[AMRadio] 1930's Old Buzzard Open Rack Transmitter Finally


Mike Dorworth, K4XM k4xm at arczip.com
Tue Feb 21 22:16:17 EST 2006


811/812 are 6.3 volt filament, the T-55 is 7.5 volts..otherwise similar to
the 811 NOT A.. Mike
----- Original Message -----
From: "Brett gazdzinski" <brett.gazdzinski at verizonbusiness.com>
To: "'Discussion of AM Radio'" <amradio at mailman.qth.net>
Sent: Tuesday, February 21, 2006 4:08 PM
Subject: RE: [AMRadio] 1930's Old Buzzard Open Rack Transmitter Finally
SeestheAirwaves! (Long)


> What is the difference between a T55 and an 812/812A?
> I know a V70D is good for more power, but has different filament
voltages...
>
> I think I have some T55 tubes, never knew they would do as 812
substitutes.
> I thought they looked lower power (smaller plates).
>
> The amount of turns on the link coil impacts what you can load up to a
LOT.
> I made my own link for the 812a rig, and had to try various link turns
> before
> it loaded up right.  I have the series cap also, and that does change how
> the
> rig loads up, but does not seem to change the efficiency or anything else.
>
> At 1500 or more volts, I run the 811a's at zero bias, the 812A's run 340ma
> cathode current, 70 ma grid drive, 275 watts out with no color on the
> plates.
> The 812A's get fixed and LOADS of grid leak bias...
>
> I have tried 811A's in the RF deck, they seemed to work just the same as
> the 812A,s.....
>
> Good deal on getting the old rig running!
>
> Brett
> N2DTS
>
>
>
> > -----Original Message-----
> > From: amradio-bounces at mailman.qth.net
> > [mailto:amradio-bounces at mailman.qth.net] On Behalf Of ne1s
> > Sent: Tuesday, February 21, 2006 3:27 PM
> > To: AM Radio Reflector; glowbugs at piobaire.mines.uidaho.edu;
> > antiquewirelessassociation at yahoogroups.com
> > Subject: [AMRadio] 1930's Old Buzzard Open Rack Transmitter
> > Finally Sees theAirwaves! (Long)
> >
> > Hello,
> >
> > Some of you may remember that a few weeks ago I posted a
> > question regarding
> > optimizing the layout for a push-pull triode HF final deck,
> > given that the
> > deck in question was already partially built (by someone
> > else). This was a
> > component of a larger project involving
> > completion/restoration of a 1930's
> > vintage homebrew open-relay rack AM transmitter I had
> > initially acquired
> > some 15-20 years ago, and has been sitting patiently in my
> > cellar shack
> > since this time waiting for me to ge a round tuit. Well, the
> > elusive and
> > highly-coveted tuit finally arrived about a year ago, when I started
> > drilling and blasting for the modulator deck. I had to take a
> > rather long
> > break from itarting last July, when I started a major effort
> > to bring a
> > majorly-hacked Johnson Valiant back from the dead for a
> > friend which led to
> > the birth of FrankenValiant. But that's another story for
> > another time. With
> > FrankenValiant finally completed, I finally got back to the
> > project at hand
> > a month ago, and to make a much longer story less so, the
> > transmitter made
> > its maiden voyage on the airwaves last Sunday evening, when I
> > used it during
> > the final 2 hours of the AWA AM QSO party, on 75M.
> >
> > The final presently uses a pair of 812s, which are modulated
> > by a pair of
> > 805s. When the debug/shakedown stage is over I'll switch over
> > to T-55s in
> > the final (harder to find, and more expensive, than 812s).
> > There are still a
> > few problems, which I hope to rectify next weekend:
> >
> > (1) My bias supply for the modulators and finals, built onto
> > the modulator
> > deck, failed. I had used a period resistor from the junkbox - it was
> > working, now it's opened up. Replacing it will involve removing the
> > modulation transformer from the deck (to make the weight more
> > manageable,
> > removing the deck from the rack, replacing the resistor, and
> > assembly in
> > reverse order of disassembly. So my on-th-air use was with
> > grid-leak bias
> > only on the finals, and zero-bias on the modulators (the 805s
> > were still
> > operating within their ratings, though).
> >
> > (2) The speech amp has an intermittent noise which modulates
> > the finals
> > quite "nicely" when it is present. No surprise: all the caps
> > in the deck are
> > original wax-paper. I did check the DC blocking caps for
> > leakage beforehand
> > (by measuring the DC grid voltages), but guess I shouldn't
> > have trusted
> > them.  A little benchwork with the 'scope should trace down
> > the faulty
> > component. It's in the first stage, 'cuz its intensity varies
> > with the mic.
> > gain control. I actually saw it on the scope when I was
> > checking out the
> > speech amp initially, and made a conscious decision to deal
> > with it later.
> >
> > (3) The link output is directly feeding coax which directly feeds the
> > antenna tuner, and I can only load the finals up to ~150 mA
> > for about 160W
> > output. Should be able to load them to nearly twice that. I
> > need to install
> > a series breadslicer to resonate the link.
> >
> > (4) I need to improve the grid drive on 10M.
> >
> > (5) I want to use a spare set of relay contacts to disable
> > the speech amp
> > (by removing the B+) on standby (receive).
> >
> > And, finally (pun intended), I would like to thank all those
> > that answered
> > my question concerning the layout of the final PA deck. If
> > you'll recall,
> > What I had to work with was the two tube sockets mounted
> > parallel to the
> > front panel, with the output tank 2-section variable
> > capacitor mounted
> > perpendicular to the panel between them, and the final tank
> > coil jack bar
> > parallel to the panel, near the back of the chassis. The
> > plate tank is all
> > above-chassis. There was no grid tank, and I was puzzling
> > over the best way
> > to add it. I ended up putting the grid 2-section variable cap
> > centered below
> > the chassis with its shaft protruding through the front
> > panel. I mounted the
> > 5-pin socked for the grid coil a few inches behind that, also
> > below the
> > chassis, so that its axis was also perpendicular to the panel
> > (and therefore
> > also to the plate tank coil). I had to be careful where I put
> > these, not
> > only for the obvious electrical design considerations, but
> > also because the
> > cap rotor (when fully open) and coils (when plugged in) extend below
> > chassis, and into the area of the exciter deck. But the grid
> > coil is also
> > perpendicular to all the coils in the exciter deck. It's not
> > a perfectly
> > symmetrical layout, but it's pretty close, and I was able to
> > neutralize it
> > using the 10M coils. Engineering is full of compromises, and
> > if You've made
> > them correctly AND you're lucky it works when you're done. I
> > seem to have
> > been lucky, at least...
> >
> > 73,
> >  -Larry/NE1S
> > ______________________________________________________________
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> >
>
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