Why such a high bias current on output valves ?

[nigelwright7557]

My EL34 output valve amplifier runs about 50mA standing bias on the output EL34 tubes.

Why does this need to be so high ?

 

[Johan Potgieter]

Nigel,

If a single-ended (single) EL34, it needs to always be in class-A, which will normally mean a high enough current for that to happen plus some. If a p.p. EL34 topology in class-A, the same applies. You do not mention the h.t., but 50mA is actually lowish for the EL34. If one takes a maximum allowed dissipation of 26W for plate plus screen, 50mA total cathode current will still allow up to 500V h.t. to be applied, other matters allowing.

 

[Miles Prower]

Help improve x-over distortion performance if Class AB.

If class A. it needs to be run at 0.5I_p(max) to stay in Class A.

 

[nigelwright7557]

Its a quad el34 output in push pull.
340VDC B+.

 

[azazello]

Your +B is low and decision for Your PP is 50 ma plate current. If You increase +B to 400-450-500 v, You can decrease I plate to 40 ma, but sound will be "dry"...

 

[DF96]

How is the bias applied? That affects how much the bias shifts when signals are present.

50mA per valve is cool, especially at such a low voltage. Mullard use something like 70mA cathode current at 400V.

 

[famousmockingbird]

I agree 50mA is cold especially with lower voltage. My 5-20's are at 33v at the cathode so 70mA and 404 at the plate (437-33), my line voltage is around 123vac today.

 

[nigelwright7557]

How is the bias applied? That affects how much the bias shifts when signals are present.

50mA per valve is cool, especially at such a low voltage. Mullard use something like 70mA cathode current at 400V.

I use a grid leak resistor and 180R cathode resistor (shared by 2 el34's).

 

[HHead]

I'm not familiar with EL34s but the bias on my PP-UL KT88 monoblock(s) is 70ma at 480v B+(actually 506v with the high line voltage we have here)

 

[jbwebst]

Just as another data point; my EL34 P-P ultralinear amp runs around 60-62mA per tube with 395V on the plates & 30V self bias on the cathodes (425V B+). Seems to be quite happy there and gives an easy, clean, 25W into 8ohms...