[AMRadio] Home-brewing construction considerations

geoff ars.w5omr at gmail.com
Thu Jan 17 23:56:19 EST 2008

D. Chester wrote:

> I don't think it makes much difference, particularly on 160-40m and I
> have seen it done both ways.  

Having these two 250TH rf decks, side-by-side, allows me to see how
things are done, both ways.  The RF deck in what has been affectionately
dubbed the 'Titanic' by K5SWK (I was a Johnny novice, experimenting and
playing, and it was down more than it was up) has a metal bar that
screws into the inside HDVL sockets, and the top of the RF choke screws
into the center of that bar.  B+ from the RF choke is applied to each of
the two coils evenly.

The new (to me) 250TH RF deck, built by Mike Spaan/W5IMF has a
home-brewed HDVL spaced jack-bar, but the coil has only three banana
plugs on the bottom side, and the inside of the two coils are tied together.

> Actually, tying the two halves of the coil together in the middle
> would introduce less stray capacitance and inductance to the coil.  I
> think the reason for the split coil and separate connectors for each
> half is to give you the option of  metering each tube separately with
> the series fed circuit in which the coil is hot with HV and carries
> the DC plate current to the final.

It would seem, at first glance, that dual plate meters would certainly
help in neutralizing the final.

> As for the vacuum capacitor, the tank should still remain balanced if
> care is taken with the layout.  I suspect it would be more sensitive
> to unbalance due to stray capacitance and differences in lead length,
> than with all the capacitance in a split capacitor.

The plug-in coil for this rig, with 3-pins on the bottom side of the
jack-bar, also has a 50uuf vacuum capacitor across the entire coil,
presumably for  resonating on 75m.   Each side is identical in construction.

I'll take pictures, and put 'em somewhere.  It's sometimes difficult to
grasp the correct/proper words for description purposes.

As an aside, I wired the thing up, and had it producing RF this
evening.   Even plate modulated the final, and made a good contact with
Steve/WA1QIX there in Massachusetts.   HE said the thing sounded fine,
and audio was good, clean and clear.  What concerned me was that the
grid meter was dropping on voice peaks.  Now, to be fair I was driving
that final with a rice-box (TS-450S) at around 30w.  It drove the grids
to around 125mA.  I never had a problem with the Viking II driving the
other rig, using the same modulator.  I also didn't hear the modulation
transformer talk-back when I'm on the original rig, like I heard this
evening.   Was only running 200w.

I'll do some more checking, tomorrow.

Good night Mr and Mrs Ship and all the Americans at sea.
from radio station KA5THB... no wait.. wrong call ;-)

73 de W5OMR

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