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Old 27th September 2009, 10:31 AM   #1
abj1 is offline abj1  Australia
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Default Biasing G3 in beamies

Hi All,

Just wondering whether anyone has done any work with putting a bias on G3 in a beam tetrode. Putting around +60V on the suppressor grid on a transmitting pentode with a separate pin out for G3 does nice things to the curves of the big-ish transmitting true pentodes (eg 814, 828, 4E27 etc) according to the datasheets. Is a similar phenomenon possible (or likely) with the 813 beamie, which does have a separate terminal for the beam forming plates, even though the data sheet I have keeps it at 0V for all applications listed? I'm thinking primarily of 'pentode' connection here.

Having said that, I'm also interested in the possibility of triode mode with 813 and here the beam plates could be at anode potential, or filament potential, or even a small +ve or -ve bias (but close to 0V).

Any experience and/or thoughts?
TIA,
Andrew
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Old 27th September 2009, 09:10 PM   #2
m6tt is offline m6tt  United States
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I have seen this mentioned with the larger sweep tubes, ostensibly to help reduce the many hilariously named oscillations like "snivets". I have also heard of this being done with vari-mu pentodes to make them sharp-cutoff. For what it's worth, it can't hurt to try.
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Old 28th September 2009, 11:26 PM   #3
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Abj1,

I do not have the asked for experience, but have difficulty to imagine why I would apply a high voltage to beam-forming electrodes. They are there after all to limit electrons going to the 'sides' of the emission bundle where they do more harm than good. To now draw some electrons towards them does not make sense to me. But perhaps someone has experience as you ask.
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Old 28th September 2009, 11:54 PM   #4
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Quote:
Originally Posted by abj1 View Post
Just wondering whether anyone has done any work with putting a bias on G3 in a beam tetrode.
Not at all easy to do since the vast majority of these types have the beam formers internally connected to the cathode. The only ones that bring these out to a separate pin that I know of are the 6KD6 and 814. Neither spec sheet recommends connecting the beam formers to anything other than the cathode.

Quote:
Putting around +60V on the suppressor grid on a transmitting pentode with a separate pin out for G3 does nice things to the curves of the big-ish transmitting true pentodes (eg 814, 828, 4E27 etc) according to the datasheets. Is a similar phenomenon possible (or likely) with the 813 beamie, which does have a separate terminal for the beam forming plates, even though the data sheet I have keeps it at 0V for all applications listed? I'm thinking primarily of 'pentode' connection here.
The 813 is not a beam former. It's a "real" pentode especially designed for use with suppressor modulation for AM. You can put a high negative bias on it without causing Barkhausen oscillations to set it up for that. (Most pentodes can not tolerate any negative bias on the suppressor/beam formers since this will cause a negative resistance characteristic at the screen, Useful if you're trying to make a negative resistance oscillator, otherwise, a must avoid.) Types that recommend positive voltages on the suppressor/beam formers (e.g. 802) do so to prevent "snivets" -- Damped Barkhausen oscillations that occur when the plate current cuts off in Class AB, B, or C. Unless that's a problem, then always connect to the cathode. There are other types (807) that could benefit from a positve bias, but it's not possible. In that case, you just have to include screen stoppers.

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Having said that, I'm also interested in the possibility of triode mode with 813 and here the beam plates could be at anode potential, or filament potential, or even a small +ve or -ve bias (but close to 0V).
That's long been the question when trioding: what to do with the suppressor -- connect to the plate or cathode? Lots of arguements for doing it either way.
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Old 29th September 2009, 12:01 AM   #5
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Originally Posted by Johan Potgieter View Post
I do not have the asked for experience, but have difficulty to imagine why I would apply a high voltage to beam-forming electrodes. They are there after all to limit electrons going to the 'sides' of the emission bundle where they do more harm than good. To now draw some electrons towards them does not make sense to me. But perhaps someone has experience as you ask.
It's never a "high voltage", usually no more than 20 -- 30V, significantly less than either the screen or plate voltages. It doesn't take much to break up that Barkhausen negative screen resistance that can cause instability at plate current cutoff. It's not high enough to prevent the suppressor/beam formers from doing what they're supposed to do: prevent that secondary emission back current that kinks up the plate characteristic.
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Old 29th September 2009, 11:02 AM   #6
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Originally Posted by Miles Prower View Post

The 813 is not a beam former. It's a "real" pentode especially designed for use with suppressor modulation for AM.
So how is it that you define 'real pentode'? Last I looked it required a third grid, aka supressor. The 813 certainly has no such grid structure; g1, g2 and the beam plate.
cheers,
Douglas
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Old 30th September 2009, 02:19 AM   #7
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The 813 certainly looks like a beam tetrode, according to RCA http://r-type.org/addtext/add064.htm
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Old 30th September 2009, 02:31 AM   #8
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Originally Posted by Bandersnatch View Post
So how is it that you define 'real pentode'? Last I looked it required a third grid, aka supressor. The 813 certainly has no such grid structure; g1, g2 and the beam plate.
cheers,
Douglas
I don't have any 813s here to break open and check. If what you're saying is true, then the ARRL Handbook has been wrong for many years now, because that is what they've been saying for most of the 1960s issues.
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Old 30th September 2009, 03:01 AM   #9
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I don't have any 813s here to break open and check.
No need to break any glass. The RCA data sheet calls the 813 a "Beam Power Tube"

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ly ones that bring these out to a separate pin that I know of are the 6KD6 and 814
Many sweep tubes put the beam plates on a seperate pin so that a small positive voltage can be applied to kill the Barkhausen oscillation.
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Old 30th September 2009, 05:22 AM   #10
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Default 813

Pin # Five is the supressor grid on the 813.
The physical distance was increased as much as possible from G2 to the Anode on Beam Tetrodes.....that helped low velocity Electrons from migrating "backwards".
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