Cathode biasing and high plate voltage?


Feb 1, 2007
Hamilton, Ontario - Canada
I have an original Blues Deville from, I think, 1996. It had some of the usual troubles over the years, such as failed input jacks, plate load resistors, and what finally made me gave up on it - channel switching relay failures. I put up with it for a long time because I am just that type but, honestly it just doesn't suit me anymore if it ever did. Very stiff and sterile I would say. So, I have definitely decided to use it as a donor to build something different.

So, what to build? I am inclined to make it into something Vox-ish. Specifically I am looking to the Mojotone NC3015 as a template to start from. Two channels; one is a volume-only single gain stage while the other is a top boost style preamp with CF-driven tone stack. I would add a preamp master to the top boost channel and also a post-PI master; either a simple crossline, or a Type 2/Lar-Mar style. The NC3015 already includes a Vox style 'Cut'. Also, no negative feedback.

Here is the thing stuck in my mind. I want to use the Blues Deville transformers. I want to replace the EL84 power section of the NC3015 with the 6L6GC tubes from the Deville. I want to use cathode biasing in the Vox style for its inherent compression. BUT the Deville power transformer, through diode rectification (which I would keep because, 2 octal holes and no 5V winding) delivers about 470V to the 6L6GC plates. How do I determine if this is feasible? Can cathode biasing be used at that plate voltage? I could maybe reduce plate voltage by 30V or so with zener diodes but, is that necessary or sufficient?

I look forward to input, just don't bother suggesting repairing the Deville circuit ;-)

(btw, I am aware the power supply dropping resistors would require component value changes to get the preamp values in the right range but I don't see that as a roadblock yet)


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Poster Extraordinaire
Dec 30, 2019
Once the cathode biasing is implemented, the *Plate to Cathode* voltage will be the voltage on the plate. That 470v minus the cathode voltage will be *seen* as ~440 to ~450v on the plates. I would probably start with a 330R cathode bias resistor on the pair of 6L6 to make sure the tubes do not redplate on start up. When the amp is up and running, adjust the cathode resistance to achieve an acceptable plate dissipation.