Re: Cathode biasing in an Audio Research amp.
- From: "Phil Allison" <philallison@xxxxxxxxxx>
- Date: Thu, 21 Jun 2007 13:05:20 +1000
"Patrick Turneroid Maniac "
I am working on an Audio Research two channel VT 100 watt
amp with bias two bias adjustments, one for each channel of 4 x KT88.
I tried the original biasing, and for -53V bias setting, Ia
for 4 tubes was 47, 52, 40, 23 mA for the 4 tubes.
** So it is normally a grid biased design.
With cathode bias, I got 57, 58, 56, 53 mA for the same four tubes in
the same positions,
** At idle.
What about operation at the clipping point ?
The point I make is that even with tubes that are obviously so very
unmatched for a given
fixed value of grid bas voltage, when the individual biasing is addopted
it reduces a 50% variation in Ia down to less than 10%.
** Only works out for pure class A amplifiers.
Not class AB ones - like the Vox AC30 is.
BTW:
Changing the VT100 to cathode bias meant a loss of nearly 50 volts in the
overall DC supply available for the tubes to use.
I once converted a old 2x EL34 guitar amp from cathode bias to grid bais,
also removed the rectifier valve and fitted SS diodes.
Doing that DOUBLED its power output, went from 20 to 40 watts rms.
........ Phil
.
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