop problem don't really resemble the problem I'm getting.
eg A very slow envelope on the first signal it receives any time I stopped played for a brief moment - one or two beats in length for a momentary dead stop, followed by a big power chord - that faded rather than crashed in due to the technical issue.

No built in ADSL style effects - fairly basic valve amp with channel switching and reverb.

I take your point but as it's next to no effort to plug a patch cable across the loop (thereby eliminating any problem caused by the switching jack), it should still be the 1st step.
2nd should be to reseat all tubes a time or 2.
3rd to try 'known good' tubes.
4th to take the chassis out and reseat all fuses, connectors, eg between boards etc, a time or 2.

I can't think of any failure modes whose symptom is a fast fade in; problem with dodgy connections in a heater circuit cause a slow fade in and out. So if it's none of the above, I'd look to check the heater circuit for failing connections, eg dry solder joints on the tube socket.

[sohosteve]

I take your point but as it's next to no effort to plug a patch cable across the loop (thereby eliminating any problem caused by the switching jack), it should still be the 1st step.
2nd should be to reseat all tubes a time or 2.
3rd to try 'known good' tubes.
4th to take the chassis out and reseat all fuses, connectors, eg between boards etc, a time or 2.

I can't think of any failure modes whose symptom is a fast fade in; problem with dodgy connections in a heater circuit cause a slow fade in and out. So if it's none of the above, I'd look to check the heater circuit for failing connections, eg dry solder joints on the tube socket.

Thanks for your kind advice.
In that case I shall try all your suggestions and see how I go from there.
Cheers!

[Enzo]

In my head I see a DC offset somewhere, likely on a grid. As if the grid resistor opened. It takes teh signal voltage to wake it up.

[glebert]

I would at least try to figure out if the problem was preamp or power amp. Plugging guitar into effects return jack (or driving from other preamp) will tell you if the power amp is the issue. Vice versa taking the send signal to another amp or mixer will tell you if it is in the preamp.

[glebert]

May not be relevant, but I picked up a non-working DSL15 last year and the only thing wrong with it was ribbon cable from preamp to power amp boards had come unseated. Don't know if the DSL40 has something like that but is easy to check.