84' 2204 Fritzing Out.

[Lummox]

Last night at rehearsal, my JCM 800 suddenly lost volume, and started humming. I shut it down and let it cool, then checked tubes, speaker, input jack with all checking out fine. I'm going to bring it home tonight, and I'm wondering what next to check. The filter caps were replaced about 5 years ago, but everything else is original, save a pair of sand block resistors on the power tube sockets when I converted to KT88's.
Could this be leaky caps, or might it be a bad OT ?

[Mark]

It could be several things but loss of volume with hum would make me look at the output tubes. I've had this happen in the past where the hum gradually increased till it was very noticeable. I looked in the back of the amp and one of the tubes was red plating. I turned it on a bit later and it worked okay, I later experimented to ensure the tube was the problem, basically I swapped the tubes around (50 watt amp) and made sure the same tube red plated. I still needed to replaced both tubes though.

What is the value of your grid leak resistor?
I'd be looking for a 82K to 100K pair of resistors.

I'm sure others will chime in with alternative ideas. As it has occurred to me a PI tube with a dead triode would also cause your fault. Mind you I'd look long and hard at your output tubes as they can cause the most damage.

[Lummox]

The grid leak resistors are 56k, which I think is a bit low. Something else I found odd was P1 has the 5w sandblock jumping pins 6 and 4, it's not there on P2. Could this be the problem ?
Power tubes were my first thought too, but tube swaps yielded no changes.
I appreciate your thoughts.

[Mark]

Perhaps a photo would help?

[Lummox]

Here's a pic of P1 and P2. There are chunks of what I assume are the other sandblock on my kitchen table which fell out when I pulled the chassis.

[IMG:960:720]http://i.imgur.com/bwYmSNQ.jpg[/img]

[Reeltarded]

Yikes!

Awesome!

[Mark]

Code: Select all

Yikes!

Awesome!

I agree with Yikes, not so sure about awesome. Yeah, your going to need another resistor, that's a problem. I really needed to see that to believe it!

I really would have a good look at this amp and see if there are any other faults/omissions, this omission is quite basic and fairly serious in that it implies the guy who did the work has either no attention to detail or no idea.

[Lummox]

Well... I figured out why there was a missing screen resistor. I soldered a new 1k 5 watt wirewound resistor across 4 and 6, plugged in a speaker and powered it up, thinking I would check the bias... Poof ! Burned up quick. (the screen resistor, not the amp)
Something else is amiss here.
[IMG:960:720]http://i.imgur.com/a2sZIjD.jpg[/img]

[Mark]

Sure is, there is a short circuit in that amp.

http://www.ampbooks.com/home/classic-ci ... power-amp/

This an illustration of the circuit notice that there isn't other components.

http://www.ceriatone.com/images/layoutP ... iatone.jpg

So the resistor burnt up with a tube in the socket?
Normally I'd say you'd need a bad tube to destroy the screen grid resistor or a burn tube socket. With the amp off, use your multimeter to check the filter caps have drained and there is no voltage still present in the amp. Measure from side of the resistor without wires attached to ground. Do this with and without the valve. I'd expect to measure open circuit.

[Lummox]

Occam's razor strikes again. Turns out it was a bad tube after all, that cooked the screen resistor. Here she is with fresh tubes and new resistors, re-biased and happy.

All's well that ends well.

[IMG:960:720]http://i.imgur.com/yoZR8F3.jpg[/img]

[M Fowler]

Good job getting the JCM back up and running we can't have a good amp down can we.

[Mark]

Good stuff glad to hear you're up and running again.

[Lummox]

I appreciate the help ! In the not too distant future, I'll probably convert this to EL34's, as I've always preferred their tone and feel.