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Suggestions for modulator/method for 813 Rig




 
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Author Topic: Suggestions for modulator/method for 813 Rig  (Read 448 times)
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KU8L
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« on: November 02, 2017, 02:24:55 PM »

GA List:

Recently, I acquired an exquisite example of a HB transmitter from the mid 50's.  It is a copy of the QST 6-Band, Three Control 500W (input) CW Transmitter article by W1JEQ.   Basically a built-in VFO with 6146 driving an 813 Final.  THe build is one of the best I have seen in 50 years

THe original article mentions using a pair of 811a's in Class B as a plate modulator, or Simple Cathode modulation.   

Essentially this is like my ART-13.

Looking for ideas for utilizing this for a perked-up AM rig.  Out of the box suggestions welcomed Smiley

Curt
KU8L
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w1vtp
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« Reply #1 on: November 02, 2017, 03:07:36 PM »

GA List:

Recently, I acquired an exquisite example of a HB transmitter from the mid 50's.  It is a copy of the QST 6-Band, Three Control 500W (input) CW Transmitter article by W1JEQ.   Basically a built-in VFO with 6146 driving an 813 Final.  THe build is one of the best I have seen in 50 years

THe original article mentions using a pair of 811a's in Class B as a plate modulator, or Simple Cathode modulation.   

Essentially this is like my ART-13.

Looking for ideas for utilizing this for a perked-up AM rig.  Out of the box suggestions welcomed Smiley

Curt
KU8L


The push pull 811a's is the only way to go - that is unless you want overkill. Depends on what you have for iron.  Bob, W2ICQ Uses this approach with his version of the Vernon Chambers 813 transmitter.

https://www.qrz.com/db/w2icq  Scroll down to the 6th and 7th pictures.

Al
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Patrick J. / KD5OEI
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« Reply #2 on: November 04, 2017, 12:58:50 PM »

I really like those "3 control" kinds of things - the same idea that the MB-40 used. The old articles that started the full range tuning efforts.
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KL7OF
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« Reply #3 on: November 04, 2017, 09:52:15 PM »

a pair of triode connected 813's would give you plenty audio .  the added advantage is that you may use just about any mod transformer that is reasonable for the power level ... 813 as modulators (in my experience) seem to have a lot of latitude as far as  mod xfmr impedance ratios go...  good luck..
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K1JJ
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« Reply #4 on: November 04, 2017, 11:18:49 PM »

Wow, Bob did a beautiful job on that rig!

Yes, a pair of p-p triode-connected 813s would be an excellent tube choice to modulate a single 813 final.  Use a 1:1 ratio mod transformer, though a 1.5:1 or 2:1 step down would be OK too.

I have run a pair of 813s modulated by a pair and this combo was possibly the cleanest plate modulated rig I've ever run. No driver transformers, less phase shift - just the mod transformer with plenty of NFB from modulator tube to early low level stage.   I used a solid state WA1GFZ FET driver for the 813 modulators.  SEE for FET audio driver details:   http://amfone.net/Amforum/index.php?topic=23632.0

A pair of tubes of the same type modulating a single final has always been a choice which gives plenty of audio headroom.  You want overkill so that the modulators are just loafing along which keeps them in the cleanest part of their transfer curves and far away from saturation.

The 813, 4-400A and 4-1000A (tetrodes, pentodes) are a select group of tubes that operate as well as real triodes when connected as triode-connected. Connect the grid and screen together of each tube and drive them p-p. Add a few diodes in the filament CT for bias to adjust the plate idle current. 813s are super rugged and plentiful. They are rated at 125 watts, but I believe the carbon plate versions are really more like 150+ watt tubes and a little more with some air in a chimney.

Check out this sample rig using 813s, a pair modulating a pair:
Under extreme testing conditions with 3KV on the plates, this combo put out 900 watts of carrier and modulated cleanly 140%. Unbelievable tube, the 813. 2200 volts would be a nice level for everyday use.

http://www.amwindow.org/tech/htm/813/813.htm

Tom, K1JJ
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« Reply #5 on: November 04, 2017, 11:47:36 PM »

813s or other screen grid tubes in the "special class B connection", where the screens are what's driven by the audio and no grid bias is used, would eliminate the screen and bias supplies for the mod stage, but not so often are the designs transferred from paper to a chassis. Pair of 6550s may be a bit light, but 4D32s about right - pick some with the same 45W plate dissipation as the 811. The point is that the use of screen grid tubes expands the choices. Just saying, the 811 is perfect but there are not too many other inexpensive Triode choices.

PDF file of this circuit's article and also the 811 modulator article from the same RHB-14 of 1956.
ARRL Articles have suggested reverse connected push pull output transformers to drive this from a 4-8 Ohm audio amp, etc. and 5.5W is wanted at the grids.

Just adding to the thoughtful and fun possibilities.

* special class B RHB-14-1956.pdf (408.23 KB - downloaded 50 times.)
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