[HAM] What are the appropriate ohms values for the cathode bias

[Kon Zissis]

Hi Geoff.
Thank you for your reply.
The plate voltage of the EL-34 valves in my Marshall JTM-45 is around
402 volts and the screen grid voltage is 398 volts.
The plate voltage of the EL-34 valves on the Matchless Chieftain  is 415
volts and the  screen grid voltage is 369 volts .
 The two EL34's each have a separate 270 ohm cathode resistor and a 250
uf cathode bypass capacitor.
 
The Moody BA-40 has a single 130 ohms cathode resistor with a 50 uf
cathode bypass resistor for both EL34 valves.
 The Australian  Valve Amps  website is  quite useful for learning about
vintage Australian built amplifiers.:
www.ozvalveamps.elands.com <http://www.ozvalveamps.elands.com/> 
 
The various  output valve data specs  that I have do not show the
cathode resistor ohms values.
 
I have  Aspen Pittman's book "The Tube Amp Book  4th Edition' and this
has schematics of  some of the Mesa Boogie amplifiers with the
"Simulclass"; feature. There are four output valves  in push pull
configuration. All four output valves receive the grid bias negative
voltage. One valve of the ''push  ''  set and the other valve of the
''pull'' set have the grounded cathodes but the cathodes of the second
":push " valve and the second "pull " valve are connected together to a
SPST switch  and this switch is connected to the ground. In the "Class
A" setting the ground is simply disconnected from these two cathodes. I
have never heard one of these amplifiers .   
 
On Hammond Wiki there is a modification showing how to use EL34 valves
in the Leslie 122 and this modification is a switch that brings in a 100
ohms resistor in series with the stock 150 ohms resistor  thus making
the total cathode resistance  250 ohms  for when using the EL-34  valves
instead of the 6550 valves.
Considering that the Leslie 122 plate voltage is 415 volts and the plate
voltage of my Marshall JTMK-45 is 402 volts , then I think that a
cathode  resistor value somewhere in the 250 ohms - 270 ohms region
might be OK for the Marshall JTM-45 cathode bias switch modification.
 
Several years ago my friend lent me his old and very mediocre sounding
Australian built Rex Bass King BA - 60  amplifier that has two EL-34
valves in it  so that I can play around with it and modify it as a
learning experience  .
I rewired  everything inside to have the  plexi Marshall 1987 specs
and this really improved the sound of the Rex Bass King  and made it
great for electric guitar. With this you could now get the typical
Marshall sound at less than ear splitting volume levels . 
 
It was at this same  time that I was browsing through my copy of the
book " A desktop reference of hip  vintage guitar amps " by Gerald Weber
and this book explained that  the cathode biased amplifiers  produce a
sweeter more singing sound with sustain , and after reading this I was
curious to hear the cathode bias sound so therefore I read up more and I
learned how to wire up a DPDT switch that would create the cathode bias
/ fixed bias  switching  configuration.
 
 After I tried out the amplifier in the cathode bias setting and then
the fixed bias setting , I noticed that  the cathode bias setting
produced a nicer guitar sound with a more compressed "sponginess '' and
fatness  even at clean volume levels. This was a real classic , warmer
1960's type of sound . The fixed bias setting produced a "stiffer" sound
with a harder attack that would appeal more to punk rockers  or modern
heavy metal rockers. 
I have always preferred the fatter  , more sweet and compressed sounding
1960's / 70's guitar sounds  both in clean and heavily overdriven mode
instead of the modern harsher and stiffer and often very trebly guitar
amplifier  sounds.
 
I eventually removed the cathode bias switch modification from the Rex
Bass King because I was not sure if I had used the correct cathode bias
resistor ohms value and my friend planned on selling the amplifier after
I had finished experimenting with it so therefore I did not want someone
to buy the amplifier and then have it break down.    I don't remember
exactly what cathode resistor ohms value I used but it would most likely
have been somewhere in the 150 to 270 ohms range. I think that I also
used a 220 uf cathode bypass capacitor.
 
A few times I did  play  my guitar preamp through my Leslie 122
amplifier ( when it was still a 147 amplifier )  and then connected the
output  to my Marshall quad box and this also had some of that nice
spongier sound. I assume that the sponginess is because the 122 / 147
amplifier is a cathode biased amplifier.  
All the best.
Kon
 
Geoff Williamson wrote:
>the value of the cathode resistor is dependent on the plate voltage  
>applied to the particular valve type