How do you swap tubes in a JCM 800, biasing, and the works?

[Naps]

We were checking out my bassist's JCM 800 100w last night and he was asking me about tubes (as I just tube my Mesa) and he wants to retube. However, I know you're supposed to bias these amps and there is more work to it than just swapping. So, can anyone give me a full tutorial on what we'd need tool wise to check and/or set the bias and anything else we should know?

We pulled a couple of the tubes out and compared them to the tubes I got from eurotube. My JJ El34L's had one less pin than his GT-EL34Ls. Can anyone explain this?

Finally, do you have to bias power tubes also? I was thinking no, but I'm not sure why.

Thanks!

[Gearjoneser]

Re: How do you swap tubes in a JCM 800, biasing, and the works?

Do a forum search on biasing tubes, and you'll get a lot of info quickly.
I'd type, but I've already written better stuff in the past than I can do now.

[Lake Placid Blues]

Re: How do you swap tubes in a JCM 800, biasing, and the works?

We pulled a couple of the tubes out and compared them to the tubes I got from eurotube. My JJ El34L's had one less pin than his GT-EL34Ls. Can anyone explain this?

The ones with one less pin have an internal connection like a 6550. They are probably more direct copies of the old KT77. It doesn't matter either way on a Marshall amp.

Here's brief summeray of what you need to do, but do do a search and get more complete data.

Get a 2-5 watt precision 1 ohm resistor and solder it between pin 8 and ground with the tubes pulled. Take the plate voltage with a multi meter, with the old tubes in. Divide the plate voltage into the max plate dissapation rating of the new tubes. 25watts for most EL34 types. Multiply the ma rating you got by .70. Measure across the 1 ohm resistor with your multi meter set to milli volts, and adjust the bias pot untill it gives you the mv you determined mathmatically. Check the plate voltage again and re-adjust. Play the amp. You can set the mv one or two higher or lower, to fine tune your tone.
(Use an insulated mini-screw driver and don't let it touch anything but the bias pot. Don't touch the terminals or any of the leads going to and from the blue cans. Use one hand and wear shoes. )