I found this one amp guy that says to do the following:
From Liam on Marshall Forum:
http://www.blamepro.com/CGI-BIN/cute...=31&thread=163
In order to run EL-34's in a JCM900 equipped with 5881/6L6's, you'll
need to change a few things.
****If you don't know what you're doing, DON'T DO IT!!!!****
Discharge the caps before starting.
Change R31, R32, R33, and R34 to 1kohm 5-watt resistor
Change R30 to 22kohm 1-watt metal oxide resistor
Change R29 to 15kohm 1/4 watt resistor
Change R28 to 56kohm 1/4 watt resistor
Change R23 to 91kohm 1/4 watt resistor
Change R17 to 100kohm 1/4 watt resistor
Change R18 to 10kohm 1/4 watt resistor
Change R19 to 470ohm 1/4 watt resistor
Change R20 to 100kohm 1/4 watt resistor
Change R26, R27 to 220kohm 1/4 watt resistor
Remove C14 100pf capacitor
Change C9, C10 to 100n 250v capacitor
Fit link across large HT capacitor with red and brown wires on it,
thus increasing it from 50 to 100uf
Remove the 56k resistor in parallel with a 10nf capacitor which is
connected between the 4 ohm tap of the output transformer and the
violet wire, then connect the violet wire directly back to the 4 ohm
tap of output transformer.
Remove the 4k7ohm resistor paralleled with a 22nf capacitor (if
fitted) that sits across R6.
Fit you EL-34's, turn it on and bias the amp for around 35ma per tube.
(No signal condition)
The above mods are for the dual reverb series amps (100w and 50w)
The mods for the 2100 (100w) and 2500 (50w) SL-X amps are almost
exactly the same, except:
Change R28 to a 68K 1/4w resistor (instead of the 56k)
Change R29 to a 27k 1/4w resistor (instead of the 15k)
Leave out the change to R17