I am loading the amp for maximum output with either tank coil installed.
Plate voltage is right at 3000 volts at 700 ma. Plate current may have been
slightly different in one case but not much if any.
I know what you mean about plate capacitance with 813s as I have an amp with
a pair that I built about 40 years ago. It is not set up to cover 10 meters!
One of these days I am going to try to get it to work on 10. That is one of
those things that gets put on the bench every few years and a few
improvements made. I have several of those type of projects around here. :>)
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Manfred Mornhinweg
> Sent: Tuesday, April 17, 2007 2:54 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Tank circuit Q
>
> 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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