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Re: [Amps] How high can I go with this amp?

To: Steve Flood <>
Subject: Re: [Amps] How high can I go with this amp?
From: jeff millar <>
Date: Sat, 10 Oct 2009 10:54:28 -0400
List-post: <">>

A different inductance will only change the Q of the tank, not the 
match, assuming you tune and load the amp properly. Making the inductor 
smaller will require more tune capacitance to resonant the tank, create 
more circulating current and require more load C to properly tap off the 
proper amount of voltage and current of for the output.

This increase in Q will actually increase tank losses a bit and reduce 
amp output by a few tenths of a percent.

The tube has a "preferred" RL for maximum power output. You 
automatically adjust the Pi Net to provide that optimum RL by adjusting 
the amp for max output. If you give the tube a lower RL than it prefers, 
it tries to put out more current, can't do it and output voltage 
suffers...which reduces power output.

The easiest way to get more output for class C would be to bias the tube 
deep into cutoff and drive it harder, this will make the tube more like 
a switch and increase efficiency.

jeff, wa1hco

Steve Flood wrote:
> Can I get more output power/efficiency with a smaller inductance coil in my 
> pi-net?
> Power supply is 2700v key-down and capable of 800mA (it is built around an 
> SB-220 transformer).
> Ip meter max is 750mA.
> Amp is 3 x 813.
> Currently...
> Pi-net designed around RL of 2333 ohms.  Pi tank is variable caps and fixed 
> coil. Design Q is 12.
> Currently, my input power is 2700v x 600mA = 1620w.
> Output measured on scope is 1111w.
> Efficiency is 68-69%.
> Plate dissipation is 176w per tube.
> These figures are based on CW with adjustable bias set for 0mA ZSAC.
> Grid current is 150mA (50mA per tube).
> Drive is 100w through input pi net.
> What if...?
> I decrease the coil inductance to lower the RL to 1800 ohms.
> It seems this will allow me to generate higher Ip and input power - say 2700v 
> at 750mA = 2025w.
> If I  can maintain 68% efficiency, I could theoretically get around 1370w out.
> Or does the tank modification allow me to go well into class C - say 75% 
> efficiency or close to 1500w out?
> Or will my drive power limit how high I can go?
> I'm just talking about CW here.
> What am I overlooking?  I am trying to figure out what the amp is capable of. 
>  I know the extra power makes little difference on the other end, and I'll 
> have extra heat and tube stress.
> Steve, KK7UV
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