JCM900 100w Dual Reverb - v3 and v4 red plating

luxspring

Hi there.

i have a JCM900 where v3 and v4 are red plating while v1 and v2 look fine.

The amp sounds normal at first but after a few minutes the volume starts to drop. 

At that point the amp is very hot and those two tubes are glowing red.

Any suggestions for possible causes for this and where I should start looking ?

I am no expert but have built a few plexi amp kits so I’d like to try and repair it myself.

many thanks
Paul

RiftAmps

Start with the basics...get a full voltage map for each output valve. Anodes, Screens, Grids, Cathodes, and see what looks out of place.

I'm guessing the bias voltage for the red plating pair is less negative than the other side, possibly a leaky coupling cap, but you'll need to take measurements to confirm.

You might also try swapping each pair side to side to see if the problem follows the valves or stays with the sockets. Unlikely, but best not to assume.

BTW the output valves are V4-V7.

luxspring

Great info.
Thanks a lot Chris, I will start exactly with those suggestions.

ecc83

A rare but possible cause of thermal runaway in OP valves is a grid leak resistor gone high. I see Rg max for the 5881 is 100k but some makers push their luck and go up to 270k, might work for well burned in old  valves but modern stock? 

Especially prone are carbon composition resistors. Always replace  if needed with metal film. 

But yes, a voltage table is the first line of attack. 

Dave.

ICBM

First swap the *outer* valves - only one valve from each side. A cause of this can be a faulty valve with a grid to cathode leak - it will not only redplate itself, it will pull down the bias voltage for its companion on the same side and make that redplate as well even when it's not faulty.

If it stays on the same side, swap the inner pair and repeat. If the fault stays on the same side after both swaps, it's a fault with the amp not the valves, and as RiftAmps said a coupling cap leak is the most likely. If the fault moved with a valve, you know which is the faulty one - although the other one from the same side may also be damaged if it's been running like that for a while.

luxspring

Thanks for all the suggestions.
Will report back when I get a chance to check.
Cheers

luxspring

Update:
I swapped the outer valves as discussed and now there is no red plating at all!!
All working normally now.
i played a loop through it for a few hours and all sounded normal.
Any ideas why that have fixed it ?
Could a dirty/bad contact on one of the tubes have caused the red plating ?

When I have more time I will remove the chassis and check the bias but the amp now appears to be working perfectly fine.

ICBM

Probably an intermittently faulty valve which has temporarily cured itself after being disturbed. Keep a look out for the problem coming back - leave the valves where they are now, then if it does you’ll know which one is the culprit.

luxspring

Will do.
Thanks again for the great suggestion

ecc83

luxspring said:

Will do.
Thanks again for the great suggestion

Also, get a Sharpy and number the tops of the valves and the chassis. 

Dave.

luxspring

Aha, new development.
I did another test just by putting a looper through the amp. All still sounded fine.
Then I did a heat check with a temperature gun on the power valves , only to discover V4 is completely cold. Hadn’t noticed previously as the room is quite bright.
That is one of the previously red plating valves that i moved from V7.
So I will need to now go deeper and open the amp
up and check voltages on the sockets etc.
The valve was obviously functional 
before I moved it as it was red plating.
hopefully it is just the valve that is the issue.
Will update during the weekend probably.
Thanks again for all the help so far.

ICBM

That sounds like the previously failing valve has died completely when moved - also not unlikely. If so, hopefully there's nothing wrong with the amp at all. You can easily use both the definitely good valves - the ones from the side that wasn't redplating - to test both the inner and outer pair of sockets separately. If those valves work OK in both positions and the amp sounds right - it's not harmful to run it like that, at least for a short time - then really you just need a set of four valves, keep the two good ones as spares, and chuck the other two, since the one that was redplating when paired with the duff one is probably damaged as well.

luxspring

Tested today with the two non red plating tubes in positions V4 and V7, then in V5 and V6 with impedance selector at 8ohm into a 16ohm cabinet.
Both tests successful and amp sounds fine now.
Thanks again to all for the great suggestions.

ICBM

That’s excellent .

The bad news is that valves can do this sort of thing, and it isn’t uncommon.

The good news is that the problems are usually relatively easy to troubleshoot and fix.

Although rarer, if this happens to a power transistor in a solid-state amp, it’s more likely to damage other parts, and will be a bench job to repair.

luxspring

Hi again.
So now that the amp is working well on two valves I decided to remove the chassis to clean some noisy pots. While I was in there I happened to test the continuity on the triode/pentode switch and then on the 8ohm/16ohm impedance switch.
Pentode/triode switch appears completely normal but when i test the continuity on the (top and bottom) three legs of the impedance switch, they are all connected whichever position the switch is in !!
Is there any reason(other than a faulty switch) that this could happen ?

I suspect I will have to remove the switch and test it out of circuit.

Thanks again

ICBM

luxspring said:

While I was in there I happened to test the continuity on the triode/pentode switch and then on the 8ohm/16ohm impedance switch.
Pentode/triode switch appears completely normal but when i test the continuity on the (top and bottom) three legs of the switch, they are all connected whichever position the switch is in !!
Is there any reason(other than a faulty switch) that this could happen ?

I suspect I will have to remove the switch and test it out of circuit.

Don’t. The impedance of the transformer secondary is so low it looks like a dead short to a multimeter - well under 1 ohm usually - so you’re reading continuity via the winding. If the amp works at both impedances, the switch is fine.

luxspring

Aha, I was wondering if there was something causing that.
indeed it works in both positions.
i was just concerned I might be damaging something.
That’s great information again.
Very many thanks.

ecc83

luxspring said:

Aha, I was wondering if there was something causing that.
indeed it works in both positions.
i was just concerned I might be damaging something.
That’s great information again.
Very many thanks.

If'n you REALLY need to know if an impedance switch is working you can do it by a voltage check. Slap a digital meter across the speaker (better a 10W load R) and feed in a 400Hz sine tone (Audacity or any such software will do this) Adjust for 1 volt rms at the 8 Ohm position and then flip it to 16. The voltage should rise to about 1.3-1.4V. 

Dave.

luxspring

Ah cool. I will try that indeed just out of curiosity.
Many thanks for the suggestion

Nerine

If it’s working I’d probably just leave it alone at this point.