JCM 900 - power tubes not drawing enough current.

[beasleybodyshop]

What could cause power tubes to not draw much current? I am working on a JCM 900 100W with no sound issue. Power tubes are only drawing something like 9ma - should be somewhere in the 22-25 mark right? Plate voltages/screens are in the 500-520 range (which is about right from my research)

Edit:

Someone put EL34's in this amp. I read that most JCM900s (later ones) came with 5881s. I pop in a set of those and I am getting 27ma.

Still no sound. When I engage the channel switch button, the clipper LEDS inside lite up and there is this faint squealing.......I swear its a noise coming from the chassis and not my test speaker. So odd...

[Stevem]

When you pop in the 5881s and play thru the amp do you see that 27 ma idle current number go up big time? if so than the output transformer is shorted and toast!

[cbass]

what voltage is on pin 5 of your 34's.If someone didnt modify the bias circuit whrn the they swapped tubes the 34's would be running cold.

[beasleybodyshop]

what voltage is on pin 5 of your 34's.If someone didnt modify the bias circuit whrn the they swapped tubes the 34's would be running cold.

with EL34s it was somewhere in the -50v range, yeah super cold.

this is a 100 watter, but for the sake of gettting the damn thing to put out noise right now im just running two tubes (dont have enough burner EL34s and 5881/6L6s right now.

This amp has those cathode fuses - it keeps popping them, so Im going to look for shorts in the bias supply and cathodes tonight.

[Stevem]

I do not understand, you say that the most current you have seen is 27ma with the 5881s in, yet you are popping cathode fuses??

[beasleybodyshop]

Steve, I was getting a reading initially, then the fuse popped for one of the cathodes. Sorry for the confusion.

[billyz]

First if the voltages all look ok. Then pull the power tubes and test the OT. If that checks out then start testing for signal and where your losing it.
When I say voltages I mean all the tubes .
I have seen quite a few of these with el34' when it was biased for 5881. You can either put in the correct tube or change the bias circuit. Usually you can parallel a resistor or jump one to get the range you want. Recently I had to change one out completely to get in range. It's the one that drops to ground.

[Leo_Gnardo]

Looks like multiple problems. #1: As cbass said, re setting the bias voltage for EL34 wasn't done, so when you try to run them they're choked off & at best you get a couple highly distorted watts.

#2 BUT you get nothing out of the OT - that's worrisome - and fuses popping - yes it might be toast. What I'd do - clip a known working OT in and let's see if we don't hear something then. Doesn't have to be the correct OT for the amp. Any competent OT say 40 to 100W will do, just to prove the output section can work.

#3: Might be some awful noises if your clipper LED's are already lit with no signal applied to the amp's input - some sort of self generated racket. Apply your test signal input to the FX return and hopefully you'll see & hear what the power section's doing, and you can sort out the preamp problem separately.

[beasleybodyshop]

Looks like multiple problems. #1: As cbass said, re setting the bias voltage for EL34 wasn't done, so when you try to run them they're choked off & at best you get a couple highly distorted watts.

#2 BUT you get nothing out of the OT - that's worrisome - and fuses popping - yes it might be toast. What I'd do - clip a known working OT in and let's see if we don't hear something then. Doesn't have to be the correct OT for the amp. Any competent OT say 40 to 100W will do, just to prove the output section can work.

#3: Might be some awful noises if your clipper LED's are already lit with no signal applied to the amp's input - some sort of self generated racket. Apply your test signal input to the FX return and hopefully you'll see & hear what the power section's doing, and you can sort out the preamp problem separately.

Injecting signal into the FX return is sending it to the power amp stage right? If so, I should send a 1Khz signal or whatever shows up easiest on my scope, and start with the phase inverter and work back to the OT?

[cbass]

You can just plug a guitar into the fx return jack to see if the PA works.
If you still aint blowin fuses

[beasleybodyshop]

I checked resistance from the secondary side of the OT to ground.

22ohms on all taps. Yep, tranny is bad.

[Leo_Gnardo]

I checked resistance from the secondary side of the OT to ground.

22ohms on all taps. Yep, tranny is bad.

ohms to ground or to 0 winding? Possible there's 22 ohms from audio ground to chassis, something unusual like that? If that's not it . . .

22 ohms from 4, 8, 16 to 0 ? That is unusual. OT secondaries I usually see 0.5 to 1.2 ohms.

[Phil_S]

FWIW, I think Leo is pointing in the right direction here. To be sure, I'd run low AC voltage on the OT and check the turns ratio. If that tests OK, then, http://www.premierguitar.com/articles/t ... r-tester-1 I would not be too quick to conclude a transformer is toasted.

[beasleybodyshop]

I checked resistance from the secondary side of the OT to ground.

22ohms on all taps. Yep, tranny is bad.

ohms to ground or to 0 winding? Possible there's 22 ohms from audio ground to chassis, something unusual like that? If that's not it . . .

22 ohms from 4, 8, 16 to 0 ? That is unusual. OT secondaries I usually see 0.5 to 1.2 ohms.

from common to the 4/8/16 taps, im getting .4 ohms. from 4/8/16 to chassis im getting 22 ohms. So does this mean its ok? I will try to drag out my bench supply and check the turns ratio.

On the other end of things, Ive tracked down the preamp issue. This amp uses several op amps as gain stages for the reverb among other things. All of the ICs aren't getting voltage from the Low Tension winding. Back tracking this issue, i discovered these weird fuses soldered onto the board. Continuity check confirms they are open - why the hell do you put a fuse in an amp if you can't take it out???? i bridged them temporarily to suss out the issue more.

Preamp schematic is attached.

Low Tension power goes to opposite sides of a bridge rectifier. From a schematic Ive found one side becomes negative voltage, the other positive. Measuring DC voltages on all four pins of the rectifier gets me something like 16v, -16, -14, -43............huh? I dont get that. Does this mean the bridge rectifier is bad? looking closer, the filter caps for this power supply must have been taken out, as they are rated at like 30v.

EDIT - i found a new replacement rectifier in my stash of random parts. after swapping it in, the voltage doesn't change, still -42v...

Thank you so much everyone for helping me. I am really knee deep in this sonofabitch and I feel like im just now beginning to wrestle it back into shape.

[Littlewyan]

I'd be wary about bridging those fuses, fuses blow for a reason. Which fuses are they? With regards to the rectifier it looks like there is a seperate supply for the ICs as they run at a much lower voltage, so this is normal.