Grid Bias or Cathode Bias

[duderduderini]

Hi Again
I have searched threads and am wondering pros and cons of grid biasing a PSE of 6550s vs Cathode Biasing. My Understanding is that grid leak resistors change in either setup as do coupling caps. You also have a bit more freedom with b+ in that you have more available in grid bias scenario but the thought of Bias supply voltage failure sounds scary Can anyone recommend a circuit/implementation (Bias winding from transformer or voltage divider from b+) for grid bias that can be adjusted and a good operating point for two 6550's in a pse configuration.
Does one sound different to the other?
Many thanks
Nick

[el156]

Originally Posted by duderduderini

Hi Again
I have searched threads and am wondering pros and cons of grid biasing a PSE of 6550s vs Cathode Biasing. My Understanding is that grid leak resistors change in either setup as do coupling caps. You also have a bit more freedom with b+ in that you have more available in grid bias scenario but the thought of Bias supply voltage failure sounds scary Can anyone recommend a circuit/implementation (Bias winding from transformer or voltage divider from b+) for grid bias that can be adjusted and a good operating point for two 6550's in a pse configuration.
Does one sound different to the other?
Many thanks
Nick

HI !
I think if you prefer grid biasing and to make sure it doesnt fail, you must add in serial with the tube supply, a fuse of about 160ma/tube. But i prefer cathode bias,wich makes an automatic ajustment of tube current. I tested both and didnt notice a big difference in sound quality, and i always use a decoupling capacitor in paralell with cathode resistor to avoid feedback,because it sounds better without it!
But it is only my opinion! some people think different!
Good Luck

[Eli Duttman]

When you get beyond the needs of "12" W. types and 7591s, cathode bias can become problematic due to I²R heating in the cathode resistor.

A well sized (wattage wise) 10 Ω metal film resistor under each cathode, when employing "fixed" bias, serves both as an "idle" current test point and a fuse. If the bias supply fails, the resistor burns out, before tube damage occurs.

An interesting option that has much (IMO) going for it is combination bias. Stand the cathode on an unbypassed 100 Ω resistor and apply a negative voltage to the control grid. The cathode resistor provides a convenient test point, part of the total bias voltage, stabilizes the operating point, and provides a small amount of distortion control via degeneration (local current NFB).

[sstulken80]

Originally Posted by Eli Duttman

A well sized (wattage wise) 10 Ω metal film resistor under each cathode, when employing "fixed" bias, serves both as an "idle" current test point and a fuse. If the bias supply fails, the resistor burns out, before tube damage occurs.

An interesting option that has much (IMO) going for it is combination bias. Stand the cathode on an unbypassed 100 Ω resistor and apply a negative voltage to the control grid. The cathode resistor provides a convenient test point, part of the total bias voltage, stabilizes the operating point, and provides a small amount of distortion control via degeneration (local current NFB).

+1. "Mixed bias" gets a bad rep from certain CBS-era Fender amps, but done properly, it should allow more stability without sacrificing very much power. Depending on how you size the cathode resistors, you get a little "tube balancing" in the deal, too.

- Scott

[duderduderini]

Hi Eli
Thanks for the replies ( i mean to thank all of you)
I see a lot of different ways to supply the bias voltage. transformer tap, suitably rectified etc... lm317 with lots of components (that could fail?). Where could i look to find as bulletproof a design for grid biasing (along with partial cathode biasing)
Thanks again to all of you

[chrish]

The power amp schematics at AX84 have grid bias with the power supply taken from the B+ with a very simple circuit. Look here at one of the PP amp to see AX84.com - The Cooperative Tube Guitar Amp Project