Losing the sound

[solskjaer95]

Hello my name is kacper and i have a problem with my Marshall DSL 401.There has been a post from MR. Kryzsky about Fading out and i think i've got the same problem but i can't really understand what exactly you were talking about because i'm not an american . But the thing is that couple days ago my amp started losing the sound just like i would turn it down but when i moved it or waited a couple moments it would run again . Then i realised that 3 of my preamp tubes werent on when the amp died and yesterday it turned off and id didnt want to turn on as id did before i dont know what to do with it . i would be pleased if someone could explain to me what is going on or tell me if its the same problem as Mr Kryzky's about fading out and if it was that could someone explain me about that, I would be very thankful.

[überfuzz]

It's hard to help if we don't know what you're feeling comfortable in doing yourself...

How long have does preamp tubes been sitting there. Is it possible that they're just gone south due to long service or bad quality? If it's not bad tubes and you're into gutting amps you should start measuring the voltages around those sockets, specially heaters.

If it's the first time you're tinkering inside an amp, read about DEADLY VOLTAGES IN AMP, EVEN IF THEY ARE OFF. Read about draining amps as well.

Good luck!

Edit, by the way, welcome to this haven.

[solskjaer95]

The tubes are sitting there for 4 years now but its weird because 1 tube is one but the res of the preamp tubes are off i think it might not be just bad quality, i took thevalves out and i cleaned the legs . Thanks for your help , i will measure the voltage and if that doesnt help i will give the amp to an electrician and maybe he will know whats going on, and thanks for the greetings;]

[überfuzz]

Actually, your tubes can be the cause. I've seen many tubes gone bad in shorter time than that, specially bad quality tubes. If I were to fix it knowing what I know now I'd go for new preamp tubes, all new. Fire it up to see if the heaters AC, somewhere around 6 VAC, is good and dandy, with the old tubes still in place. If the heaters are good change to the new set of tubes and see if that does the trick.

[Jazz P Bass]

The heaters of tubes V1, V4 & the output tubes are fed by Fil 1 & Fil 2.
Tubes V2 & V3 have bridge rectifier feeding the heaters.
If V2 & V3 are not lighting then I would look at the bridge rectifier circuit and/ or the connectors.
(after swapping tubes around.)

[solskjaer95]

Could you tell me where the heaters are and what is their function?? i gues its something with heating but like i said im not american and i dont know what does it exactly mean.

[Jazz P Bass]

Uhmmm.
Maybe you should not be in there
The heaters "heat" the cathodes inside the tubes themselves.
Once heated, the cathode "emits" electrons.
Actually the heater is a filament.
Somewhat like what is in a light bulb.
12AX7 Pin 4-5 & pin 9 (4 & 5 are soldered together in the amp) are the filament connections.
EL84 output tubes pin 4 & pin5 are the filament connections.
Look on the schematic at the EL84 output tubes.
It shows the cathode (pin 3) as a bracket.
Under that bracket is Fil 1 & Fil 2 connected through a line that looks like a resistor.
That is the heater.
Fil1 & Fil2 are designations for the filament supply connections.
AS I said, Valve2 & Valve 3 are operating at dc potential.
All other valves connect to Fil 1 & Fil 2 which is 6.3 Volts AC.

[überfuzz]

Hmm, I guess measuring the voltages might be aiming to high. Have you tried flipping the tubes around. Turn the amplifier off and change places between the tube that's alive and one of the tubes that's dead. I take it you haven't done this, or?

[solskjaer95]

ive just done it with one tube but iwill try it out.

[überfuzz]

Does the problem follow the valve or is the sockets dead regardless of which valve is sitting there.

[km6xz]

When the sound is lost, does it decay over a period of a few seconds, 7-8 seconds? Or does it drop suddenly in volume? When looking for the glow in the center of each tube is one not lighting when the problem occurs or several? What position are the tubes in that are not glowing? If more than one tube is not glowing when the problem is present, it is not likely to be a tube that is faulty. If only one tube is dark when the problem occurs, but glowing in the center when it is working, that one tube is probably at fault. Moving the tube to a new positions(mark the tubes so you do not confuse them.) Does the tube that does not glow stay in the same location or does the tube you moved continue to stay dark when the problem occurs?
If more than one tube is dark when the problem is present, take the amp to a qualified technician. It is a simple to find problem but dealing with live high voltage is not for people without proper test instruments. If the problem is isolated to a single tube, regardless of the socket it is in, replace that one tube with a known good one and that should take care of the problem.
Good luck

[Guitarist]

You can also narrow down what half of the amp is working. Plug the guitar into the effects return. Sound? Then the output is working. (now you can test the 12ax7s by subbing one by one into that last PI tube socket before the el34s.

You can also tap/sample out from the preamp at the effects send.