[HAM] What are the appropriate ohms values for the cathode bias resistors for the different output valve types ?

[Geoff Williamson]

Kon,

the value of the cathode resistor is dependent on the plate voltage  
applied to the particular valve type.  For experimental purposes you  
could look at some schematics of guitar amp designs that use the same  
plate voltage on self-biased EL-34s.  The Matchless Chieftan springs  
to mind for example.  Various older Australian amps used this  
configuration also, such as early Moody BA40 amps and some early high  
power Goldentones too I think.  Just google the schematics, you'll  
find the ballpark resistor value that way.  You could calculate it of  
course, so if you haven't already done so you should download a  
datasheet for the EL-34.  If you mess with valve gear you really  
should have a valve data book anyhow, which often list examples of  
cathode resistor values for particular output valve pairs in AB1  
biased configuration.  There are other factors to consider here too,  
such as the output transformer design (I don't know enough about that  
stuff to advise you here) which might limit the success of your  
planned mod, or it might work out fine.  If you want to check out a  
proprietary design that switches from fixed bias to self-biased output  
configuration, I think there was a Boogie amp that had this idea  
incorporated.  You might check that one out because if I recall  
correctly there was more to that design than simply switching the way  
you suggest.  Perhaps it had additional features so that the amp could  
be switched back and forth whilst playing, I don't know.  Don't be  
surprised if the mod doesn't give you the sound you were hoping for.   
I know some of these designs give quite a 'dark' bluesy sound with  
self-biased EL-34 outputs.  Not so the Matchless Chieftan however.

Arguably, this is OT for a Hammond list, unless of course you are  
going for a variation on the Jon Lord sound...BTW, you'll probably  
find a wealth more information on a guitar amp group, there used to be  
plenty of hot rodders on those groups, tube amps groups etc.  I'd be  
surprised if you can't find good info in those archives.

Geoff


On 07/01/2008, at 1:30 AM, Kon Zissis wrote:

> Hi everyone.
> I would like to know what the correct  ohms values and wattage ratings
> are  for the cathode bias  resistors used  in valve amplifiers that  
> have
> cathode biased output stages.
> What would be the appropriate  cathode bias resistor values for the
> following power output valves :
> EL34 , 6L6,  5881,  EL84 ( 6BQ5 )  , 6V6 ,  KT-66 ,  KT-88.