Help 2555SL no sound, no light in preamp tubes

[siav]

Hi,

So I fixed my problem with impedance from the cab (showed 16ohm in stereo) but now I don’t get sound at all. The red lead channel indicator turns off even when the lead channel is engaged.

I noticed that preamp tubes won’t light and v4 power tube is not as bright as the others … help!

 

[Pete Farrington]

Maybe there’s a bad connection in the heater circuit.
Do the preamp valves warm up at all?

 

[siav]

No they don’t heat up at all

 

[FleshOnGear]

It’s odd for the power tubes to light up but not the preamp tubes. That seems to indicate a bad connection along the string of heater wires, like Pete said.

You said the LED for the lead channel doesn’t come on, right? That LED and the channel switching relay are powered off the heater supply, too.

 

[siav]

The LED lights up but fades off.

 

[siav]

The LED for the lead channel I mean.

 

[FleshOnGear]

The LED lights up but fades off.

So if you switch to clean, and then back to lead, does the LED come on again, and then fade again? Or does it only do this one time, when you first power the amp on? Do the channels change normally?

 

[siav]

The lead LED doesn’t turns on onde it’s off, it only gets on sometimes when I power in the amp. And I don’t get any sound at clean.

I played the amp yesterday

 

[siav]

It seems like the problem is from the string of heater wires. I remember that I had an usual hum when I turned in the amp yesterday

 

[Sledge Johnson]

I'm unsure of your abilities with tube amp circuits so my advice will be intentionally vague.

There are lethal voltages in a tube amp, do not attempt repairs if you are not qualified to work with high voltage circuits

Check the fuses, replace if needed with the exact same values and types.

Sounds like the heater circuit doesn't have good continuity or appropriate voltage (and/or/both?) current.

--or--

The 6.3V tap of the power transformer may be partially fried.

I'm unable to determine the source of the problem from here.


If unsure take it to a tech !!

 

[Sledge Johnson]

The next step would be to replace the unlit pre-amp tubes with a known good one -- one by one.

Turn off the amp and wait for the tubes to cool before swapping.
If the known good tube lights up, then the problem is with the tubes. If the known good tube Does Not light up, the heater circuit has a problem.
No need to check v4 yet.

 

[siav]

The three preamp tubes can’t go wrong or go bad at once …

 

[siav]

I bias my amps myself and I’m comfortable with measuring current and voltage.
Can you advise me on where I should measure what please?

 

[Pete Farrington]

Starting at the wires from the PT, check each pair of terminals on the heater circuit for 6.3VAC.
So one probe to pin2 of the 1st output valve socket, the other to pin7 of that socket.
If ok, move along to the next socket.

It’s a very good idea to use probe clips, ie securely clip the probes onto the terminals then power the amp up.
Take the measurement then power down and clip on to the next pair of terminals.

 

[Chris-in-LA]

Check to see if the preamp tubes are getting 6.3v heater voltage

 

[siav]

Thanks. Got it for the power tubes.
Which pins for the preamp tubes ?

 

[FleshOnGear]

Measure between pin 9 and pin 5.

 

[Spanngitter]

I only stumbled once over a 2555SL and this is years ago but didn't they use DC Heating for the first 3 PreAmp Tubes in that amp, analog to the 2100/2500 SL Series?
The used rectifier BR3 is severly underrated (due to a too large filter cap behind it) and mounted flush on the PCB which prevents thermal conduction so it usually either burns itself thru the PCB or at least unsolder itself before it dies. V4 (PI) is heated using common 6.3Vsc, same supply as the Poweramp tubes...

 

[siav]

This isn’t a question for me lol I’m not that advanced in amplification electronics. Perhaps you can look at the schematics?
That might be the reason why I see some cables that have melted their isolation.

 

[Spanngitter]

I don't have the Schematics for a 2555SL available and Marshall won't share them as I am not an official repair center.
But maybe you could open your amp and provide a few pictures, BR3 normally sits on the small PCB attached to the backside of the amp where the fuses and speaker jacks are residing. Usually the BR is quite well destinctive due to the charring on the PCB, so maybe you can take a picture, also same for the cabling of which you think it might be heat damaged...