JCM900 4100 high plate voltage! Any help

[saranden]

Hello.

Ok, I put a wire in the wrong place on one of the caps....corrected it and she didn't blow the R35 this time. Getting there.

Plate voltage is 530V before coming out of standby. I let it warm up for 15 minutes before turning the amp on to see what the bias reading was. Not far out, it was at 39mV for which I need 33mV by my calculations. Set that, then plugged a guitar in.. SOUND at last..however

After playing for 10 minutes, V5 (tube V4 being nearest the ECC83's) was hotter that a poker and 5 times brighter than the other 3. I didn't notice it getting hotter as I was too busy having a bash on the guitar.

What are your thoughts on this?

 

[RussBert]

You have a tube in "runaway" you need to find out if it's the tube, or the socket. Try that tube in another socket. If it still red plates when moved, it's the tube. If the tube that's now in that socket red plates, it's the socket

with the amp running and all tubes in, measure the voltages of each power tube. Also measure the current draw of each power tube.


The cap you wired in backwards must be disposed of. A cap installed backwards can not be re-used

 

[saranden]

Thanks Russbert.

You must tell me it in plain language...ie measure the current draw!?
What pin? If you tell me where to measure, I can do just that.
BTW, what does PI stand for.? "Branches off to the PI and the preamp tube plates"

I changed v4 and v5 for a different pair, and it is coming through very low volume and over modulating on clean.

I've just discovered the fuses in the back of the amp are blown. Have to wait now until I've got the new 500mA fuses before I can go on. I'm not even going to try it with fuses above 500mA..boo hoo

 

[saranden]

Thanks to RussBert and Plexifier for you guidance and help so I could fix my amp. It works and sounds fab! There were no shorts or arcing on the PCB, I had to replace R35, the pico fuse, the screen resistors R31-R34 and the 50uF/50uF 500V capacitors which were constantly taking out R35. I would like to know how I could reduce the plate voltage to around 470V which seems the norm for this amplifier. It also transpired for general information that the pcb is definitely wired for EL34A's as standard. I don't know if that's a good thing!
Anyway....thanks.

One thing, I can't get the bias below 47.5mV and slowly creeping up. R28 is a 47k resistor. Would it help if I changed this, and to what?

 

[RussBert]

Is there any arcing on the power tube sockets? Are the tube pin contacts in each power tube socket tight, clean & secure? I might be prudent to replace the bias supply capacitors. The bias should be steady. Monitoring the voltage coming out of the bias supply and on pin 5 would be a good idea at this point. You should be able to get around -30 to -45 volts with the sweep of the bias pot

What voltages are you getting on all the power tube pins?

 

[saranden]

Hmmmm! A bit premature I think.
The bias is creeping up all the time. The plate voltage is static at 486V
Tubes are 25w...25/486=0.0514 x 0.7=36mV. It is running at well over 50.

R28 is a 47k. I read that 5881 tubes have a different resistor here. My board is populated way different than a 5881 so this is definitely built as a EL34A tube board.
Date stamp is Aug 92
What next?

suggestions welcome

 

[saranden]

Fully warmed up it is staying at 54mV on any of the valves

 

[Jonathan Wilder]

54mV is WAY too hot! Even for 5881's.

Most guitar amps should be running right around 35-40mV at the MOST (if biasing via cathode current, which it appears that you are). There's absolutely no reason to take them any hotter than this. Furthermore, if you place 5881's in an amp that's set up for EL34's, it will idle too hot as this one appears to be doing.

Install some EL34's and see where you end up.

 

[saranden]

Hi Jon.
It is definitely an EL34 board and I have 34's in it. I've got 2 new filter caps and 4 new screen resistors in. I can't get the bias below 54mV. Do you think there's a possibility I may have to replace some of the other electrolytics?

I'm using the proper octal bias probe to measure.

 

[Jonathan Wilder]

Hi Jon.
It is definitely an EL34 board and I have 34's in it. I've got 2 new filter caps and 4 new screen resistors in. I can't get the bias below 54mV. Do you think there's a possibility I may have to replace some of the other electrolytics?

I'm using the proper octal bias probe to measure.

I'm suspecting a bias supply issue (not uncommon on the JCM900 series).

How much voltage do you see on pin 5 of the power valves?

 

[saranden]

.

 

[saranden]

The voltage in standby is -77V and turned on is -47V

 

[saranden]

The voltages are -77v in standby and -47 turned on

 

[Jonathan Wilder]

Hi Jon. I can't seem to get my answer to display.
The voltages are -77v in standby and -47 turned on

Try a different set of valves. EL34s shouldn't idle that hot with -47 at the control grids (unless your B+ is in the 500V+ range).

 

[saranden]

B+=356v

 

[saranden]

Sorry ..I had those values the wrong way around. It is -47V with the amp turned on in play mode

 

[saranden]

OK. So the B+ is 356V. I measured this at the first point of HT at the filter cap.

 

[Jonathan Wilder]

OK. So the B+ is 356V. I measured this at the first point of HT at the filter cap.

B+ should not be that low. If B+ is 356V then those valves should not be idling at 54mV with -47V at the control grid. Something is definitely amiss here. Can you post up some pics of your cap wiring?

 

[saranden]

Here you go....Jon, you must ask me things in simple to understand directives.
As you can see along the way, I have had some great advice, and managed to get the amplifier working with sound....of which it sounds great.
So, I put the orange wire on the wrong pole to start with, I soldered it on with the red wire....and I turned it on!!!!!!!! Anyway, it blew a fuse, then I realised what I had done, and hoped that I had not damaged my brand new caps.. as the amp worked, I assumed I hadnt damaged them, and when I measured them after discharging them, I am getting just over 50uF on each + pole.

 

[Jonathan Wilder]

OK I need more detailed pics than that. Take some close up as well as some zoomed out a bit so I can see where the other ends of the wires are going..