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Author Topic: RCA 813 150 watt oscillator  (Read 13570 times)
G3RZP
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« Reply #15 on: November 09, 2011, 03:03:47 PM »

One of the IERE Confernces in the 80s had a paper from people at STC about drive levels for filter crystals: the inferences are suggestive. At +10 dBm or so, with crystals inthe glass version of HC/6U, they could see a blue glow, and the metallisation gradually left the crystals. Even at 100 mW in the crystal, we're talkiing of about 40dB gain in the stage. To quote Domine Sampson, "Prodigious!"

73

Peter G3RZP
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W8JI
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« Reply #16 on: November 09, 2011, 05:12:20 PM »

Dave,

I'm not trying to kill anyone's spirit, but that circuit is awful.

Adding a little tetrode oscillator is minimal, as is cathode keying.

Peter,

It is worse than the resonant frequency..... look at the RF path back to the filament transformer. Now what is the resonant frequency?


I've had some success with a crystal from screen back to control in a high power stage. This circuit though looks like it needs nichrome shields to block photon bombardment of Rp.

73 Tom



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G3UUR
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« Reply #17 on: November 10, 2011, 02:10:43 AM »

Tom,

I don't know what RCA's original intention was. The article in "Ham Tips" is not really an application note and it may have only been intended to give amateurs a hint of what could be done with a single 813. Whatever their motive for publishing it, a number of amateurs have built transmitters based on this design in recent years and are using them on the air. They don't sound that bad and I haven't heard of anyone electrocuting themselves yet. The thing that was surprising to me was that some of them are using miniature crystals, and that's why I set up a Pierce power oscillator to see how much stick these little crystals can handle.

Personally, I would go for a 6AG7 crystal oscillator because that would easily drive an 813, produce a clean signal, and I have lots of them. However, there are some amateurs who get great pleasure out of using primitive equipment and my view is each to his own.

Peter, I imagine the glow that the STC engineers observed was a plasma generated in the gas that they back-filled the case with. Undoubtedly the removal of the electrode material was due to plasma ashing. I would estimate that the crystals I tested were operating for 20 to 30 seconds at in excess of 50-100mW because the room temperature ESR values were 5 to 10 ohms. I wouldn't expect the ESR to be that low at elevated temps. 700Hz shift down followed by up to 7kHz shift higher was typical at 100mA crystal current. You can tell when you're overtaxing the crystal because it begins to self-heat and whoop as you key. If it's not whooping, then it's probably okay for typical amateur service.

Is greater than 40dB unreasonable for a tube with positive feedback? I wouldn't have thought so, but then I haven't analysed it in detail. I'll leave that to you professional guys

Dave.     
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G3RZP
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« Reply #18 on: November 10, 2011, 03:38:43 AM »

It would have been plasma, although the crystal housing was evacuated - but not, I suspect to the amount that a vacuum tube is. Not much gas, but enough!
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W8JI
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« Reply #19 on: November 10, 2011, 05:16:14 AM »

I can see I'm going to have to build an 813. After I null my TX signals out of the RX antenna paths.
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G3UUR
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« Reply #20 on: November 10, 2011, 11:24:14 AM »

Peter,

A partial vacuum of around 0.01 Torr would have been ideal for setting up a plasma in air, but what puzzles me is that the electric field across the crystal with 10mW dissipation was large enough to initiate a plasma discharge. In modern crystals where the active volume (trapped energy) is between the electrodes and the strain falls off towards the edges, the field across the unplated edges is miniscule. The field across the electrodes has a long path across the unplated crystal to the edges. Either they miscalculated the dissipation or there was something else going on there!

Tom, there's obviously a need for a "safe version" of the 813 150-watt single tube transmitter.

Dave.
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G3RZP
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« Reply #21 on: November 10, 2011, 11:46:47 AM »

They were talking about crystals in a filter. There may have been a big field between physically adjacent crystals of course. But I only report what has stuck in my memory for over 30 years, although I can't remember their names. Somewhere I probably have the conference proceedings. I have a feeling it was on frequency control and synthesis, so I was the Chairman, but it's a long time ago.

