> Hi gang--
> 
> This weekend I finally finished the prototype 6AG7/6L6 rig I've spoken of
> here in recent weeks.  It generated power right off the bat--with the
> downside that the pi net will not tune or load properly on 40m.  As an 80m
> rig it's killer, but the intent was to create a pi net that would work on
> both 80 and 40 without taps or switching.  I'm pretty sure at this point
> that the tank coil is too big, and the next step is to knock a few turns off
> of it and try it again.

Jeff.... as a loose rule of thumb, it is possible to tune two bands with
one pinet coil, but you need to cut the coil for the higher band with about
100pf of the tuning capacitor.  Then use the rest of the tuning capacitor
to pad it down to 80M from 40M....(it will take about a 250-365 pf cap in
the plate input side to do this with a 40M coil that has a couple of extra
turns compared to normal).  Note that when you do this, the efficiency will
be off a bit, because of the increased capacitance on 80M in the input tank.
That will not cause too much problem on a breadboard set, but might on a
big rig.  It affects the plate impedance of the final (remember there are
optimum values of C1 and L and C2 in a pinet design for any given plate
volatge and plate current).  I don't have the correct formulas in memory,
but they are in any late model handbook.  For a practical glowbug, it 
really does not matter that much... just make sure there is enough capacity
in the input and output to handle the tuning with good harmonic suppression.
As a general rule of thumb, on the input side of the pinet use a 365pf if
you have it and on the output use a 3 or 5 section 365pf cap.  That way,
you should always have sufficient range on 160/80/40/maybe 30 meters.
 
> But I stuck to 80m and did something interesting: I rummaged through my tube
> collection and drug out every 7S power tube I could find (and hey, anybody
> know what the difference between a 7AC and 7S base is?  I see no difference
> in the '65 Handbook illos...) and tried them all in the rig.  They all
> worked...and there were some surprises.

Yes, isn't that a most wonderful and marvelous finding?  There is a whole
string of similarly based power tubes that you can basically just plug and
play, within certain limits.

> The rig was powered by a junkbox homebrew ps I got at a hamfest in a
> $50-takes-it-all kind of deal.  It's got a key-down voltage of 350 or so,
> but the regulation is terrible (key up is about 475) so I didn't put the rig
> on the air for fear of chirping, tempted tho I was.  I will try other and
> better power supplies I have on hand once I can get some Jones plug cables
> together for them.

For general use, I would tend to stick to 450 volts and less and prefer
a 350 volt power supply for tube longevity, unless a good cooling fan is
about.  That will still give plenty of play power.  Some of the lesser
tubes in the series are hard put to push 450 volts without glowing mighty
red on their plates!

> 6550       63        14W
> 
> The 6550, by the way, is a BEAUTIFUL tube.

Yes it is the King 6L6 in its highest and mightiest configuration!

Tuffy 6L6 will run forever...... but don't let the audio folks know you
have that 6550, they will come a'runnin' ta snap 'er up!

> So...the circuit works, with the caution that it won't load on 40m yet.
> I'll work on that and post new specs here for the coil when I get it licked.
> 
> --73--
> --Jeff Duntemann KG7JF

Do keep us posted, Jeff..... that is wat globuggin' be all about!

73/ZUT DE NA4G/Bob UP