Cathode bias verses adjustible bias thoughts on sound

[Tone Junkie]

Have any of you built the same amp with both what are the differances.
Thanks Bill

[navdave]

I have a Fender Prosonic amp it switches from adjustable bias
to class A cathode bias. In cathode mode the amp sounds sweeter with
a great boost in the mid-range. The down side is the loss a head room and the
increase in hum.

[rzenc]

Have any of you built the same amp with both what are the differances.
Thanks Bill

Why not install a switch?

[sluckey]

I built ax84.com's November amp with a fixed/cathode bias switch. I didn't hear much difference with this amp. I built a second November amp and didn't even put the switch in it.

http://home.comcast.net/~seluckey/amps/november/november.htm

[Colas LeGrippa]

I just modded a 2204 with both bias. 150 R resistor on each cathode to ground and -25V on the grids. No switch to toggle between the 2 modes, but both at the same time.
Browner tone than only fixed bias and still adjustable 'cause I have hooked up 2 pots to a double negative voltage supply, one for each tube. This way I can adjust the output tubes individually that gives me the chance of using dissimilar tubes without any compromise. I can use an EL34 with a 6550 , with both running at their max. Note that I could have put two different values resistors tied to the cathodes to meet each tube requirement, but then with less flexibility. To get back to your question, cathode bias equals browner tone, and more interesting to me, warmer and richer, closer to SE tone.

[Shrapnel]

To get back to your question, cathode bias equals browner tone, and more interesting to me, warmer and richer, closer to SE tone.

Hence the misnomer in marketing calling practically all P-P cathode biased amps Class-A amps, even when it's far from the truth. Well, actually marketing grabbed and ran with the term, not knowing what it really meant, and sold the hype.

[jjasilli]

Ditto to sluckey

[Tone Junkie]

So then im building a 4 tube 6ca7 amp, from past reads obviously 2 differant resisters and caps one to each pair whats a good area to start or is it math from B+that gives me the best equation. Im still tossing around Tweed bluesmiester or TOS.
Thanks Bill

[jjasilli]

Use the math from your B+.  You may be able to simply plagiarize from similar builds.  But with the math you can determine how hot or cold you want your bias in this amp.  I built a substitution box (Schematic posted) using a rheostat from Weber and switchable bypass caps, to A/B different R value settings.  BTW:  I would really prefer to believe that cathode bias sounds better; but that has not been my actual experience. 

However with cathode bias, I find that the B+ supply voltage is THE significant factor.  Changing K-resistor values tends to push B+ voltage around, with little change in plate diss.

[sluckey]

So then im building a 4 tube 6ca7 amp, from past reads obviously 2 differant resisters and caps one to each pair whats a good area to start

That's a big amp for cathode bias. Usually when an amp design requires the amount of power that a quad of EL34s can deliver, fixed bias is used.

The only fixed bias EL34 amps I'm familiar with use a 270 ohm resistor under each tube. That would imply using a single 135 ohm resistor under a pair. So I'd grab a pair of 130 to 150 ohm resistors for your idea.

Cap values are wide open. Use anything from 25uF to about 500uF.

[rzenc]

... with cathode bias, I find that the B+ supply voltage is THE significant factor.  Changing K-resistor values tends to push B+ voltage around, with little change in plate diss.

Had the same exp while building 80W 4x6L6GC...Ik kept almost the same..increase/decrease Rk changed Vk, thus changing Vp-k..current flowing thru Rk was almost the same..

One of my amps have a F/K bias switch and when playing at low levels I don't hear much difference either..at higher levels (pair of 6550), this amp gets very agressive when cathode biased..

BTW, with 4x6L6GC @ 465V (plate and screen - ~ 2K load) Rk for each pair 220R/20W. Vk 42.xxVDC.

Best Regards,
R.

[Tone Junkie]

Thanks guys