Seeking for info about Mixed Bias theory

[kagliostro]

I want to increase my knowledge about the Mixed Bias on tube amps

Unfortunately I've find explanations that go deep about Fixed Bias and Cathode Bias, no tecnical info about the use of the Mixed Bias

Yes. some info are disposable about what is the Mixed Bias but I'm not able to find any study about It

Things like the difference in using more Fixed Bias vs Cathode Bias I mean, seems very difficult to find such disquisitions

Does someone knows if in books like radiotron or similar there are disquisitions about ?

Many Thanks

Franco

[tubeswell]

Try RDHB 4 Ch 13. Not much to glean there in relation to mixed bias, except commentary on the need for better regulation of plate and screen power supply if designing for Class AB1.


My take on mixed bias, is it's like designing fixed bias with a little bit of 'auto-bias' function (from the cathode resistor) as a form of 'protection' from running too hot. So you should be able to whack the fixed bias up to 90% of Pmax (or even higher?) at idle and be 'protected' from over-dissipation by the cathode resistor under signal conditions. I wouldn't run too much cathode resistance because it'll compromise the potential gain to be had from the fixed bias. (i.e., the advantage of fixed bias is more plate-to-cathode voltage, and hence more gain, for any given B+ voltage, so why would you want to lose that?). YMMV

[kagliostro]

Many Thanks Tubesweel

Considerations as yours are what I'm trying to find in a tecnical study about

Franco

[Backwoods Joe]

What amps used mixed bias besides the early AC50?

[_Steve]

Interested to learn more too!

[kagliostro]

and who knows how many others

Franco

[Williamblake]

Is this a belts and braces kind of approach or can you have it all, i wonder.

[kagliostro]

Not able to understand what you mean

I can translate but english isn't my language, so I don't know what It means

Franco

[tubeswell]

‘Belts and braces’ is a metaphor for ‘taking all precautions’ or ‘being thorough’ or ‘covering all bases’. It literally means you wear both belts and braces to keep your pants on (so you have a backup system if one of those things lets you down).

[kagliostro]

Many Thanks Tubeswell

Now all is clear, I understand perfectly

An italian friend that used the Mixed Bias and is helping me to implement it in a project

he say that that it increases in a positive way the nuances of the sound adding good harmonics

If the guy for whom I'm planning the amp (is a Geloso G1 1020 A conversion) bring the project to fruition then we will know more

in any case I will report here on the forum on the results

BTW

belts and suspenders is what I've been wearing for a long time now I've lost some weight so just belt 

Have a nice day my friend

Franco

[66Strat]

I think that in the case of the AC50, the cathode resistors may have been implemented to inject degenerative feedback, rather than for bias purposes.

[tubeswell]

Yes, if there’s no cathode bypass cap, the cathode resistor will provide cathode current feedback, which will lame-out the signal.