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813 Class C question




 
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W4RFM
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« on: November 28, 2019, 09:37:33 PM »

I know that 813's usually run with 1700 to 2K or more on them, but I want to run a single tube Class C in a transmitter I am going to construct.  Any reason why I can't run one at say 1250 or 1500 vdc, pushed by the ol' classic 6AG7 and 6L6 ahead of it?  Kinda sorts ART 13 ish style.

I also have a bunch of new and used 4-125's, that I could use.  Opinions Please.

Thanks again.
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BOB / W4RFM  "I have looked far and wide, (I also checked near and narrow)"
Steve - K4HX
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« Reply #1 on: November 28, 2019, 10:11:13 PM »

Seems you cold. Found this spec sheet, with conditions for 1250 volts.


* 813_rf_C_telephony_specs.png (1067.57 KB, 1412x1004 - viewed 71 times.)
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K8DI
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« Reply #2 on: November 29, 2019, 07:30:38 AM »

https://frank.pocnet.net/sheets/049/8/813.pdf
https://frank.pocnet.net/sheets/079/8/813.pdf

Frank.pocnet.net is a fantastic resource for tube data!

Ed
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« Reply #3 on: November 29, 2019, 08:36:39 AM »

hi Bob .... the 813 with a 50 Watt filament is not as emission limited as a 4-125 with a 37 Watt filament .... the 4-125 is a higher impedance tube working well at 2.5 to 3 kV .... putting 2 in parallel will cure a lot of ills and could take to 600W out if you wanted to push them .... remember the Collins 30K something with its single 4-125 and 300W carrier and no forced air .... mind the modulation impedance  tho .... very high at 25kOhms .... tough to find a replacement
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W4RFM
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« Reply #4 on: November 29, 2019, 03:45:22 PM »

Thank everyone, I think i will do the 813 on this one, and maybe build a bigger one later and used a pair of 4-125's.
By the way BFS, I bought that Who album at an Army PX  in Germany, Loved it.

Bob
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« Reply #5 on: November 29, 2019, 07:56:31 PM »

Bob,

If you want a good reference single 813 design, look at the Chambers rig.  It appeared in QST and many of the ARRL Handbooks.  I am looking at a 55 ARRL handbook and its titled, "A 500W Multiband VFO Transmitter".   Vernon Chambers was the designer.  Back in the day, 500W referred to input power and not output power.  Typically you can easily get 300W out of the rig.  Bob - W2ICQ has built maybe 3 of these over the years and uses his from time to time.  Dave - KA2J uses his occasionally as well.  Joe - WA2PJP built one over the last year or so.  The typically modulator used are a pair of  811A's.   

73,
Joe-W3GMS 
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W4RFM
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« Reply #6 on: November 29, 2019, 09:20:20 PM »

Hey Joe,
I found it in my 1956 ARRL book, I will scrutinize this, looks good. I plan to build one for 80 meters only with a selector for 3880 or 3885  KC.
Thanks Joe, good to hear from you again.
Bob W4RFM
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« Reply #7 on: November 30, 2019, 03:05:19 PM »

Here is a suggested 813 final modified from my 175W SG transmitter:

Phil - AC0OB

* AC0OB Plate Modulated 813 Stage for Bob.pdf (117.39 KB - downloaded 61 times.)
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« Reply #8 on: November 30, 2019, 06:19:49 PM »

Hi Bob,

If you keep the 813 HV down at your proposed level, you could have 811A modulators utilizing a common HV PS. If you can find mod iron with a fairly small step-down ratio you'll have a rig that will run cool and have lots of audio headroom. It should run forever.


Don
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« Reply #9 on: December 03, 2019, 09:19:41 PM »

At 1250 VDC plate u need -160 bias in AM Plate modulation, class "C" mode. I'd use a fixed screen supply with a small 10 hy series choke for complete modulation. I like that better than no fixed bias and using a clamper tube. If you use a series resistor for screen voltage and no fixed bias, tube wastes a lot of heat and never really cuts off if keying in CW.

The bias can be derived from a VR105 plus a grid leak resistor of about 4.5 K @ 1 Watt to make -160 volts. The fixed bias is more than 2 X cutoff and resting current with no excitation should be zero. Grid current runs around 12 - 13 ma. with a drive of about 3 watts. Also fixed bias of 2 x cutoff reduces the back wave when keying.


Seth KC2WE

Note: Been doing this since 1959 L o L
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Seth Taylor
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« Reply #10 on: December 04, 2019, 04:40:39 PM »

Thanks Seth,
Can I also achieve similar results with a 4-125A.  You see, I have plenty of those and a filament transformer for one.  Makes a little more compact rig in the rack as well.  Any help is appreciated. Please check my POWER SUPPLY QUESTION near this for a print of the final I have been planning.
Thanks.
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« Reply #11 on: December 04, 2019, 07:20:56 PM »

Thanks Seth,
Can I also achieve similar results with a 4-125A.  You see, I have plenty of those and a filament transformer for one.  Makes a little more compact rig in the rack as well.  Any help is appreciated. Please check my POWER SUPPLY QUESTION near this for a print of the final I have been planning.
Thanks.

see Page 1 for a 4-125A FInal circuit

Phil - AC0OB

* AC0OB Plate Modulated 813 Stage for Bob.pdf (159.59 KB - downloaded 34 times.)
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« Reply #12 on: December 04, 2019, 08:51:19 PM »

Hmmm, they look amazing similar. Smiley Gonna study this over night. Would you approve of an audio choke in the screen supply instead of a dropping resistor? I have a separate supply already built for the bias, screen and plate supply large enough to power the 2x 811a's (modulators) and the 4-125A.

Thanks Phil
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« Reply #13 on: December 04, 2019, 09:09:32 PM »

Hmmm, they look amazing similar. Smiley Gonna study this over night. Would you approve of an audio choke in the screen supply instead of a dropping resistor? I have a separate supply already built for the bias, screen and plate supply large enough to power the 2x 811a's (modulators) and the 4-125A.

Thanks Phil

Vsg and Vcg were the only changes.

Phil
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