Help 2555SL no sound, no light in preamp tubes

[FleshOnGear]

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...

The 2555SL is the same as the Silver Jubilee. All the heaters are AC, but the 6.3V supply is rectified to power the channel switching relay and the LED indicator.

 

[Pete Farrington]

I was basing my responses on this schematic https://el34world.com/charts/Schema...arshall_25aniv_silverjubilee_50_100w_2555.pdf
Sorry I should’ve linked it previously.

 

[siav]

I have a hard time posting pictures here because I can’t change the resolution of my smartphones camera.
Here’s the inside the chassis. The previous owner has recalled the heater cable (in black) and has isolated the black wire connecting the foot switch to the preamp tube.

184993-C3-8-BB1-4-C90-89-B5-BEB9-E7384-CE9

Image 184993-C3-8-BB1-4-C90-89-B5-BEB9-E7384-CE9 hosted in ImgBB

ibb.co

8-E57-CF82-CD15-4-C7-A-8719-15-C0500-A121-E

Image 8-E57-CF82-CD15-4-C7-A-8719-15-C0500-A121-E hosted in ImgBB

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[Pete Farrington]

I’m not surprised that it’s failed.

 

[siav]

Why the black cable linking the footswitch to one of the preamp tubes melted?

 

[J Hoeve]

Ik heb het schema van de 2555sl-60-02 en de voedingen van de verschillende landen, ik zal proberen deze ook te posten, als dat niet werkt, kunt u mij een persoonlijke e-mail sturen, zodat ik ze naar u kan sturen.
pe1rfe@outlook.com, groeten, Jan Hoeve, pe1rfe.
Holland.

 

[Sledge Johnson]

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?

Ok then measure the heater voltage of the pre amp tubes (look up the pin #s)
Should be 6.3V +/- 0.2
Look for cracked solder joints with magnification
Re-flow where necessary.

My best guess without seeing your amp is a continuity break in the heater circuit.
There is also the possibility of a bad switching relay (if you have those, idk right now)

I'll come back later when I get home and look at the schematic.

 

[Sledge Johnson]

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.

Melted insulation ? Hmmm, got too hot
The question is Why ?

 

[siav]

I only know resistors that heat up for the purpose of heating. The wires got too much power?

 

[Sledge Johnson]

Had a quick look at the gut shots (pictures)
Haven't had time to look at the schematics yet.

The black electrical tape disturbs me.
What mods have been done to this amp ?

Just a shot in the dark ---> Triode-Pentode switching via footswitch ?

 

[siav]

Had a quick look at the gut shots (pictures)
Haven't had time to look at the schematics yet.

The black electrical tape disturbs me.
What mods have been done to this amp ?

Just a shot in the dark ---> Triode-Pentode switching via footswitch ?

No mods, it was like that and I put electric cable around the place it had melted

 

[Pete Farrington]

No mods, it was like that

It didn’t come out of the factory with the heater wiring anything like that.
The amp needs to be taken to a competent person to get put right.
Sorry I didn’t state that earlier, I kinda assumed it was self evident.

 

[siav]

It didn’t come out of the factory with the heater wiring anything like that.
The amp needs to be taken to a competent person to get put right.
Sorry I didn’t state that earlier, I kinda assumed it was self evident.

Thanks for the advise but I’d rather fix it myself

 

[PelliX]

Ik heb het schema van de 2555sl-60-02 en de voedingen van de verschillende landen, ik zal proberen deze ook te posten, als dat niet werkt, kunt u mij een persoonlijke e-mail sturen, zodat ik ze naar u kan sturen.
pe1rfe@outlook.com, groeten, Jan Hoeve, pe1rfe.
Holland.

Top, hartelijk dank voor de documenten, maar zou je misschien je bericht voortaan ff door de Google translate kunnen gooien? Dan hebben er meer wat aan

No mods, it was like that and I put electric cable around the place it had melted

Eh, where you did install this cable exactly?

The amp needs to be taken to a competent person to get put right.
Sorry I didn’t state that earlier, I kinda assumed it was self evident.

All things taken into account, I think you should consider taking Pete's advice here. The amp has obviously been "modified in a way that is not optimal for its correct operation" to put it mildly.

