For what it's worth; quasi fixed bias

[George61]

I am fairly new to amp building and I have a 36W Ceriatone which is my 'test bed'. I read a post somewhere about limiting the cathode bias voltage using zener diodes so I gave it a try. The post suggested using zeners that are 1 volt greater than the voltage at idle. I have 11.0 across the cathode bias resistor at idle so I used two 6V 5W zeners in series, as per the post.

The mod didn't make a drastic change but it did have some effect. With the amp cranked the amp seems to be slightly less distorted with more of a Marshall dirt sound which, for my tastes, is an improvement.

[xtian]

It doesn't matter how it sounds, it only matters that it looks awesome. And that looks awesome.

[seedlings]

Have you measured Vk when you strum a chord? Does the bias voltage still drift significantly? I have a PP 6L6 cathode-bias build that idles at 35V, but goes to 45V once a chord is played.

CHAD

[ChrisM]

That sounds cool, I'll have to try.

What are the brown bias resistors? Make?

[Lindz]

I found that using this mod helps kill the "18 watt buzz" similar to the way Paul Ruby's diode clamp mod does on the EL84 grids - even more effective on a couple problematic builds I had actually

I also came across a similar variation for "quasi-fixed bias" by Phil Zaphod - http://www.ax84.com/drums/bbs/dm.php?thread=254479 for those that want tighten up a cathode biased amp a little bit