jcm bias quick help.

[murkat]

JCM 800 20th anny. had KT66's. one blew.
I am trying to rebias with a set of el34's, using a bias right, I can only get the bias up to 10.2 mv with the marshall bias pot full clockwise.
I notice added resisters in line at power tube sockets, and possibly a changed one as well in front of the bias pot. see pics.
are the added resisters bumping down the mv for the KT66's?
[IMG:800:600]http://i13.photobucket.com/albums/a257/ ... amp002.jpg[/img]
[IMG:800:600]http://i13.photobucket.com/albums/a257/ ... amp001.jpg[/img]

[Firestorm]

Is that a 120K dropping resistor connected to the bias diode? EL34-based Marshalls usually have a 220K there. 6550-based models used 150K. Maybe KT66 amps used 120K. That's probably it.

[murkat]

yeah, you can see that it has been changed, and the added ones on each power tube socket....

[Firestorm]

The grid stoppers on the output tubes look legit. 5K6? That's the same value Marshall would have used with EL34s. KT66s like a little resistance in front of them, too. That won't affect your bias at all -- almost zero current, hence no voltage drop.
Try changing the 120K to 220K and you'll reduce the negative bias voltage to the point where you can adjust it for EL34s. The grid return resistors also look like they're set for 6550s (or maybe KT66s). With EL34s, Marshall would have used 220Ks there too. You can leave 'em alone. The lower values are kinder to the tubes.

[murkat]

The grid stoppers on the output tubes look legit. 5K6?
Try changing the 120K to 220K and you'll reduce the negative bias voltage to the point where you can adjust it for EL34s. The grid return resistors also look like they're set for 6550s (or maybe KT66s). With EL34s, Marshall would have used 220Ks there too. You can leave 'em alone. The lower values are kinder to the tubes.

yes, 5k6 on the grid stoppers.

the resister in front of the bias adjust pot has been changed, evidents of that.
looks like Y/V/R/G... 4k7? and should be 220k?

[Firestorm]

NO. That's probably Y/V/O (red and orange are hard to distinguish on some resistors). That would be 47K, which is correct. The one I'm taking about is between the turret with the white wire and the banded side of the bias diode -- brown/red/yellow (120K). That should be red/red/yellow (220K) with EL34s.
You sure you're good to do this? Scary voltages and stuff in there.

[murkat]

yeah, I'm cool. And I thankyou for your concerns!!!!
I gotchya now. thanks. will report back..

edit... Thanks for the guidence. She sounds great, pure jcm grind.