Fixed Autobias?

[M Gregg]

Simple question,

I have never tried this I guess someone has..

Is it possible to fix bias an output tube close to bias point and then autobias it to the correct value? So you have fixed bias not adjustable and its auto biased into correct operation?
I guess this dosen't work or I would have seen it done?

Regards
M. Gregg

[M Gregg]

I am told it works..LOL

OK how low can you get the cathode resistor and still have effective auto bias?


Regards
M. Gregg

[hidnplayr]

I guess you need a servo..

[dcgillespie]

The result would be to effectively and significantly reduce the value of the cathode bias resistor over that traditionally used for pure cathode bias. From a pure performance standpoint, fixed bias usually delivers the best overall performance, although cathode bias can deliver very good performance as well when designed correctly. However, if any deviation from pure fixed bias represents a move from optimum, then the combination of the two bias schemes would have to represent something less than optimum conditions.

One way to easily try your idea would be to combine my Enhanced Fixed Bias (EFB)(tm) design with the addition of an appropriately small cathode bias resistor between the output of the EFB regulator and the cathodes. In this scenario, the EFB regulator would be set for (possibly) a couple of volts less bias than required, and then the small cathode resistor could make up the last few volts.

In the end, I'm afraid that to have enough flexibility with such a design so as to allow the cathode bias portion let each tube properly self adjust to an optimum bias condition, the cathode bias resistor would have to be so large as to largely negate the qualities that the fixed bias portion was attempting to provide. All you can do is try it and see. If you do, let us know how it works out!

Dave

[M Gregg]

Originally Posted by hidnplayr

I guess you need a servo..

I don't want to use a servo....I am wondering about Grid bias and standard auto bias combined..

Regards
M. Gregg

[M Gregg]

Originally Posted by dcgillespie

In the end, I'm afraid that to have enough flexibility with such a design so as to allow the cathode bias portion let each tube properly self adjust to an optimum bias condition, the cathode bias resistor would have to be so large as to largely negate the qualities that the fixed bias portion was attempting to provide. All you can do is try it and see. If you do, let us know how it works out!

Dave

If I can get a reduction of resistor to quarter of the value or even half the value it will be useful..

Regards
M. Gregg

[DF96]

This is sometimes known as mixed bias. I have used it for an EL34 UL PP output. My 'fixed' bias was variable. If it is really fixed then you can't take it too far as it won't adjust for valve variations and ageing. Halving the cathode resistor value is about as far as you can go, unless you can test and select the valves and hope they don't age too quickly.

[hidnplayr]

Oh, so you mean a plain combination of cathode resistor & fixed bias? It will work yes, but regulation will obviously be not as good as with simple auto bias..

[M Gregg]

Originally Posted by DF96

This is sometimes known as mixed bias. I have used it for an EL34 UL PP output. My 'fixed' bias was variable. If it is really fixed then you can't take it too far as it won't adjust for valve variations and ageing. Halving the cathode resistor value is about as far as you can go, unless you can test and select the valves and hope they don't age too quickly.

looks interesting, I guess it could be one dual pot to just shift the Fixed value..(If needed)
Just reducing the resistor would reduce the discharge time of the capacitor after transient..worth a try...

Thank's for your thoughts...

Regards
M. Gregg

[Printer2]

Fixed bias with cathode bias.

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