Ulf,
To make the pi calculations, you should consider using the LOADED plate
voltage. Idle plate voltage is not a factor. The difference, however,
is not very much. Maybe 10-20%...
And on #3, you are right. The voltage and current are unimportant when
considered SEPERATELY. Unfortunately, there is not very much emperical
data published on the Russian tubes so picking the optimal B+ is hard to
do. Generally speaking, higher B+ is better as the efficiency and gain
is better. But in my experience with the GI7b, at a certain voltage
point, the efficiency of the amp starts to drop. At low B+, the PO is
fine - the trouble is the Ig gets high due to the low gain of the tube
at that lower B+.
73/jeff.ac0c
Ulf Tjerneld wrote:
> Hi all,
>
> I'm collecting parts for an amp-project. And I have som newbie
> questions:
>
> 1, Maybe this question is silly: When calculating the Pi network for
> an amplifier with GI7-B's I'm aiming for an unloaded plate voltage of
> 2500 volts. With one tube this means that the plate current would be
> 350 mA for an input power of about 875 watts and not dissipating more
> than 350 watts. If I'm instead going to use a pair of tubes, this
> means that I would input 700 mA in the Pi network formula. Right?
>
> 2, I have a perfect capacitor for the Pi-tank in the above example.
> But is rated exactly 2500 volts (Johnson something...). Would that be
> to little for an unloaded plate voltage of 2500 volts?
>
> 3, Most people that builds amplifiers based on the GS35B use a plate
> voltage of around 3500 volts. This would mean a plate current of about
> 700 mA for 2500 watts input. But let's say I only have 2500 volts, but
> enough iron in the transformer. Would I achieve equal results if I let
> the the tube draw more current? I.e. let the tube run with 2500 volts
> and a plate current of 1 A? And to extrapolate further, even go to a
> plate voltage of 2000 volts resulting in a current of 1.3 A? To
> summarize: does it matter what combination of voltage and current I
> use as long as it is within the specs of the tube?
>
> All the best
>
> Ulf/SM0NOR
>
>
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