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Old 28th Nov 2022, 5:15 pm   #1
G6Tanuki
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Default Combined power-pentode and double-diodes?

The likes of the octal EBL31 [essentially an EL33 and two small-signal diodes] were popular in early poverty-superhets as an alternative to using a separate double-diode-triode [EBC33/6Q7] and less-high-slope output-valve [6V6 or something like a Cossor 42OT].

The idea seems to have been consigned to the history-books post-WWII; I have never come across a B8 or B9 glass-button-base equivalent of the double-diode-output-pentode.

Does anyone know different? I guess that after WWII there was greater wealth and the need for 3-valve-plus-rectifier poverty-superhets, along with the emergence of more modern valves made the idea of a power-tetrode/pentode with diodes obsolete.
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Old 28th Nov 2022, 6:00 pm   #2
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Default Re: Combined power-pentode and double-diodes?

EBL21? Quite a few Continental sets featured an ECH21, ECH21, EBL21, rectifier line up, taking advantage of the second ECH21 triode as first AF amp. Others just used the high gain of the EBL21 pentode to continue the short-superhet theme. Admittedly, this series of glass base valves was superceded fairly quickly post-war without an EBL21 successor.
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Old 28th Nov 2022, 6:19 pm   #3
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Default Re: Combined power-pentode and double-diodes?

The diodes just migrated to the IF amp instead (usually an EBF80 or 89).
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Old 28th Nov 2022, 6:26 pm   #4
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Default Re: Combined power-pentode and double-diodes?

Yes, but I don't remember coming across any radios with the EBL21 or its series-connected-heater version the UBL21 [55V@0.1A] that was intended for use in AC/DC radios.

I guess the whole poverty-superhet idea was doomed to oblivion.

I was always fascinated by the ECH21/UCH21 as an example of coercing a valve-design into a sub-optimal base-pattern; to provide enough connections they had to use the centre locating-spigot [which would normally be grounded] as an active connection.
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Old 28th Nov 2022, 6:32 pm   #5
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Default Re: Combined power-pentode and double-diodes?

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Originally Posted by paulsherwin View Post
The diodes just migrated to the IF amp instead (usually an EBF80 or 89).
But were there ever any consumer-radio-targeted 'high slope' output-tetrodes/pentodes that let you do a povertyhet using the diodes of an EBF80/89 without needing an intervening triode before the power-stage?
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Old 28th Nov 2022, 6:47 pm   #6
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Default Re: Combined power-pentode and double-diodes?

Pye made loads of them in the 50s (they were keen on short superhets in the octal era too). Their designs were initially mixed B8A/B9A arrangements with an EL41 for output, but I think they also used EL84s later on. They perform quite well.

https://www.radiomuseum.org/r/pye_p43p_4.html
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Old 28th Nov 2022, 8:21 pm   #7
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Default Re: Combined power-pentode and double-diodes?

Philips produced radios with various seemingly randomly chosen valve line ups, the 209U has UCH21, UCH21, UBL21, UY1N. Only 3 valves but not a short superhet.
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Old 28th Nov 2022, 8:48 pm   #8
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Default Re: Combined power-pentode and double-diodes?

I seem to recall that Ekco made some switch-tuned radio's U195? that used, UBF80 and UL41. There was also a clock-radio with a similar valve line-up.
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Old 29th Nov 2022, 12:58 am   #9
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Default Re: Combined power-pentode and double-diodes?

Quote:
Originally Posted by G6Tanuki View Post

The idea seems to have been consigned to the history-books post-WWII; I have never come across a B8 or B9 glass-button-base equivalent of the double-diode-output-pentode.

Does anyone know different?
AWV 6BV7.

6BV7 AWV 195207.pdf


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Old 29th Nov 2022, 11:51 am   #10
Leon Crampin
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Default Re: Combined power-pentode and double-diodes?

Possibly one reason why double diode pentode output valves fell from favour was their restricted supply during the war. They were deemed to be superfluous to the war effort and supplies were limited. A further factor was the effective fitment of pickup sockets. Although it's possible to re-arrange the IF stage (Philips, Murphy) or the oscillator triode (Bush) as an AF preamp, switching is more complex and the available gain is limited.

Post war, economic constraints may have limited the sales of radiograms, so perhaps the provision of pickup sockets on a cheaper table set may have been a sales advantage. Thus, the conventional 4 + R superhet became the norm.

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Old 29th Nov 2022, 1:23 pm   #11
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Default Re: Combined power-pentode and double-diodes?

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Originally Posted by G6Tanuki View Post
Yes, but I don't remember coming across any radios with the EBL21 or its series-connected-heater version the UBL21 [55V@0.1A] that was intended for use in AC/DC radios.
The EBL21 is extremely common here in Poland. I do wonder if there was an extended period after WWII when the valve factories of the People's Republic only knew how to make EBL21s and ECH21s because they turn up absolutely everywhere, even in the most unexpected places in commercial equipment. I saw a 600W PA amplifier with some monster transmitting triodes in the output stage, but the preceding stages were entirely ECH21s and EBL21s.

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Old 29th Nov 2022, 2:00 pm   #12
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Default Re: Combined power-pentode and double-diodes?

I believe that the Czechoslovak TESLA company continued to produce radios with the ECH21, ECH21 EBL21 plus rectifier line-up and / or series heater variants long after Philips had moved on from this type of design.
Perhaps Polish or other eastern manufacturers did likewise.

