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Old 10th Jan 2020, 10:21 pm   #1
G6Tanuki
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Default "Double anode" triodes/tetrodes.

Looking through an old copy of the "Babani" valve-specx book, I came across the strange world of triodes and tetrodes that had two anodes.

12FQ8 being a dual-triode with each triode having 2 anodes.

6FH8 is a dual-anode tetrode along with a triode and diode

EF816 is a dual-anode pentode.

6FG7 is a dual-anode-tetrode-weith-a-diode.

What were these for? Initially I imagined some sort of single-balanced-modulator, but that wouldn't make sense since there's no way to supply balanced drive.
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Old 10th Jan 2020, 10:31 pm   #2
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Default Re: "Double anode" triodes/tetrodes.

According to Radiomuseum

https://www.radiomuseum.org/tubes/tube_12fq8.html
Tone generators for musical instruments.

A sync separator
https://www.radiomuseum.org/tubes/tube_ef816.html
Cheers

6FH8
https://www.radiomuseum.org/r/chicagomus_cordovox.html

Mike T
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Old 10th Jan 2020, 11:07 pm   #3
Synchrodyne
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Default Re: "Double anode" triodes/tetrodes.

The EF816 dual-anode pentode at least followed the precedent of the 6BU8, which was a joint development by Zenith and GE for used as a noise-gated sync separator and noise-gated AGC rectifier for TV receivers. This was chronicled in “Electronics” magazine 1957 May:

Electronics 195705 Noise Gating Tube for AGC and Sync.pdf

(This is a highly compressed .pdf, so of questionable readability. The original article is available here: https://www.americanradiohistory.com...aster_Page.htm,)

The back-story here starts with another Zenith-GE joint development of the 1940s, namely the 6BN6 gated beam valve intended for use as an FM quadrature demodulator. Zenith also used the 6BN6 as a TV sync separator, then as a noise-gated sync separator. It was expensive for the job, though, and the existing 6BE6 heptode was found to be suitable and adequate, although not quite as good. The steps from there to the 6BU8 are covered in the above article. Following Zenith’s deployment of the 6BE6 in the noise-gated sync separator role, the industry developed sharp cutoff heptodes that were better for the purpose, namely the 6AY6 and 6CS6. These were also slated for use as colour subcarrier demodulators. (And were used as mixers in some HF receivers.)

As an aside, whilst Zenith and GE had co-operated on some circuit and valve developments, and both were staunch supporters of FM during the late 1940s and 1950s, the Zenith-GE pilot-tone FM stereo system was not a joint effort. Each submitted its own system for evaluation by the NSRC and FCC; the FCC saw that they were very similar and combined them as a single system for evaluation purposes. Post facto there were apparently claims and counterclaims as to who had “really” invented the system. (There was an earlier thread about this: https://www.vintage-radio.net/forum/...d.php?p=803619.)

No doubt the 6BU8 and its successors such as the EF816 found several other uses. Mullard proposed the 6BU8 for use in a stereo decoder for its own TDM system detailed in Practical Wireless 1960 May, p.57ff.

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In contrast, Mullard’s early circuit for decoding of the Zenith-GE system used an ECH84 pair, or alternatively an EH90 (6CS6) pair.


Cheers,

Last edited by Synchrodyne; 10th Jan 2020 at 11:15 pm.
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Old 10th Jan 2020, 11:14 pm   #4
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Default Re: "Double anode" triodes/tetrodes.

The amateur fraternity was quick to adopt the 6BU8:

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Cheers,
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Old 11th Jan 2020, 1:06 pm   #5
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Default Re: "Double anode" triodes/tetrodes.

Maybe the best known double anode valve especially with vintage TV enthusiasts is the Cossor 4TSA used as a sync separator in various pre war models. Line pulses were extracted from one anode and frame from the other.
Clever guys those Cossor designers at Highbury Corner. John.
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Old 11th Jan 2020, 5:28 pm   #6
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Default Re: "Double anode" triodes/tetrodes.

The 6BU8 seems to be a different beastie to the 12FQ8.

The latter shares the same grids between both of a pair of anodes, so uses are where anodes have differing loads... say one tuned to a fundamental, the other to a harmonic.

The 6BU8 has electrode structures to move the device current between the anodes. Rather like the 7360 beam deflection valve intended for colour TV coding/decoding, stereo decoders etc. They made good mixers (G3PDM receiver) and good product detectors.

However, the valve in the original post can't vary current between the pair of anodes.

David
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Old 11th Jan 2020, 5:36 pm   #7
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Default Re: "Double anode" triodes/tetrodes.

They are fascinating - thanks for the various links and usage-suggestions - Organ dividers was never something I even thought about!

