JCM900 RED PLATE and volume loss..

[sergio]

Hi , My friend got a JCM900 100w, after a few minutes he plays, there is a big loss of volume, he noted that a 6L6 turns red.
Any thoughts?

[breakfastbuddy]

the power tubes are probely worn out .

[paddy]

Sounds like you may have a problem with bias to that tube.
I would check all wiring and connections in the bias circuit.
Also check the tube sockets as they may be making poor contact
with the tubes,particularly pin 5.
You could also have trouble with the coupling capacitor which
feeds the offending tubes' grid.If this capacitor is slightly leaky
it can cause the tube to red plate.

good luck

paddy

[jaysg]

Hi , My friend got a JCM900 100w, after a few minutes he plays, there is a big loss of volume, he noted that a 6L6 turns red.
Any thoughts?

I thought those had fused pairs of output tubes.

[sergio]

I tried to replace all the valves but the problem is always with the same tube.
I have to check the coupling capacitors.

[John_P_WI]

I will suggest this is a thermal related issue. Is it the same tube or tube socket? Many years ago 15? I had a carvin head on my bench that did the exact thing.

Turned out to be the wire in the tube base pin would lose contact when the tube warmed up and would wildly red plate when it lost the bias voltage.

If it is the same socket, look for a trace or component that when it is heating up loses continuity, resulting in loss of bias voltage to that socket.

John

[Wayne]

A couple of quick chime-ins...

- Was the amp modded to run 6L6? '900s, as far as I know, run EL34s stock.

- My JCM 900 red plated both tubes on one "side" last time I put new tubes in it. I never did find out why, and it's never done it since despite a year of regular use (practice once/week & a gig every other week or so). FWIW, I'm still using those tubes!!! Maybe they come factory-equipped with a ghost

W

[Wayne]

Hi , My friend got a JCM900 100w, after a few minutes he plays, there is a big loss of volume, he noted that a 6L6 turns red.
Any thoughts?

I thought those had fused pairs of output tubes.

Oh, and each pair is fused @ 500 mA.