[HAM] Capacitors

[Kon Zissis]

Hi David and everyone.
When the aged filter electrolytic capacitors have decreased in mfd value
then I assume that this would allow the plate ( anode ) resistors  to
act as partial feedback resistors  between the various valve stages and
because the aged filter capacitors have now decreased in mfd value and
no longer blocking the lower frequencies  from passing  between the
various valve stages through  the plate resistors , then this would
cause  the lower  frequencies to be partially attenuated by the feedback
effect of the plate resistors wired on the various valves.
 
I have built a guitar preamp  that uses three 12AX7 valves  and a high
DC voltage power supply, and I have placed six switches that select
either 100 K ohms  or 1 Mega ohms  as the plate resistor values of all
six stages of the 12AX7 valves. 
100 K ohms is a very commonly used plate resistance value in valve
circuits  but when  I set the switches to the 1 Mega ohms  settings ,
this lowers the plate voltage in order to allow an earlier onset of
distortion  but  the 1 Mega ohms resistances also cause the bass
response to be warmer and meatier compared to the 100 K ohms settings. 
 
This warmer bass response seems to  be happening because the 1 Mega ohm
plate resistances  lessen the feedback effect  of the plate resistors
between the various valve stages .
Because of this I think that a valve amplifier  would sound warmer and
fatter if the plate resistor values were very high in order to minimise
the feedback effect and then have a very high DC voltage  power supply
so that a proper DC voltage to allow the clean headroom still goes to
the valve plates even with the very high plate resistor values such as 1
Mega ohms instead of the more common 100 K ohms.
 
The 12AU7 valve in the Leslie  122 and 147  uses  56 K plate resistors
so therefore it would be interesting to see how the bass response is
affected if the 56 K resistors were replaced with 100 K or 200 K or 1
Mega ohms values.  Disregarding the fact that the reduced voltage would
lessen the clean headroom of the 12AU7 , I expect that the higher plate
resistor values might increase the  bass response to some extent.
All the best.
Kon
 
David Anderson wrote:
Aged electrolytics can act as tone-shaping components in and of  
themselves.