Biasing G3 in beamies

[abj1]

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

[m6tt]

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.

[Johan Potgieter]

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.

[Miles Prower]

Originally Posted by abj1

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.

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.

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.

[Miles Prower]

Originally Posted by Johan Potgieter

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.

[Bandersnatch]

Originally Posted by Miles Prower

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

[ray_moth]

The 813 certainly looks like a beam tetrode, according to RCA http://r-type.org/addtext/add064.htm

[Miles Prower]

Originally Posted by Bandersnatch

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.

[tubelab.com]

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"

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.

[Richard Ellis]

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".
__________________________________________________ ___Rick......