EMRFD Message Archive 4760

Message Date From Subject
4760 2010-06-08 11:55:34 kilocycles Oscillator-multiplier tube rigs - Was: Re: AGC audio chain
Tim and Allison, thanks for your comments on this topic. Quoting Allison:

"For the lower bands usually the first stage after the osc was a buffer and for bands 40M and higher it was a multiplier followed by a driver to a final."

That would be the situation I'm interested in. Take 15m operation as an example. The buffer/multiplier stage is the key. Feed 7 MHz into the VFO input jack, and a tuned circuit at 21 MHz selects the 3rd harmonic and passes it on to the subsequent stages. Is that oversimplifying it? The key question is would that tuned circuit at 21 MHz select and pass a 21MHz signal injected at the VFO jack? If the grid circuit of the buffer/multiplier is in fact a tuned circuit at 7 MHz and the plate circuit is tuned to 14, 21, or 28 MHz, that would not appear to be true.

The Hallicrafters HT-40 and Eico 720 are simpler designs. They look to be "MOPA-with-a-buffer" transmitters; no driver stage. In the HT-40, the Oscillator/buffer is a triode-pentode, and the pentode grid is resistive. The frequency multiplication appears to be a "Drive" capacitor in parallel with bandswitch-selected coils at the PA tube grid.

In Tim's case of the Eico 720, the oscillator is an electron-coupled pentode oscillator. The 6AQ5 buffer's grid is resistive and the plate is likewise tuned by a Drive capacitor and bandswitched coils at the input to the PA. The 720 also has a screen grid pot to the 6AQ5 that contols drive to the PA.

It seems to me that these two designs should pass the synthesizer (or heterodyne VFO) output that is on the operating frequency, depending on the range of the Drive capacitor.

Ted, KX4OM
4761 2010-06-08 12:55:15 ajparent1 Oscillator-multiplier tube rigs - Was: Re: AGC audio chain
HI Ted,

Looked up the HT40 and that would be a winner with
4762 2010-06-08 13:28:40 Bob Macklin Re: Oscillator-multiplier tube rigs - Was: Re: AGC audio chain
Many rigs of that period used a RF choke that resonated between 80M and 40M on the input to the buffer/multiplier stage.

Bob Macklin
Seattle, Wa.
"Real Radios Glow In The Dark"
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4763 2010-06-08 14:47:56 kilocycles Oscillator-multiplier tube rigs - Was: Re: AGC audio chain
Hi Allison,
I had an HT-40 that was bought new by my folks back in 1961 that I used on 6 meters with a converter and SP-400. Lots of fun. Out of nostalgia I got one on eBay a few years ago, and I restored it nicely. Then I went all stupid and sold it last year, along with an SX-140 that I also restored. I've got to get another one of those rigs.

Ted, KX4OM

4764 2010-06-08 16:28:42 Nick Kennedy Re: Oscillator-multiplier tube rigs - Was: Re: AGC audio chain
Speaking of HT-40s and multiplication ...

The HT-40 was my first commercial rig so when I stated acquiring a few
boatanchors it was near the top of the list. Now I have one that gets dusted
off every SKN or so.

A couple years ago I thought it would be fun to get it to run on 6-meters, a
band it supposedly covers but I'd never tried. I tried a crystal in the 8.3
MHz range first with no luck. Then I tried a DDS with a pretty good MOSFET
amplifier boosting its output but still no luck. I saw output around 25 MHz
and once on 33 MHz but never got 6 meters out of it, despite plenty of grid
current indicated and trying different final tubes.


Nick, WA5BDU

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4773 2010-06-10 06:03:52 davidpnewkirk Oscillator-multiplier tube rigs - Was: Re: AGC audio chain