The only circuit I can remember seeing where one of these was used was a 'feed-forward' ham radio speech-processor: one anode was used as the normal audio-amp, the output from the other anode was fed to a 2-diode voltage-doubler whose output was then used to vary the degree of forward-bias of a pair of PIN-diodes which were fed from the first anode's output.

I guess you could have used both halves of a 12AU7 or similar to achieve the same result.
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Old 11th Jan 2020, 11:47 pm   #8
Synchrodyne
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Default Re: "Double anode" triodes/tetrodes.

The ability to swing the current between the two anodes of the 6BU8 may have been an incidental or secondary feature.

Zenith’s starting point was two heptodes, one for sync separation and the other for agc, that used the same gating signal (low-level demodulated video) on their respective grids 1. They had different signals on their respective grids 3, though. The first step was noting that heptodes were not essential, but that dual-control pentodes would work. In fact this was already established, bearing in mind that the starting point was a noise-gated sync separator circuit using the 6BN6, which was an ersatz form of the dual-control pentode. Also, in looking for lower cost alternatives, Zenith had considered the 6SA6 dual-control pentode, but the 6BE6 was lower cost still. What was needed was to combine both dual-control pentodes in a single envelope in a B9A base. Given that the same signal was used on both grids 1, then these could share a pinout. A shared cathode pinout was easy enough, as was a shared grid 2 (screen) pinout. That got the pinout count down to 9. Thus the 6BU8 was conceptually a twin, dual-control pentode, but the common pinouts made it look like a split-anode or twin-anode pentode. With the EF816, one could infer that in terms of the Pro-Electron designation system, it was seen as being basically a pentode (EF) and not a dual pentode (EFF).

Nonetheless, Zenith and GE must have been aware of the current swinging possibility, given that GE had developed the 6AR8 dual anode sheet beam tube in 1954 primarily for use as a colour TV subcarrier demodulator. (The RCA 7360 followed in 1959, and appears to have been aimed primarily at the SSB market; it was suitable for use up to 100 MHz).

And curiously, Zenith proposed using the 6AR8A as the demodulator in its early FM stereo decoder circuit:

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I am not sure that the 6AR8(A) (or its 6JH8 successor) was ever used this way in a commercial product, though. GE noted that this valve was subject to stray magnetic fields, so pickup of 50/60/100/120 Hz hum could have been a problem in audio applications.

Be that as it may, clearly the 6BU8 et al and the 6AR8/7360 were interchangeable for some applications.

The 6FG7 was mentioned in the original post. This though appears to have been a VHF TV triode pentode frequency changer of the third kind.


Cheers,
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Old 24th Jan 2020, 4:50 am   #9
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Default Re: "Double anode" triodes/tetrodes.

Zenith’s proposal to use the 6AR8 sheet beam valve in FM stereo decoding might trace back to its earlier colour TV decoding work, wherein it appears to have been the main proponent of the sheet beam valve (see “Electronics” magazine 1954 May.) Whilst there is no hard evidence that Zenith worked with GE on the development of the 6AR8, given the antecedent of the 6BN6 gated beam valve and the post-cedent of the 6BU8, it would not be surprising.

The split-anode pentode (6BU8, EF815, etc.) was certainly an unusual valve, but more so would have been the split-anode heptode. This was experimented with by Philco for use as a colour TV subcarrier decoder, but evidently not put into production. (See “Electronics” 1954 June). Which valve supplier it worked with is unknown, but previously Philco had worked with Sylvania on the development of the FM1000 heptode for use as a locked-oscillator FM demodulator. So there was a joint heptode history. Philco’s colour decoder heptode work at the time also included the 6AJ8 (ECH81), which would have been yet another application for this multi-use valve.


Cheers,
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Old 24th Jan 2020, 10:40 am   #10
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Default Re: "Double anode" triodes/tetrodes.

I just gave two away to a lad in Portugal, along with suitable transformers.
https://www.radiomuseum.org/tubes/tube_ecll800.html

It also has a triode for the phase splitter.

Joe
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Old 24th Jan 2020, 12:31 pm   #11
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Default Re: "Double anode" triodes/tetrodes.

The ECLL800 is a double-pentode-plus-a-triode; though there are two pentode-anodes, there are also two pentode control-grids (so it's essentially two pentodes in one envelope, sharing a common cathode).

The 'double-anode' valves I'm really pondering on have one control-grid but two anodes, whose anode-currents are controlled by that one grid.
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Old 24th Jan 2020, 6:17 pm   #12
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Default Re: "Double anode" triodes/tetrodes.

The 6CD6 is the other way round with two of everything except for the anode.
They are all linked in parallel internally though.
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Old 24th Jan 2020, 6:33 pm   #13
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Default Re: "Double anode" triodes/tetrodes.

