Fixed bias design question

[Sheldon]

Originally posted by Rob11966
Sheldon, I have seen some designs without the capacitor to ground. I think the simple SE is an example of this. Is it absolutely required?

It's there on the Simple SE. It's C7, the final cap in the bias supply. At 100uf it's plenty big and should give a quiet supply to less than a couple Hz.

Sheldon

BTW, the grid leak resistor in that design is 20k (R-22). It also illustrates the value of the source follower. You can load your driver lightly, use a small coupling cap, and use a low value grid leak (and push a little A2 for peaks). This costs you one FET, a couple of resistors, and a few milliamps current from your B+ supply.

Here's mine. The 2.2uf cap is bypassed with a 100nf film cap (not shown): http://www.diyaudio.com/forums/attac...amp=1192137373

[Rob11966]

Thanks Sheldon,

I overlooked that cap thinking that it was just a filter cap.

Rob

[kevinkr]

Originally posted by jmbee
hello,
original spec. from Western Electric for 300B are:
maximum grid circuit resistance for
fixed bias ......... 0.05 megohm
self bias............0.25 megohm
very different!!

crd
jmb

I have never, ever had a problem with 100K and fixed bias with good tubes, (WE, JJ, EH, KR and Valve Arts) and I have designed and built a lot of 300B PP and SE amplifiers, however I would very strongly recommend not ever going above 100K.

[Rob11966]

Originally posted by kevinkr


I have never, ever had a problem with 100K and fixed bias with good tubes, (WE, JJ, EH, KR and Valve Arts) and I have designed and built a lot of 300B PP and SE amplifiers, however I would very strongly recommend not ever going above 100K.

Hi Kevin, which resistor are you talking about, the resistor between the cap and the grid or the one between the the bias supply and he grid?

Rob

[jmbee]

hello,
the resistor between the cap and the grid is optionnal .
the critical grid resistor we talk is between the grid and the bias supply
for fixed bias, or between grid and ground for self bias.

crd
jmb

[kevinkr]

Originally posted by Rob11966


Hi Kevin, which resistor are you talking about, the resistor between the cap and the grid or the one between the the bias supply and he grid?

Rob

Originally posted by jmbee
hello,
the resistor between the cap and the grid is optionnal .
the critical grid resistor we talk is between the grid and the bias supply
for fixed bias, or between grid and ground for self bias.

crd
jmb

Exactly as jmb indicated in his post above. The resistor between cap and grid is frequently referred to as a grid stopper resistor..