I can remember that as Chairman, at the conference dinner, there was no stinting of the free wine, unlike the rest of those present!
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K1ZJH
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« Reply #22 on: November 12, 2011, 07:07:13 AM »

Shunt feed the tank, to keep the B+ off the tank components. Two RFCs and a few RF rated coupling caps.
Add a 6AG7 buffer stage. No reason not to modifiy the design to make it safer and more useful.

Pete
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G3RZP
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« Reply #23 on: November 13, 2011, 05:11:48 AM »

Use an 807 oscillator and an 807 as buffer multiplier and add an 813. It's in the 1949 ARRL handbook!
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N2EY
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« Reply #24 on: November 14, 2011, 03:26:10 AM »

Shunt feed the tank, to keep the B+ off the tank components. Two RFCs and a few RF rated coupling caps.
Add a 6AG7 buffer stage. No reason not to modifiy the design to make it safer and more useful.

The problem is that what you've done is to turn what was a "simple" one-stage oscillator transmitter into a MOPA.

I am sure that it is possible to get the 813 oscillator tx to work. My point is that it takes a bit of doing:

1) Suitable keying relay
2) Suitable crystals
3) Proper insulation on the output circuit
4) Careful adjustment and montioring of xtal current
5) Suitable 1000-1500 volt power supply

and the result is a transmitter that isn't so simple any more, and which requires a crystal on each output frequency. And it puts out maybe 100 watts RF on a good day.

It seems to me that there are much simpler and easier ways to build a 100 watt output crystal controlled HF CW transmitter using tubes.

That's all I was trying to say.

73 de Jim, N2EY
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W8JI
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« Reply #25 on: November 14, 2011, 05:07:11 AM »

I think it is quite possible to build a safe stable 813 single tube PA. The circuit shown is just not very good, even for the technology of the period.
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N2EY
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« Reply #26 on: November 14, 2011, 06:16:32 AM »

I think it is quite possible to build a safe stable 813 single tube PA. The circuit shown is just not very good, even for the technology of the period.

PA? Did you mean "power oscillator"?

I agree that there are/were much better ways to do the job, even in 1938. I strongly suspect the thing was tossed together by the folks at RCA as a way to show off what the 813 could do, without a lot of testing nor asking "what could possibly go wrong?"

There's also the effect of "fashion". For example, series feed was common in those days, even though it meant coils and capacitor rotors hot with DC, because it saved two parts and some folks just didn't like RFCs. A little chirp or click was no big deal in many circles, too. Safety was looked at differently then, too.

I wonder if the article was written before or after Ross Hull died?

73 de Jim, N2EY
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W8JI
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« Reply #27 on: November 14, 2011, 07:15:14 AM »

I meant PO, not PA.

Who was Ross Hull?
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K1ZJH
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Posts: 1087




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« Reply #28 on: November 14, 2011, 07:34:46 AM »

Shunt feed the tank, to keep the B+ off the tank components. Two RFCs and a few RF rated coupling caps.
Add a 6AG7 buffer stage. No reason not to modifiy the design to make it safer and more useful.

The problem is that what you've done is to turn what was a "simple" one-stage oscillator transmitter into a MOPA.


 73 de Jim, N2EY


Why is that a problem?   Grin Grin
It is still a relatively simple circuit.
And you have two purty tubes glowing.

Pete 
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N2EY
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« Reply #29 on: November 14, 2011, 07:43:59 AM »

I meant PO, not PA.

OK no problem.

Who was Ross Hull?

A ham from Australia who spent some years at ARRL Hq in the 1930s. By the rules of the time he couldn't get a US license, so he focused on technical stuff. Lots of good articles by him or to which he contributed.

He was electrocuted one night while working on an experimental TV set. Was using a conventional 3000 volts supply for the CRT, haywired together. There were people in the house who heard an odd noise and went immediately to his aid but it was too late.

His death was a great loss and touched off the "Switch to Safety" campaign at ARRL.

73 de Jim, N2EY
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