JCM 600 bias problem

[01redcobra]

Customer brought in this JCM 600 that would power up, but no sound. Found a blown t500ma fuse and checked the power tubes. They were bad, so I replaced them with new and checked the bias. They are running at 50-52mv which after doing some quick math, I found that they should be running at about 35 or so. There appears to be a bias pot between the last 12ax and the first el34, but I can't see a screw slot to adjust it. It's labeled DB101 on the board, but on the schematic I have it should be labeled RV101. Plate voltage is at 489-490. Were these amps built with no bias adjustment or am I in the wrong place?

[Amp Kat]

Try looking in the back of the amp by the test jacks on a little pcb board. They are hard to see,very small.

Oops edit: was thinkig of the 2000


Marshall Amps Info & SchematicsThis amp does not have a bias pot, and you'll have a hard time fitting .... See the Marshall Amps JCM600 page for more "official" information on the JCM600 amps. ... Rear jack PCB circuit diagram, TL-60-61-02 Issue 3, (Marshall, 1999).

[01redcobra]

There's no test jacks on the amp. I believe they are on the JCM2000 series. I had to use a bias probe to check it.

[Amp Kat]

There's no test jacks on the amp. I believe they are on the JCM2000 series. I had to use a bias probe to check it.

Check this out

jcm600 bias pot symbol - Marshall Amp Forum

[Enzo]

Make sure you are looking at the right part. DB on a Marshall refers to "diode bridge." So DB101 would in my mind refer to one of those little round bridge rectifiers.

[01redcobra]

Yep, wrong part there. Found it hiding on the very corner of the board behind 2 radial caps. Duh! Couldn't see the forest for the trees. Thanks guys!

[Jared Purdy]

totally unrelated...

Enzo, tell me about this tube holder that you've got...please (I tried to see if you had a private email address, but couldn't find it)

Regards, Jared

[Amp Kat]

Why don't you start a seperate thread so he will see it. I know he has an EL-84 derattler for the C-30 that is real nice.

[Enzo]

Amen, please let's not stir Peavey tube rattle problems into a thread about a Marshall JCM600.

I turned off my email and PM buttons because too many people abused them. I do my best to help people on this forum, yet I still got tons of email expecting personal private repair lessons.

[Jared Purdy]

Enzo, how can I get a look at theis tube holder of your's??

[Enzo]

Do what Amp Kat suggested, start a thread.

[Jared Purdy]

done!