Marshall JCM900 Red Plating and Bias Issue

[earache]

Hi Folks

I am looking at a JCM900 model 4500 50watt that my friend complained that “it got real quiet”.

Upon taking it out of the cab it became evident that one of the JJ E34L tubes had blown, lost vacuum and the getter was all chalky white. It has also blown the .5 mA power tube fuse.

There appears to be nothing blatantly burnt or melted inside the chassis.

I have a used pair of EL34s so after replacing the tube I put them in and turned the amp on. Wasn’t long after I switched off the standby that one of them started to red plate so I shut it down.

Then I put a bias probe across one of the power tubes and hooked another meter to pin 3 of the tube to get an idea of what’s going on.

Turning the Amp from standby, pin three showed 328 V and the bias probe was reading 280 mA !
Right about then the amp squealed loudly and I shut it all down again. The squealing was with all the volumes and gains off!

I tried to turn it on again to turn the bias pot to see what would happen but the squealing was unnerving so I shut it down once again.

What to check with the amp off? Where to start?

Schematic is here

https://www.thetubestore.com/lib/the...-Schematic.pdf

Thanks in advance!

[The Dude]

Pull the power tubes and see what your bias voltage is doing. My bet is that C15 is bad and you are losing bias. Check the cap if bias is not negative enough.

[Enzo]

In any case, pull power tubes and see what voltages are sitting on pins 3,4,5.

[Jazz P Bass]

C11 or C12 is leaking?

[earache]

Thanks for the quick reply. I will look at it again tonite after work.

[g1]

My bet is that C15 is bad and you are losing bias.

That would be the first place I would look too. And recommend replacing with a part rated for at least 630V.

[earache]

So - the first thing I went at was C15. It tested ok out of circuit but I replaced it with a Mallory 150 rated at 630v anyway per your suggestion, G1. The diode next to it “D1” which is a 1N4007 also tested bad in circuit but when pulled tested alright. Must be the particular circuit that it’s in that caused it to appear bad, as I was able to test all of the rectifier diodes in circuit successfully, which are all also 1N4007s by the way.

Now after those parts are replaced and with the power tubes pulled, here are the voltages

V4: P3 464V P4 463V P5 -36.59

V5: P3 464V P4 464V P5 -36.90

[The Dude]

I would have checked the bias voltage before changing any parts. You'd then know if you fixed anything or not. That said, your voltages look close. Can you get any more neg voltage with the bias pot?

[alexradium]

So - the first thing I went at was C15. It tested ok out of circuit but I replaced it with a Mallory 150 rated at 630v anyway per your suggestion, G1. The diode next to it “D1” which is a 1N4007 also tested bad in circuit but when pulled tested alright. Must be the particular circuit that it’s in that caused it to appear bad, as I was able to test all of the rectifier diodes in circuit successfully, which are all also 1N4007s by the way.

Now after those parts are replaced and with the power tubes pulled, here are the voltages

V4: P3 464V P4 463V P5 -36.59

V5: P3 464V P4 464V P5 -36.90

C15 has to be X2 rated

[g1]

C15 has to be X2 rated

For noise suppression? The problem with the X2 is that it is only 250VAC rated and seems at least a bit prone to failure in this application. If you go with class X, I would suggest at least upping to X1 which is 400VAC rated.
edit: the specific X2 used by Marshall in these amps was 250VAC. This is not true for all X2 caps.
In any case, suggest upping voltage rating on whatever you use for replacement.

[earache]

I’m not familiar with the “X1” or “X2” rating of capacitors, can you please elaborate or share a spec sheet describing this spec?

Thanks in advance

[66 Kicks]

For noise suppression? The problem with the X2 is that it is only 250VAC rated and seems at least a bit prone to failure in this application. If you go with class X, I would suggest at least upping to X1 which is 400VAC rated.

That's not right.

X Class has ratings up to 660Vac. 47nF at 660Vac capacitors are readily available.

I know the schematic calls for Class X, but does anyone think they are really necessary?

[The Dude]

Class X caps are designed to fail short, which would prevent a complete loss of bias. However, my experience says that these particular caps (in Marshall amps) fail towards open more often than they should.

[The Dude]

......I know the schematic calls for Class X, but does anyone think they are really necessary?

I don't. A quality cap rated sufficiently is ok, IMO. Truthfully, I've replaced more X caps in that circuit than in amps that don't use X caps for bias feed, which makes me doubt the effectiveness of the 'X'. A regular cap is no more or less likely to fail than say a bias filter cap. I've never had one come back. That's just my $.02. You don't have to like it.

[TimmyP1955]

How does the bias supply even work? C15 is in series, not in parallel. There should be no CD getting to R7 or beyond.