[Amps] Tank circuit Q

[Gary Schafer]

Hi Tom,

Thanks for your reply. Yes it does get confusing when you try to understand
the full meaning of what really 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?

As long as I am getting into this thing I thought I might as well understand
more of how all this works. I know that it doesn't need to be too precise to
make the amp work though.

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!

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.
My air wound coil made of 1/4" copper tubing does get warm.

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.

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.

It looks like they may have had problems with minimum capacitance from the
start in this thing and stuck those series caps in there.

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. But I still need to find a way to get a lower minimum plate tune
capacitance even for 20 meters.

73
Gary  K4FMX


> -----Original Message-----
> From: Tom W8JI [mailto:w8ji at w8ji.com]
> Sent: Monday, April 16, 2007 6:27 AM
> To: garyschafer at comcast.net; amps at contesting.com
> Subject: Re: [Amps] Tank circuit Q
> 
> I think what confuses people is that commonly used amateur
> radio formulas for tank Q are really just rough shortcuts or
> approximations for describing real world systems. The saving
> grace is the system, unless Q is below a minimum value,
> doesn't change nearly as much as many might think when
> operating Q is not the magical numbers (like the popular "Q
> must be 12") that we are so overly fond of zeroing in on.
> 
> The formulas are all approximations and should not be taken
> as a description of what precisely happens in the network or
> in the real world.
> 
> I wouldn't spend too much time worrying about making an
> approximation fit a non-critical goal precisely!
> 
> The capacitor current, assuming a sine wave from a purely
> resistive source driving the tank, would be simply the RMS
> voltage at the tank input over the capacitor's impedance.
> This is true regardless of inductor unloaded Q.  Current in
> the inductor is slightly different  than that in the real
> world because there are other shunt impedances at work in
> the system, and the waveform is not a pure sinewave. But it
> is close enough.
> 
> As for capacitor current, be sure to read the manufacturer's
> detailed application guidelines carefully. There are many
> people who apply a base number that is designed to be used
> in a series of approximations directly as a limit, and who
> wrongly assume any current beyond the number that was never
> intended to be an absolute limit results in some component
> life or operating catastrophy.
> 
> The two parallel capacitors in series with your tank
> variable C clearly need to go. They are a sign of bad
> engineering, just as the shorted taps on the toroid were.
> 
> 73 Tom
>