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811A's, Dual Grid and Class B triodes

I was browsing through Terman (2nd edition) last night and read a paragraph about "Dual Grid and Class B triodes". According to Terman, if two concentric (but similar pitch) grids are put in the triode and then connected to each other as a single control grid, the result is a high-mu triode that needs zero bias for Class B operation.

I do not see dual-grid tubes mentioned in Terman's 3rd edition in the same way.

Googling the term, I see that the 52 tube seems to be an example where both grids are brought out to individual terminals.

I also see that sometimes "dual grid" is used to describe RF tubes where there are two pins for a single grid (to decrease inductance I guess), I'm not talking about these tubes.

The two-grids-connected-together characteristics remind me a lot of, for example, the 811A (the most familiar Class B triode I'm familiar with), but that only has a single grid terminal. Am I correct that the internal grid structures of an 811A are essentially that of two connected grids? If not, what inside an 811A makes it zero-bias high- mu class B triode, as opposed to say its externally similar non- identical-twin the 812A (a low-to-medium-mu triode that needs bias)?

I also note that Terman claims that the dual-grid structure forms a very good electrostatic shield between heater and plate, and see that

811A's are often used in grounded-grid connection in RF amps. (Must be a bitch to neutralize in common-cathode).

I've been intermittently playing around with SPICE to model 811A curves (including grid current at positive grid voltage) and none of the conventional triode models work right at all - its curves are more like a pentode (in fact it's pretty trivial to fit it this way if you let the diode characteristics take over at low plate voltage).

Tim.

Reply to
shoppa
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======================================= The #46 is another. The 46 was popular in a number of Atwater Kent radios circa 1932-33. Class B Push-Pull output stages in high-end radios had a brief run in the early 1930s, then faded into history. -- Ed

Reply to
Ed Engelken

Dual-grid tubes were popular for a short while. They were touted as being versatile -- connect grid 2 to grid 1 and you had a high-mu triode, connect grid 2 to the plate and you had a low-mu triode. Judging from my tube data books, it didn't take folks long to decide they just wanted one or the other, not two in one package.

The curves look pretty triode-like to me, once you take into account the fact that the thing is running almost exclusively at positive grid voltages. Perhaps the SPICE triode models aren't taking this into account?

--

Tim Wescott
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Reply to
Tim Wescott

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Hi RATs!

I have a huge P-P 46 vintage amp. It is not yet functioning. It is clearly very well built :)

Grow up, Class "A" does not mean good, it means simple ... which, despite some journalists' daydreams, is not exactly the same thing ...

Happy Ears! Al

Reply to
tubegarden

A push-pull pair of zero-bias class B triodes is even more remarkably simple.

When I first saw the schematic of a 811A P-P audio amp, I was astonished to find not a single resistor or capacitor in the circuit. Two tubes, input and output transformers, and connections to a filament supply and a plate supply. That's all there is, folks!

Tim.

Reply to
shoppa

This is very true and interesting. RCA figured out you could strap the control and screen grids of certain power pentodes and use them as low or zero bias triodes, and a couple of RCA theater amps do this. This is also seen in AM era editions of the W6SAI Radio Handbook for modulators. It is simple. It works. Based on long time experiments it was concluded it doesn't sound very good at all.

I have never tried it, on the theoryu if it worked very well it would be popular.

The 811 is certainly an attractive tube economically. It has a mu of

160 as I remember. The Altec 1570B used them as did the Gotham Audio cutter head amp using a GR toroid for output.
Reply to
Bret Ludwig

Several of the last of the RCA MI-series cinema amps did this with

6L6s and I believe 6146s. None are desireable to the audiophile in stock form.

They showed why toroids are a bad idea for opt's very well, although there are still a lot of people not listening. With a conventional or C-core transformer and a revised feedback loop the basic design is well worth study, though.

The 811 can be substituted in the MI200 McIntosh if the proper intermediate transformer is put in so as to get the filament voltage at the 6.3 rather than the 10 volts of the, I think, 8005s. Although the amplification factor of the 811 is way too high it works anyway. Distortion still meets Mc specs. We wound one, in fact, where I worked on a toroid, because we had plenty of scrap surplus transformers we could dewind (and take the copper home.) And a Gorman winder, which makes winding toroids pretty fast and simple. The 811s pull more current but the Mc filament transformers never even got hot.

Reply to
RapidRonnie

Because using conventional circuits the distortion was terrible. True Class B operation works well only at high continuous levels without extreme measures such as Wiggins and Mcintosh/Gow/Corderman afforded. Al Bereskin at Baldwin designed the "poor man's Mc" for organ use and it was published in an extremely good DIY article in the IRE journal in '55 or '56. It never got below 2% THD.

Reply to
RapidRonnie

Bugger... HTML.

Al said: ... Grow up, Class "A" does not mean good, it means simple ... which, despite some journalists' daydreams, is not exactly the same thing

*Saxophones are really complicated.

cheers, Ian.

Reply to
Ian Iveson

The dual grid tubes strike me as a gimmick. Any pentode or beam power tube can be connected at least 4 ways. Connect the screen to B+ and it behaves as a pentode or beam power tube. Connect the screen to the control grid and it becomes a high-mu triode. Connect the screen to the plate and it becomes a low-mu triode. Connect the control grid to a low positive voltage and drive the screen and it is a space charge triode.

The 811 has only one grid as do other zero bias tubes like the 6N7 and 1635. They just wound the grid with many turns spaced close together to get this characteristic. Low-mu tubes like the 812, 2A3, and 12B4 have few turns spaced far apart. The only triode I know of with two grids is the 6C5. The second grid is internally connected to the plate.

I have seen circuits using 6V6s with the screen tied to the control grid to make a zero bias triode.

The old ARRL handbooks (50s?, 60s?) had a listing in their tube tables for using a 12AX7 as a class B zero bias output tube. They said it could produce 7.5W. I never saw a circuit that actually used one this way, however.

-- Jim Mueller snipped-for-privacy@nospam.com

To get my real email address, replace wr>> I was browsing through Terman (2nd edition) last night and read a

Reply to
Jim Mueller

Most of those sets used push-pull class A, not B.

Pete

Reply to
Uncle Peter

There is more than one way to skin a cat. The 811A is a high mu triode. There are several ways to create such a beast.

One: place the grid closer to the cathode than to the plate. The ratio of the distance between grid and cathode vs the distance between cathode and plate is one of the factors that determine the mu of a triode.

Two: construct the grid with close spaced turns. The closer the turns of the grid wires are to each other, the higher the mu.

Three: Use two concentric grids. This is very similar to number two.

ANY triode works well in grounded grid. A high mu triode will have better screening due to the close wound grid. Tetrodes such as the

4-400 have been used in grounded grid by grounding both grids. Others such as the 4cx250 don't have enough heavy enough grids to handle the current in class AB2 or B so they are run in cathode driven service with normal grid and screen voltages, but with both grid and screen grounded for RF.
Reply to
ken scharf

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