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

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Tue Feb 21 16:08:05 EST 2006

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
it loaded up right.  I have the series cap also, and that does change how
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
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!



> -----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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