Hi Gary,
> Thanks for your reply. Yes it does get confusing when you try to understand
> the full meaning of what really happens.
It's quite simple, if you look at it analytically, instead of applying
cookbook formulas and blindly believing in the results! Unfortunately,
many engineering books use the cookbook approach rather than explaining
what happens.
> All the stuff that I am reading
> about figuring tank Q only mentions load impedance, Xc and Xl and finding
> the proper ratios. There is no mention in the formulas of where coil loss
> fits in although there is plenty of talk separately of how coil Q is the
> result of Xl/r. Wonder why that part is not included when finding circuit Q?
Because it's assumed to be negligible. When you have the PI circuit coil
significantly contributing to the loaded Q, it means you have a lousy
coil and you should make a better one!
A PI circuit designed for a loaded Q of 12 with ideal components, then
built with a coil that has an unloaded Q of 350 (typical for a decent
air-wound coil), would end up having a true loaded Q of 11.6. That's not
enough change to really worry about it. It's within the general
tolerances of the system.
> What really raises the question about coil loss is when I substituted the
> air wound coil in place of the toroid coil the output power went from 800
> watts to 1100 watts, with 2100 watts input in both cases. That's a lot of
> coil loss!
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.
You can of course load it differently now, if you have control over both
the load and tuning capacitors. Set the load capacitor to a lower value,
and the tuning capacitor to a slightly higher one to restore resonance.
That will load the tube at a lower impedance level, which will probably
decrease power a bit but improve IMD.
> It also seems, from what I am able to measure, that the Q of the system is
> rather high at around 20. I measure about 95 pf of plate tune capacitance
> where a Q of 12 calls for around 53 pf. The spec on the plate tune cap has a
> minimum capacitance of 27 but with the way things are packed in there it
> seems there is a lot of stray capacitance.
That's typical. When using tubes close to their upper frequency limit,
usually the lion's share of the tuning capacitance is actually inside
the tube, and in the wiring!
On an amplifier covering a wide range of frequencies, typically the Q is
chosen differently over the range. At the high end, Q needs to be high
to allow for the high tube capacitance. At the low end, it's nice to use
a lower Q, so you don't need such big capacitors. The price you have to
pay for this is that at the high end you need a very high coil Q, while
at the low end of the frequency range the coil Q is rather uncritical.
That's why many amps use dedicated thick tubing coils for the high
bands, and a separate, multiband wire coil for the lower ones.
> My air wound coil made of 1/4" copper tubing does get warm.
With a loaded Q of 20 and a coil Q of 400, which is about the highest
you can hope to get, at 1500 watts output you still have 75 watts
heating the coil! That would make it quite warm even if a fan blows at it!
> The 2 capacitors in series with the plate tune are switched in at the 40
> meter position and above. I am guessing the reason they did that was to make
> for broader plate tuning as this amp has motor driven capacitors that allow
> preset tuned channels and the broader tuning allows for a better chance of
> hitting the preset when automatically tuned.
And it helps lower the minimal capacitance of the combination, as long
as the stray capacitance isn't too high.
> The amp also goes down to 1.6 MHz and a fixed capacitor is switched in
> parallel with the plate tune down there. Also the capacitors in series with
> the plate tune are switched out below 40 meters.
Makes sense.
> It looks like they may have had problems with minimum capacitance from the
> start in this thing and stuck those series caps in there.
This problem always exists, except when using UHF tubes at HF! Try
designing an 813 amp that covers up to 10 meters, or even 6 meters, fed
from a voltage in the high part of its range. There you will see the
problem of tube capacitance!
Of course, take the 833 for an extreme example! Tube plus stray
capacitance alone will set the starting Q at about 30 on 10 meters, in
the best case, and in practice, to have some tuning range, you might
need 40!
> I am shooting for using the air coil on 20 meters and up and using the
> toroid for the lower bands. The 160 and 80 meter loss is not too bad with
> the toroid.
Can you fit one toroid per band? That should give good performance, if
done right. But it might not be much more compact than using a tapped
air coil. And certainly it's more expensive and heavier!
> But I still need to find a way to get a lower minimum plate tune
> capacitance even for 20 meters.
Good luck with that one! Ask Harry Potter for help.
Manfred.
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