> Indeed it seems to high to be coil loss. If your present
> coil is loosing
> 50 watts, it means the toroid was loosing 350 watts, and
> that doesn't
> seem possible. It would have molten down!
>
> So I think you are now loading the tube in a different
> way, probably
> applying a higher load impedance. This will make it more
> efficient, but
> also increase the distortion.
>
> Did you measure the IMD performance before and after the
> coil change? I
> guess it's worse now.
If he has the same grid current and same plate current at
the same drive he has not changed to operating loadline.
If he does NOT have the same grid current then he simply
needs to reach up and turn the knobs slightly.
I am not surprised by the efficiency change at all. The loss
could have been distributed through the entire toroid
winding and leads to the bandswitch as well as the core
material, and even some of it in the tube if the tube did
not see a good loadline. A resistance, which is what a lossy
tank circuit looks more like, would cause the class AB tube
to transition slowly into and out of cutoff reducing anode
efficiency. A sharply resonant circuit generally does not
soften the transition.
You can see this effect with cathode circuits in cathode
driven PA's. Use a good low loss low pass circuit at the
cathode and efficiency will improve over a circuit that does
not bypass harmonics well. IM products can do anything, but
the they most often improve over the low Q system while
efficiency also increases.
It is a complex system, but as long as he readjusts the tank
for good grid current (GG amp) IM isn't a worry at all. I'd
bet money it actually improves without the toroid.
73 tom
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