JCM900 problems - part 2

[zeegler]

I finally got around to poking around inside my JCM900 with my meter. I have come across some discrepencies, and to make matters worse, since this amp has been modded, I am not 100% sure what's going on. This amp started life as a 5881 powered amp but has allegedly been modded to take EL34s. I'm not sure exactly what is entailed in the mod. If you look at the schematics for both the 5881, and the EL34 version, there are several components that are different values. For example, in the power amp section, R30 is listed as a 22k in the EL34 version, but only 10k in the 5881 version. I checked R30 on mine, and it is definitely a 10k resistor. So, now I'm starting to wonder if this amp has been modded properly at all. I'm about ready to give in and take the amp to my local tech, and have him check everything, and make sure it is 100% compatible with EL34s.

Before I do that, do any of you guys know of a forum for amp builders/mods where I could get some really good technical feedback on this type of thing?

[Cygnus X1]

Try here:

http://ampgarage.com/forum/index.php

[zeegler]

Try here:
http://ampgarage.com/forum/index.php

This is exactly what I've been looking for. Thanks Cyg.

[charvel750]

My understanding on Marshalls with different tubes is simply the bias control. If you want 5881's in an EL-34 amp, as long as you can set the bias to those tubes all would be fine. For example, in several old JMP models that were stock with EL-34, I have successfully installed KT-88's (6550) and just had to adjust the bias, that's all. Boy do those older heads kick ass with KT-88's.