Marshall JCM 800

[keysnleslie]

I happened on this site by accident and have been reading for hours,wish I had found it long ago,Thank You for being here,I am hooked !
I have on my bench a Marshall JCM 800 that was working when it was re-tubed by the owner because "he thought it needed it" He turned it on then switched from standby to play and it made a loud pop and blew the mains fuse. I took some advise from other threads on this site and fired it up with no power tubes and the fuse held. I then tried all the tubes one at a time and they all worked up to the last one which after being on 30 to 45 seconds gave a bright internal flash,a loud pop through the speaker,and proceeded to take out the mains fuse. The owner claims these are new tubes but they have no print on them or any other marks but look to be new or have few hours on them. I checked the local supporting components with a DMM and everything seems to be within spec.I will try it in the morning with a set of "pulls" that I have that are known to be functional.
Any opinions or ideas?
Again, Great Site !

[Enzo]

Welcome to amp club. The first rule of amp club...

WHich JCM800 amp do you have?

Just because a tube is "new" doesn't mean it can't be bad. It is always a good idea to buy tubes from a seller who warrants his tubes. if I get a bad tube in an order, I can expect a replacement for it then.

WHile your one at a time is perfectly good troubleshooting, I always watch the tube the first time I fire it up after installng new ones. If one self-destructs, you can usually or at least often see it flash. That saves you the time of the easter egg hunt. besides, you also want to watch to make sure none are red-plating.

When following a bad tube, or anytime you see an amp for the first time, and your fuse blowing flasher is certainly a bad tube, always check the screen resistors. You can do it from outside even. Remove the power tubes, then probe pin 4 of each socket for B+ voltage - open resistor means no voltage. Or with the amp cold, probe one pin 4 and measure resistance to each of the other three pin 4 holes. You should get a reading of twice what one screen resistor value is - you are measuring through the one for the home socket and then through the one for the destination socket. So on that Marshall you likely get 2k between any two pins 4. You can also check for the presence of bias voltage wgile at it, but in my exsperience, burnt out control grid resistors are far less common.

ALways let the tubes warm up 15 minutes or more before a final bias setting is made.

[keysnleslie]

Oops,sorry,its a JCM 800 Super Lead model 2210

[keysnleslie]

Thanks for the great idea to check through the tube sockets,it will save a lot of time in the future,but I had already opened this one up. Ok I have 460 volts DC at pin 4 on each socket. Bias is -37. The reading I get between pin 4's is 1100 ohms,and I can only get readings between v1 and v2,and between v3 v4 not across v2 and v3. (assuming counting left to right from the back of the amp.
I also found a chip out of one socket between pins 6 and 7 with a bit of dark coloration one the remaining bakelite. I feel I should change this socket for piece of mind if nothing else,this amp is used hard and often.

[Enzo]

If you have a burnt socket, please do replace it. You are asking for arcs, if it is not arcing already.

Odd to me that your pins 4 won't all measure together, they all come from the same supply bus. But if B+ is getting everywhere it needs to, I guess it is OK.

And get rid of that flashing tube.

[keysnleslie]

Ok I have the socket ordered and the customer is getting a replacement tube for the bad one.This was supposed to be a matched set although there are no markings on them and the supplier wants to know the current draw so he can match the replacement to the 3 we have. My question at this point is;how do I determine the current draw,and is it even possible to do so without 4 tubes? (I usually am a lot quicker than this project has been but time constraints and family medical issues have this bogged down)

[Enzo]

Who is the supplier? A lot of tubes have the data on the container, only some put a label on the base. I don't think they want you to measure the current, since that would depend upon the test conditions. I suspect they are saying "current" as shorthand for the current rating number that came with the tube. With some it will be a "real" number like 29 or 36, while with others like Groove it will be a rating number like 7 or 3 (1-10 scale)

[keysnleslie]

The tubes are 11th generation Shuguang ad I believe they are from "The Tube Guru" AKA Doug's Tubes at myspace.

[keysnleslie]

oops...the supplier was The Tube Tramp,my mistake,sorry.

[keysnleslie]

Ok I finally recieved the right socket (I do know the difference between octal and Loctal but ordered the wrong stk.# so its taken a while to complete this one.
I installed the new socket,fired it up w/o tubes and all is good,460 B+ and -38 bias. I installed the tubes,brought the bias down a bit and woo hoo it works ! One oddity is that I have 430 B+ with the tubes in,but its consistant and runs and sounds great,I just don't know enough to state why there is a voltage drop.
Thank You Enzo and if there is anything to add here please do. I'm sure I will be back here often (like a bad rash) Regards, JF

[Enzo]

Voltages will; always be higher without the tubes. The power supplies are not regulated, so the more power drawn from them, the lower they will drop.

On a hot summer day sitting in your car at idle, switch on the AC, and the car engine will slow down as it has to work harder now. Exactly the same deal.

Voltage readings taken without tubes are pretty meaningless, other than to just show that the supplies are present.