There are various valves like the 6DC6 which are really multiple-valves-wired-in-parallel; the ATP4 (as used in the WWII WS18/WS38) being another of my favourite examples...

http://www.r-type.org/exhib/aat0020.htm
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Old 25th Jan 2020, 6:08 am   #14
Synchrodyne
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Default Re: "Double anode" triodes/tetrodes.

Here is some information on the prototype split anode heptode.

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As may be seen the anodes and grids 3 were split, all other electrodes being common to both “halves”. Thus it had some similarities with the 6BU8 split-anode pentode.


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Old 25th Jan 2020, 1:55 pm   #15
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Default Re: "Double anode" triodes/tetrodes.

There's an appealing simplicity to that circuit- aside from the double-anode heptode itself. "Bring me colour with minimum bottle count, let the tube itself do the colour-difference matrixing". (I'm using "tube" in the British, rather than American sense!)

Whilst FM stereo radio and TV colour transmissions brought balanced demodulation into widespread consumer use and probably inspired several double-anode propositions, I wonder when balanced mixing really took off in radio usage? VLF superhet applications must have emphasised the need to keep the LO from deafening the IF strip to designers, and Racal's VLF adaptors are nice examples of balanced mixing but used mainstream conventional devices rather than newer "exotica" like the 7360.
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Old 25th Jan 2020, 3:54 pm   #16
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Default Re: "Double anode" triodes/tetrodes.

I'm sure I recall reading a book from the early-1930s in which people working for Marconi offered designs for experimental multi-anode/multi-grid valves in balanced-modulators for use on the transatlantic long-wave radiotelephone system (which, of course, was one of the earliest applied uses of ISB) and later pre-WWII shortwave radiotelephone systems.

Yes, they also used the classic 'diode quad' approach (AT&T used this too in their early systems multiplexing loads of phonecalls onto a single coaxial cable) but it seems that the whole idea of active balanced-modulators/demodulators goes back quite a long time.

I've even got a circuit somewhere from a 1930s ARRL magazine which shows a pair of 6L6 valves used as a 'high level balanced modulator' [carrier to the control-grids, push-pull audio to the screen-grids, 400V on the anodes] directly feeding the antenna, to produce a few tens of Watts of DSB on 7MHz - an idea I'm somewhat tempted to recreate.

We had to wait a while before the likes of the 6BN6 and 7360 arrived.
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Old 26th Jan 2020, 12:51 am   #17
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Default Re: "Double anode" triodes/tetrodes.

Philco’s starting point for the split-anode heptode colour demodulator was a circuit using a pair of 6BA7 heptodes:

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The split-anode heptode was a logical simplification. But the 6BA7 had been chosen because it was a cheaper, readily available alternative to the 6AS6 dual-control pentode that had been used quite often in early colour decoders. The 6BA7 was said to be a quarter of the price of the 6AS6. It is interesting to note that Philco, having gone to the heptode because it was available, then went direct to the split-anode heptode concept, and not say to the split-anode pentode, which would have been a logical derivative of the 6AS6. But it could have been that the colour TV market, quite small at the time, was not seen as supporting new valves that would not be likely to have significant other applications. However, GE’s development of the 6AR8 beam deflection tube runs counter to that notion. Anyway, the split-anode heptode was not proceeded with, and the split-anode pentode did not appear until later on, when it was developed primarily for noise-gated sync separation and agc applications.

Single heptodes very suitable for colour demodulation were developed, namely the 6BY6 and 6CS6, but their primary motivation was the noise-gated sync separator application for both monochrome (huge market) and colour receivers (small market), where they were better than the 6BE6 but cheaper than the 6BN6. There was also the 6DB6 dual-control pentode for colour demodulation, much cheaper than the 6AS6, and also suitable for noise-gated sync separation.

Philco’s split-anode heptode proposal thus looks to have been just a bit ahead of its time. I wonder too if there was some reluctance on the part of the valve makers given the history in which Sylvania had developed the FM1000 heptode especially for Philco’s locked oscillator FM demodulator, but that circuit lasted only for a few seasons. Clearly the big market for FM demodulators was in TV receivers, and I am not sure if the FM1000 got much traction there. So the FM1000 might have been a loss-maker for Sylvania

Crosby did a lot of work on exalted carrier demodulators (product detectors) using diodes, triodes and multigrid valves including heptodes. I think he had the patent on the heptode as an exalted carrier AM and PM demodulator. One can see that a split-anode heptode could have replaced the two individual heptodes in his two-phase demodulator.

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On the other hand, given that this was aimed at the commercial/industrial market, such simplification – if it was thought of at the time - may have been ranked more lowly against separation of the I and Q circuits and also commonality of the single heptodes with those used elsewhere in the receiver.


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