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previous: J Fletcher Not really a Marshall guy myself,bu... -- 1/23/1998 7:30 AM

Re: Right bias for JCM 800?

1/23/1998 11:41 PM
GVB	Re: Right bias for JCM 800? The bias current you mentioned would be approximately right (not that there is an exact proper bias point for the tubes) for a 100W with EL34's installed. But the 800 series of Marshalls used several different plate voltages/transformers and two different types of output tubes, 6550's and EL-34's. This gives various "proper" bias points from 24ma up to 44ma. The "proper" current could be calculated as follows. Ik = PA/(2 X V+) where Ik is the bias current, Pa is the maximum plate dissipation rating for the tube in W, and V+ is the measured plate voltage. The PA for the EL34 is 25W and the 6550 is 35W. Although this method sets the bias current to about 1/2 of PA you could also set it between 1/3 and 2/3 of PA safely. However higher currents will age the tubes more quickly and obviously will require replacing more often. GVB

1/23/1998 11:41 PM

GVB

Re: Right bias for JCM 800?

The bias current you mentioned would be approximately right (not that there is an exact proper bias point for the tubes) for a 100W with EL34's installed. But the 800 series of Marshalls used several different plate voltages/transformers and two different types of output tubes, 6550's and EL-34's. This gives various "proper" bias points from 24ma up to 44ma.

The "proper" current could be calculated as follows.

Ik = PA/(2 X V+)

where Ik is the bias current, Pa is the maximum plate dissipation rating for the tube in W, and V+ is the measured plate voltage.

The PA for the EL34 is 25W and the 6550 is 35W.

Although this method sets the bias current to about 1/2 of PA you could also set it between 1/3 and 2/3 of PA safely. However higher currents will age the tubes more quickly and obviously will require replacing more often.

GVB

Replies:

J Fletcher You are right...I was wrong...I mad... -- 1/26/1998 11:54 AM