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Old 12th February 2008, 07:15 AM   #1
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Default Transformer Coupled Pentode Question

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,

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Old 12th February 2008, 07:28 AM   #2
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Default Re: Transformer Coupled Pentode Question

Quote:
Originally posted by dsavitsk
...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
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Old 12th February 2008, 07:47 AM   #3
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Default Re: Re: Transformer Coupled Pentode Question

Quote:
Originally posted by AndreasS
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?


Quote:
Originally posted by AndreasS
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 ...
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