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Old 29th Nov 2022, 2:23 pm   #13
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Default Re: Combined power-pentode and double-diodes?

Two ECH/UCH21's were often used, the triode section in the IF bottle being used as the 1st AF amp, Eg: Tesla 305U Talisman (Amongst others)

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Old 29th Nov 2022, 2:54 pm   #14
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Default Re: Combined power-pentode and double-diodes?

Ekco CR32 series car radios used the EBL21
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Old 29th Nov 2022, 4:39 pm   #15
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Default Re: Combined power-pentode and double-diodes?

Some fascinating insight has emerged; I wonder if the popularity of the EBL21/UBL21 in Eastern European radios came about as some kind of post-WWII planned-economy thing to try and minimise the diversity-of-different-valves needed for production and maintenance?

I'd forgotten about the likes of the Pye Piper and similar; didn't K-B also do a small receiver that used reflexing so the audio signal went back through the IF amp? I always wondered how well that would work when the IF-amp was subject to AGC action... still, it was probably as good or better than the ubiquitous post-WWII "Premier" 3-valve TRF radios that filled the need for a bedside/kitchen radio at low cost.

The 6BV7 - that's a new one on me!

In the same context of power-valves being in the same bottle as diodes, there were a number of US versions that combined a half-wave rectifier with a small output-valve; the 12A7 for example, or the 25A7 or the later 70L7/117M7/117N7/117P7 which could produce a Watt or so of audio as well as enough rectified current from the 117V mains to allow the feeding of the heaters of a series-string of 1.4V battery valves. I remember at least one US battery/mains portable used this approach - on batteries the output-valve was a battery-filament one but on mains one half of the battery output-valve's centre-tapped filament was turned off and it became the driver for the combined output-pentode/rectifier, giving greater audio output.

Cunning trick: on mains, the series-connected battery-valve's filaments became the output-pentode's cathode-resistor so the LT supply was essentially 'free'!
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Old 29th Nov 2022, 5:11 pm   #16
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Default Re: Combined power-pentode and double-diodes?

The 2 off ECH21 plus EBL21 or U equivalent circuit was a war-time Philips Eindhoven development intended to save cost; more info here. Tesla and Philips seemed to have some sort of commercial relationship at the time, hence Tesla's use of the circuit during WWII and continuing into the late '50s.

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Old 29th Nov 2022, 5:15 pm   #17
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Default Re: Combined power-pentode and double-diodes?

Quote:
Originally Posted by G6Tanuki View Post
But were there ever any consumer-radio-targeted 'high slope' output-tetrodes/pentodes that let you do a povertyhet using the diodes of an EBF80/89 without needing an intervening triode before the power-stage?
UL41 (and UL84) are high-slope... many cheap record players drove the UL84 directly from a crystal pickup.

The Ferguson 208U is a short-superhet, UCH42, UAF42, UL41, UY41 (all B8A based), the single diode in the UAF41 drives the UL41 with no intermediate AF amplifier.

Despite the lack of overall gain, it is a good radio - Ferguson paid a lot of attention to the coils and tracking at the front end.
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Old 29th Nov 2022, 6:28 pm   #18
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Default Re: Combined power-pentode and double-diodes?

I suspect the manufacturers had trouble cramming a power pentode and a couple of diodes into a B8A/B9A envelope, at least at first.
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Old 29th Nov 2022, 6:50 pm   #19
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Default Re: Combined power-pentode and double-diodes?

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Originally Posted by paulsherwin View Post
I suspect the manufacturers had trouble cramming a power pentode and a couple of diodes into a B8A/B9A envelope, at least at first.
Yes, it would have been a bit of a struggle - but as noted upthread they managed to fit a low-power pentode and a half-wave HT rectifier into a small space....

Even in the Octal days, I'm amused to see that the basic EL33 pentode took only 0.9A@6.3V heater but its dual-diode-pentode sibling the EBL31 needed a massive 1.5A@6.3V for its heater!

Needing 0.6A@6.3V additional heater-power just to drive a couple of signal-frequency diodes seems to be getting a bit louche and bordering on the profligate!

What was going on there?
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Old 29th Nov 2022, 8:56 pm   #20
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Default Re: Combined power-pentode and double-diodes?

Based on books by Philips dating from about 1939 to 1942 I deduce that around 1938 Philips thought that the EF22 (6.3 V/0.2 A), the ECH21 (6.3 V/0.33 A), the EBL21 (6.3 V/0.8 A), and the AZ1 (4V/1.1 A) series of valves would be all that was needed for consumer radios with 6.3 V heaters.

The same goes for their U-series of valves (UF21, UCH21, UBL21 and UY21).

For battery radios their range of valve types was much broader (DAC21, DBC21, DF21, DF22, DK21, DL21, DLL21, and DM21).

For 'special' applications Philips envisioned the "50" series of valves. Remarkable of this "50" series was that the valve bases are not identical:
- P-base: EC50, EL50, EL51
- Loctal 8-pin: EF51
- Loctal 9-pin: EC52, EC56, EC57, EE50, EF50, EF54, EF55, EFF50, EFF51
- Octal: DAH50
- Various: EA50, EA52, EC53, EC55

In some Philips publications of around 1940 more valves of the "50" series are described but they never made it to the production stage, most likely because of World War II. Some of the EF(F)5x valves mentioned above were developed by Mullard.
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