altec 436c tube biasing ?

[MagnetoSound]

Thanks, PRR.

> the plate current to aim for in Class A operation as 10mA or so ...

WHY? (Hint: no, you could do "Class A operation" with much less than 1mA...)

Yes, my interpretation of tube data sheets leaves a lot to be desired ... why do they suggest 10mA plate current? ... perhaps the assumed application is not relevant here?

> one might assume around 7k in the winding ...

And one would be wrong. You wish for zero DC resistance in the winding. You bias-down the tube somewhere between maximum and zero by setting grid-cathode voltage.

This gives terrible DC gain and output. But we don't want DC signals. The AC impedance is higher than the DC resistance.

Right, it is the reflected AC impedance that is of concern as far as the signal is concerned, not DC resistance - and the grid-cathode voltage is the stopper in the bottle for DC current ... so I guess not even the internal Rp matters then, when biasing the tube?

 

[ChrioN]

Such a few components, such a pita to build... or maybe I'm wrong?

 

[KrIVIUM2323]

why do they suggest 10mA plate current? ... perhaps the assumed application is not relevant here?

The application given for classA in datasheet is for SE (resistance capacitance coupled stage) operation so with an anode resistor. Transformer coupled design seems to works differently...

Thanks PRR! I've got the terms to look at for theory info about that kind of designs!.

 

[PRR]

> application given for classA in datasheet is for SE (resistance capacitance coupled stage) operation so with an anode resistor. Transformer coupled design seems to works differently...

The "Characteristics and Typical..." does not say resistance-coupled amplifier.

The GE datasheet page 3 does give RC-amp suggestions. Use your math. RL = 0.1Meg (100K), Ebb = 300V, *obviously* the current must be less than 300V/100K or 3mA. For 0.51meg and 90V, less than 0.17mA.

> why do they suggest 10mA

A small truck may 'boast' that it can tote 1,000 pounds. That does not mean you can't use for smaller loads.

The 10mA 250V condition actually is one possible transformer-coupled point. And it could output more power than most line-level audio systems need. I have seen a similar tube worked at 2*8mA 280V push-pull for a 1.5 Watt monitor speaker; line-level is ~~1mW nominal and rarely over 100mW (0.1W) max.

You could turn to a smaller tube. 12AU7 is the small 6CG7. Not a lot smaller, just a wee bit cheaper. We could use something a third the "size"; but tubes much smaller than 12AU7 start to cost *more* because the teeny bits get so fiddly. So we use a dozen mA tube at just one or a few mA. Like delivering popcorn in a half-ton truck.

 

[KrIVIUM2323]

Thanks for correcting PRR.

I did some research about transformer coupling and found many interesting infos but for the moment i must admit many of them just don't explain how to 'theorize' (do the math determining biasing conditions, load lines etc, etc from datasheets) or are too complicated for my brain...  :-[

PRR could you point an article or a book which explain this in a simple manner (if possible to explain this simply) a bit like articles or publication from Norman H Crowhurst (which i found really easy to understand but unfortunately not really usefull in this case for the ones i've got )?