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Information on 807 tubes

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jovica

Thu Dec 04 2008, 04:30AM

Registered Member #1790 Joined: Fri Oct 31 2008, 10:10AM
Location: Sydney, Australia
Posts: 40

Hi every one,

Well the other day i was searching through ebay for tubes, and I saw this tube which was $10us (well actualy 2 tubes

)

It only had a few minutes left before it ended so I just placed a bid on it and went of to school, later on i checked my ebay and I had won it. I was preaty happy, but now i have a problem, I originaly wanted to build a vttc but I am unsure if this tube is suitable for this kind of application.

Here is a picture of the tube

Arcstarter

Thu Dec 04 2008, 04:43AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

It will work, but only maybe a tiny bit of breakout. With 600 volts absolute max on the plate and around 100ma max current, it is not going to get you much. You could however make something like a flyback driver :P. It would only be like 60 watts but it would get you something.

...

Thu Dec 04 2008, 05:54AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

It will work fine, just keep things low duty cycle

I used something like that (806) as a direct replacement for a 811a, and as long as you didn't let it run long enough to heat the plate too much it worked great making few inches of spark off mot.

Proud Mary

Thu Dec 04 2008, 09:26AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

807 is an extremely robust valve which was the mainstay of medium power MF and HF transmitter power amplifiers from WW2 until the 1960s.

It can be run 24/7 with 600V on the anode, but in "Intermittent Commercial and Amateur Service" (ICAS) can run at 750V.

It can be driven at up to 60MHz on full power, and at 125MHz on reduced power. See:

A tip: be sure to put small anti-parasitic chokes of different values up close to the grid pin, and anode top-cap, to reduce the likelihood of parasitic (usually VHF) oscillation, which can radiate wildly, and even destroy this otherwise very tough valve.

Steve Conner

Thu Dec 04 2008, 11:51AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Indeed. The voltage ratings on tubes are like the current ratings on IGBTs: even the absolute max is more of a guideline than a deadline.

For instance, say the 807 is rated at 600V supply in a high-level modulated AM transmitter. Because of the flywheel effect of the tank circuit, the peak voltage will be around 1kV. Then at 100% modulation, the modulator will be wiggling the supply voltage between 0 and 1200V, and the peak plate voltage will scale accordingly.

So the "600V" rated tube happily deals with peak plate voltages of 2kV+ all day long, not to mention putting out 4x its "rated power" on modulation peaks.

This is why connecting an 807 to a MOT isn't as crazy as it seems. (Whether that cheesy Chinese tube has the same balls as the original tube that won WW2 is another matter, though.)

PS, the 807 is a beam tetrode, so be careful what you do with the screen grid. It's basically a 6L6GC with a top cap and parasitics.

Proud Mary

Thu Dec 04 2008, 01:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

More 807 tips:

807 has no objection to parallel operation, and WW2 transmitters like Marconi's Oceanspan MF/HF tx had four of them in parallel in its output power amplifier.

Parasitic interaction is the main worry in parallel operation, but DIY anti-parasitic chokes will bring peace and stability.

A typical grid choke might consist of 20 or more turns enamelled copper wire round a 100R 2W carbon resistor, with a similar arrangement using heavier wire - say 25 turns - and a higher wattage carbon resistor at the anode. Make the number of turns on each choke a bit different to avoid possible TATG (Tuned-Anode-Tuned-Grid) type VHF parasitics, but avoid obvious multiples like, for example, 20 turns and 40 turns, lest you make, by mistake, an unwanted harmonic generator. Layout is always important in building successful valve circuits of this kind, and a certain amount of experiment may well be needed!

As Steve says, getting the screen-grid (i.e. g2) current right is essential for valves like this if they are to operate at their best, and not to die off in hasty disappointment,

Arcstarter

Thu Dec 04 2008, 06:20PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Wow, i would not have thought it could withstand all that. I had a tube with similar ratings and i tried it on 1kv and it died, even though i tuned it pretty well. That is what i was going by, but this tube is much bigger than what i used. Either way, with 600volts in even, it is still fun, and you could get maybe an inch. But, if you are like me, you would crank that baby right up.

Proud Mary

Fri Dec 05 2008, 08:18AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A method of limiting the current in an indirectly heated valve, which is seldom used except in electrometer circuits, is to reduce the heater voltage somewhat, which reduces electron emission, and so increases anode resistance.

This technique is one of those NOT recommended by manufacturers, as it can, over time, damage the oxide coating on the cathode, but it seems to me it should allow the use of a higher anode voltage without drawing so much current as to destroy the anode.

This might allow the use of an HT supply from an MOT for 50 or 100 hours or so before the tube fails. Try it with caution!

Steve Conner

Fri Dec 05 2008, 10:30AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I wouldn't try that on an indirectly heated tube, it's likely to destroy the cathode. Lowering the screen grid voltage seems a lot safer.

For example, I have an old transformer from some piece of tube equipment that would be good for a small VTTC like this. It's 300-0-300V, and with one end grounded it would become 0-300-600V. The 600V could be level shifted ala microwave oven to get a 1200V peak plate supply, and the 300 or 600V taps could be used for the screen supply, depending on what kind of tube you have, as some need more screen voltage than others.

The 300V could be either level shifted or just plain AC, it hardly matters since every electrode in a tube comes with a free diode.

Harry, you're not an old radio ham by any chance? G3 or G4

Proud Mary

Fri Dec 05 2008, 11:31AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

Harry, you're not an old radio ham by any chance? G3 or G4

I knew I'd be rumbled as an Altzheimer's candidate with a retirement hobby sooner or later, Steve, a dinosaur left over from the age of megacycles and glowing glass! Shops stacked to the roof with war surplus wireless sets and radar electronics at giveaway prices! Gas light and cobblestone streets! Silver Surfer and Golden Oldie!

PS W/T still 25 w.p.m.

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