CJ

Fixed Bias and Cathode Bias Combination
« on: February 15, 2013, 05:18:59 AM »
has anybody ever tried using Fixed and Cathode bias at the same time?

say you had a pair of 6V6 in a git amp,

why not use maybe 20 volts fixed bias and 10 volts cathode at the same time?

when i changed to cathode bias on the Princeton Reverb, i gut the added compression,

but the power dropped a bit, fixed bias is more efficient.

so why not try to get the best of both worlds, more compression and more power?

all you do is add a smaller value than normal cathode resistor to a fixed bias amp,

so instead of the typical 270 ohm cathode resistor, use 100 ohms,

6V6 P-P amp running at 80 ma would give you 8 volts of cathode bias,

then just tweak the voltage divider for maybe 20 volts of fixed bias?

have to go thru the Trainwreck schematics to see oif Ken Fisher ever did this,


no wait, i need 36 volts fixed, and 8 volts cathode, as the resistor will eat at the fixed,

right?   :o



« Last Edit: February 15, 2013, 05:41:29 AM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


volker

Re: Fixed Bias and Cathode Bias Combination
« Reply #1 on: February 15, 2013, 05:54:24 AM »
I did, it was 2x 6L6 amp that was originally built for fixed bias. The owner thought like you to combine the sonic characteristics of both variants. We chose the resistor by ear and I adjusted the bias accordingly. He was happy, it didn't do much for me to be honest. No need to overthink this really, put the bias voltage to -max, put in the resistor and adjust to taste.

squib

Re: Fixed Bias and Cathode Bias Combination
« Reply #2 on: February 15, 2013, 06:17:03 AM »
I repaired a commercial guitar amp last week that did this .... buggered if I can remember what model is was now at the end of a long day at the bench.
but it used something like 24 v bias and a cathode resistor, then had a switch that dropped another R in parallel across the cathode R to allow for either EL34 or 6L6.
Sounded great once all the leaking power supply caps were replaced.

CJ

Re: Fixed Bias and Cathode Bias Combination
« Reply #3 on: February 15, 2013, 06:44:00 AM »
wow, what a trip, thanks guys!

what did John Lennon say,

nothing you can do that can't be done,

nothing you can sing that can't be sung, and all that,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

EmRR

Re: Fixed Bias and Cathode Bias Combination
« Reply #4 on: February 15, 2013, 10:49:44 AM »
There's at least one old RCA limiter that uses a combination at the driver tube for the output stage.   Probably the only one I've heard personally, and I haven't ever changed it up to see what happened. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Jonte Knif

Re: Fixed Bias and Cathode Bias Combination
« Reply #5 on: February 15, 2013, 12:06:14 PM »
In my home hifi set I have amps with combined bias. The reason I built them so is that I wanted to use the same supply for driver tube filaments and output tube bias. Since it was a bit too little, I put a couple of volts worth of unbypassed cathode resistor to the output tube. (Siemens F2a) That makes a convenient spot for bias monitoring LED too!

So in this case it was not about sound, just simplicity.

bockaudio

Re: Fixed Bias and Cathode Bias Combination
« Reply #6 on: February 18, 2013, 04:11:29 PM »
This combo bias is done on the P*ltec EQP and EAR EQ.


 

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