Transformer Coupled Pentode Question

[dsavitsk]

I am pretty comfortable working with triodes, but other than in guitar amps (where I just do what Leo tells me to do) pentodes remain a bit of a mystery.

So, I have a triode strapped D3a in an amp as shown. This is a really nice headphone amp, but I got to thinking that maybe it would be interesting to experiment with the d3a in pentode mode.

I attached the pentode curves with G2 at 150V. B+ is conveniently at 150V already. So, if I simply disconnect G2 from the plate and instead connect it to B+ right before the output transformer, does this get me pentode mode? From the load line I drew for a 5K load, it looks like I need to lower the bias to ~1.25V with a 33R resistor, but is that all there is to it? Is there a better way to do this? And, what sort of changes should I expect? more power? More distortion? Anything?

Thanks,

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

 

[AndreasS]

...I need to lower the bias to ~1.25V with a 33R resistor, but is that all there is to it? Is there a better way to do this? And, what sort of changes should I expect? more power? More distortion?...

Hi,

grid current begins at -1,3Volt, so the bias -1,25 ist to small. And for decreasing the autput impedance of the pentod you're needing voltage feedback, this lowers the input impedance (in this schematic).

Regards Andreas

 

[dsavitsk]

Re: Re: Transformer Coupled Pentode Question

grid current begins at -1,3Volt, so the bias -1,25 ist to small.

I'm afraid I don't know what either of these mean. How do you know that from the datasheet?

And for decreasing the autput impedance of the pentod you're needing voltage feedback, this lowers the input impedance (in this schematic).

Assuming this means I need some feedback somewhere? I'm afraid I don't even know where to start looking for how to do this ...