Cathode Bias - is it really "plug'n'play"?

welshboyo

So, just become the owner of a Bogner Atma which is a Cathode Biased EL84 amp.

Is it too good to be true that as its cathode biased then I can just replace valves with a decent matched pair of EL84's?

Its humming a bit which I'm not too pleased about but can't be arsed with chasing the previous owner about it - one of the EL84 was glowing a bit more than the other and was at the point of redplate last night I reckon.

I have ordered a full compliment of valves for it and everywhere I look it says "no bias needed - cathode"

Is this true?

kennedydream1980

Yes that is correct, as long as you have a matched pair of EL84's it will be fine. I had a Cornford Roadhouse 30 which was exactly the same.

ecc83

Yes, "in general" cathode bias (aka auto-bias) will set the anode current to the correct level. The problem is in many guitar amp designs they set it too high or, the valves they used in development were a bit on the "cold" side and other brands ran a wee bit hotter.  Set the amp up in darkened room and check the "spine" of the EL84 anodes. Even the slighest reddening shows out of spec' dissipation. This won't wreck the valves immediately but will shorten their life.  It is no great trouble for a tech to either fit a higher value cathode resistor or put a few ohms in series with the incumbent.

Since you have just bought the amp a tech check would surely be a good idea?

N.B. If anyone here has a VERY early Blackstar A15 or A30 it might redplate with some brands of 84, easy fix.

Dave.

Modulus_Amps

yes, but saying that a lot of amp manufacturers run EL84 at too high voltage and too high plate dissipation and not big enough screen resistors, worth while getting it checked out I would say.