Help with a redplating jcm900

[madmartigan]

Hi
A friend brought over his jcm900 2500 50 watt (el34). He was complaining of a sudden volume drop. I replaced the 2 el34s but when i went to bias I cant get it below 55vdc with the trim pot backed all the way down. The volume returned to normal but the tubes are redplating so hot that theyre literally smoking!
Could this be a bad ot or should I be looking at the bias supply (if I knew where to look )?
This is a 1990 model, the first year of the 900s I believe. Any help would be greatly appreciated.
Thanks

[jcmjmp]

The amp might be setup for 6550/5881 tubes, which is typical of JCM900s. To convert it to EL34s, you can follow the instructions here:
http://www.lydian.ca/Marshall_Misc_file ... ersion.pdf" onclick="window.open(this.href);return false;

[madmartigan]

The amp might be setup for 6550/5881 tubes, which is typical of JCM900s. To convert it to EL34s, you can follow the instructions here:
http://www.lydian.ca/Marshall_Misc_file ... ersion.pdf" onclick="window.open(this.href);return false;

As far as I know the amp has always had el34s. From what I understand they didnt start using 5881 until later models. How would I know for sure?

[madmartigan]

Ok so correct me if Im wrong, that manual in your link is for an el34 to 5881 conversion. If thats the case then this amp is definately set up for el34s. BTW thanks for the link

[jcmjmp]

Ok so correct me if Im wrong, that manual in your link is for an el34 to 5881 conversion. If thats the case then this amp is definately set up for el34s. BTW thanks for the link

No - the manual actually explains both.

If the screen grids are 470Ω and the bias resistors are 56k, you should be setup for EL34s.

So you tried new tubes but the bias trim pot won't allow you to adjust the idle properly?

What is your plate voltage and does it change with the bias setting?

[madmartigan]

Im going to need a bit of help if you dont mind. Which pin do I get plate voltage from? 5?

[madmartigan]

Ok I was able to take a few measurements before the label on the brand new groove tubes turned black, began smoldering, and set off the smoke detector
Hope these are the right ones:
v4 pin3 322v pin4 278v
v5 pin3 320v pin5 281v

and also measured v1 pin1 110v and pin6 151v

[jerrydyer]

470ohm is not correct for el34.
those tubes are gone now if they burned the labels off.
be careful dont just rely on posts here... read up.

[bob barcus]

470ohm is not correct for el34.
those tubes are gone now if they burned the labels off.
be careful dont just rely on posts here... read up.

Best Advice on Metro forum,There are only 3-4 people on this forum I really trust JD is one,Everone means well,but you can blow up your amp,If you don't know(like me) what your doing