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

[Kon Zissis]

Hi Brad and everyone.
I read your Hammond Wiki article called "122 amplifier modification"
http://www.arcanedevices.com/hammond/122_mod/
Using this as a basis as well as the schematic of the Matchless
Chieftain amplifier that uses cathode biased EL-34 valves , I have
decided to use a 270 ohms resistor for the cathode bias modification of
my Marshal JTM-45.

I am experimenting with this at the moment and I have wired up a  switch
to select either the stock grid bias configuration , or the cathode bias
configuration. I am also experimenting with a 220 uf  cathode bypass
capacitor connected in or out of circuit.

 Years ago I put in a dual ganged 220 K pots which is wired up as a post
phase inverter master volume control  so that I could overdrive the
Marshall at any volume levels  but the distortion  produced by the
overdriven phase splitter 12AX7 valve sounds harder and stiffer without
the enhancement produced by the fully driven EL-34 valves 

My  Marshall JTM-45  now sounds nicer when I switch to the cathode bias
setting. 
I tried this out both with the clean sound and the overdriven sound.
With  the stock grid bias setting the sound is stiffer but with the
cathode bias setting there is a nicer, more "spongy" sound.

The volume level is the same in the grid bias setting and the cathode
bias setting  with the 270 ohms cathode resistor.

There is  some difference in the tone and the overdrive characteristic
when I disconnected the 220 uf  cathode bypass capacitor.

As well as the 270 ohms resistor , I also tried out six other higher
resistor values.  
Without the 220 uf cathode bypass capacitor , the higher cathode
resistor ohms values start to lower the output level of the amplifier
whilst still allowing the output stage full overdrive effect. 
Out of curiosity I tried out the following  cathode resistance values :
270 ohms, 470 ohms , 820 ohms , 1.6 K ohms , 2.2 K ohms, 8.2 K ohms and
39K ohms.

The 470 ohms and the 820 ohms  and the 1.2K ohms resistors produced a
slight reduction in the output levels.
The 8.2 K ohms and the 39 K ohms cathode resistor values significantly
reduced the volume levels  thus allowing me to  get the full Marshall
output stage overdrive  sound at  "neighbourhood friendly "  volume
levels but the EL34's were still producing the characteristic output
stage distortion sound.

When I connected the 220 uf cathode bypass capacitor in circuit , it
worked well with cathode resistances up to around 820 ohms. With the
1.6 K ohms cathode resistance,   the 220 uf capacitor  cause a
noticeable "sag " effect  similar to that produced by a compressor set
to produce the heavy compression and  release 'breath " effect . With
the  higher resistances values,   the 220 uf caused the sound to become
''farty'' sounding.

I am planning on putting in a multi position rotary switch so that I can
choose  the different resistor values , starting with 270 ohms resistor
and then switching  to other higher  ohms value resistors. This will
allow the Marshall to be fully overdriven at much more socially
acceptable values. The overdrive sound produced  from the fully driven
EL34 valves sounds better than the stiffer, more sterile sounding
overdrive produced  by the 12AX7 phase splitter valve and this
modification allows the EL34's to produce the nicer overdrive sound
whilst running cold.

I will also put in another switch to switch the 220 uf resistor in or
out of circuit for extra  tonal versatility. 

With the success of this experiment with my Marshall JTM-45 , I will
also do a similar experiment with my Leslie 122.
I will try out higher  cathode resistor  values with the 6550's, and I
will also try this out with and without the 200 uf cathode capacitor  on
my modified Leslie 122 so that I can get the full output stage Leslie
122 distortion  at much lower output levels.

Interestingly, I saw the Mojave amplifiers website and these amplifier
have an "Output Dampening" control which allows the amplifier  to be
fully overdriven  but have a variable output level . The advert for the
Mojave Sidewinder  amplifier says:

"Because Power Dampening lowers the power being generated by the tubes,
the amp runs cooler, and with much less stress. This reduces wear on the
tubes and transformers. Power Dampening is not a built in attenuator,
nor is it a master volume control. The result is excellent retention of
both tone and clarity. Power Dampening lets you get full throttle
overdrive with lower power amp volumes."

The Mojave Sidewinder amplifier can be seen on:
http://www.mojaveampworks.com/index.php?id=11,0,0,1,0,0

On   http://www.mojaveampworks.com/index.php?id=17,45,0,0,1,0    there
is a photo of the underside of the Sidewinder amplifier and from this
photo  the Output Dampening control looks like a normal potentiometer
with two yellow wires  coming from it. These two wires are shorted out
when the potentiometer is turned up to the fully clockwise setting so
therefore I wonder if the Output Dampening control is simply a variable
cathode resistance control. 

All the best.
Kon 



Brad Baker wrote:

(as far as I know)

Pentodes wired as pentodes have a high plate resistance.  This means
that changes in plate voltage (B+ for example) cause very little
change in plate current for a fixed grid/cathode voltage.  Look at the
curves.

The cathode bias resistance is chosen to select the value of no-signal
bias current you are comfortable with for that tube.  This current
value should be reasonably insensitive to the plate voltage.