If you insist on attempting a fix yourself, the first thing to do would be to disconnect the footswitch from the heater circuit. These two should never be 'in touch' [pun vaguely intended]. Without looking at the schematic, I'm assuming that whoever did this had some kind of a 'load' in the footswitch and was thus reducing the voltage on the heater circuit. At least, that's the best I can come up with.

Anyway, you've probably got a cooked rectifier, but a (semi-) working heater tap on the PT. Check the voltage in to and out from the rectifier for the preamp valves' heater circuitry. If you're getting 6.3V-ish AC at the input, the tap is probably fine. Output voltage (if any) from the rectifier will of course DC.

 

[Chris-in-LA]

Eh, where you did install this cable exactly?

There seems to be a language problem here. I believe that OP means that he put tape on the melted insulation (not isolation) on the wire from the heater circuit to the pedal jack.

 

[smiley]

watching this thread...

I have had my 2555sl since brand new with very slight home use only. My issue has been the volume starts to lower when playing. I had it repaired the first time by an amp tech (I forget the exact board they had to replace). It also blew a tube which were not covered by warranty - they were crazy prices from Marshal and I've yet to find a good replacement that sounds the same.

The volume started fluctuating again and I basically haven't played it in years now because of that Guess I need to find an amp tech again... really disappointed with this amp.

 

[NickkiC]

Replace those damaged wires. Check continuity along heaters, as recommended. I’d check this from V5 to V1 for continuity. Reflow connections at the tube pins and check that nothing has come loose.
How do you bias, DMM or a bias meter?

 

[FleshOnGear]

Replace those damaged wires. Check continuity along heaters, as recommended. I’d check this from V5 to V1 for continuity. Reflow connections at the tube pins and check that nothing has come loose.
How do you bias, DMM or a bias meter?

Also, that wire between the preamp heaters and the footswitch jack shouldn’t be there. I’m not sure why that was done, and it indicates a possibility that the amp is messed up in a way that requires a trained technician.

Thanks for the advise but I’d rather fix it myself

I understand wanting to do it yourself. But you have to be able to trace the circuit and compare it to the schematic to see how it was altered. However you decide to proceed, I wish you luck. Make sure you discharge the filter caps so you don’t get electrocuted.

 

[PelliX]

There seems to be a language problem here. I believe that OP means that he put tape on the melted insulation (not isolation) on the wire from the heater circuit to the pedal jack.

Right, right - that makes sense, sorry. You would think a bit of tape would keep the heat contained [no, please no pitchforks, I'm joking!]

I understand wanting to do it yourself. But you have to be able to trace the circuit and compare it to the schematic to see how it was altered. However you decide to proceed, I wish you luck. Make sure you discharge the filter caps so you don’t get electrocuted.

Yup, not just that - have a little read about dealing with high voltage circuits, in particular valve amps. If you have a variac and a current limiter, these are your friends. I'll just leave it at that as you outlined that you're comfortable measuring voltages in there.

It also blew a tube which were not covered by warranty - they were crazy prices from Marshal and I've yet to find a good replacement that sounds the same.

Depending on the VLVE code they were most likely Shuguang (brown base often) or JJ (black base). There's a sticky in the Workshop section with the codes. Frankly, JJ's "do the Marshall thing" well, if you ask me - and they don't break the bank.

Replace those damaged wires.

Uh no, disconnect that footswitch.

Check continuity along heaters, as recommended.

As outlined earlier in the thread, this amp is a bit of an oddball in the sense that there's mixed AC and DC heating. I'd check the rectifier as I suggested, as that's the beginning of the DC heated section.

Reflow connections at the tube pins and check that nothing has come loose.

They look fine on the pics, I wouldn't assume that there's any issue here - at least for now. No *harm* in doing so properly, but I'd stop things melting before I worried about a dodgy contact there.

 

[siav]

Lots of great comments here, thanks guys!

I think I’ll disconnect the foot switch as I don’t use it.
Also, will check the heater voltage after recabling the heater cables between power tubes (the ones on the preamp are fine) as it is wired now.

The power tubes are JJ’s EL34.

I think the previous owner did some work in it and I don’t know what he’s done. If you see anything that shouldn’t be wired as it is (except the footswith), please tell me.

Thanks again for your help!