1978 Marshall

[Valvehead]

Ive got a 78 JMP 50w , mv , 2 input 2204 im playing with . I noticed something strange . The B+ to V1 and V2 seem too low according to all the charts Ive collected .

The 2204 schematic shows 2 , 10K/1W resistors on the B+ rail . This amp has 3 of them. Definitely looks like the original /factory resistors . Has anyone come across this on an older 2204 / 2203 ?

[Valvehead]

Paging :
Trace from Voodoo amps

[Enzo]

I see three 10k resistors in the B+ string.

http://www.schematicheaven.com/marsh..._50w_2204u.pdf

The JCM800 version of the 2204 only has two.

http://www.schematicheaven.com/marsh...l_50w_2204.pdf

[Valvehead]

Thats what i found too enzo ......After looking at all my scems.... i have an oddball amp here, maybe ?

the 76' 2204 / 2 input shows 3 10K , but a plexi style preamp

the 80's 2204 / 2 input shows 2 , 10K with a jcm800 style preamp.

This is a 78 2 input with the 3 10K , and a jcm 800 preamp

of course, i dont have a 78' scem .

[Enzo]

It is unlikely someone went in and ADDED an extra 10k resistor, so accept it as stock. That would be my approach.

How does the amp work?

[Valvehead]

Oh , its absolutely stock as far as the 3 , 10K's . I can see they are factory .

i just had problems / weird plate voltages ........and found that it was the pcb traces broken / worn . All the caps are new and i just replaced a bunch of the vintage 78' resistors


had to wire it "point to point" on the fist 4 stages . It wanted to oscillate badly at first but a shielded cable cured that . I did a couple basic gain mods.

the amp sounds GREAT , a loud , nasty old 50w JMP ....


no buzz or hum --- just ROCK.