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Old 30th Nov 2022, 1:00 pm   #23
turretslug
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Default Re: Combined power-pentode and double-diodes?

Quote:
Originally Posted by G6Tanuki View Post
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?
That strikes me, too- a modest, run-of-the-mill AF power capability in return for more heating power than the brawny KT66 of the same era is puzzling. Most AM-orientated signal diodes seemed to have been miniscule slivers of metal at one end or the other of the cathode tube, so can't have needed much share of the emissive power requirement. Compare with the 6.3V 0.15A of the 6AQ6 double-diode/signal triode, whose triode was of the EC90/half-ECC82 class. Even granting each diode a third of the cathode heating power suggests only 50mA each. The all-round spec. of the EBL21 is sufficiently close to that of the EBL31 to suggest that it might have consisted of a very similar basic electrode assembly but with a more efficient cathode coating (0.8A vs. 1.5A).

One topic that crops up periodically is the scarcity of EBL31s that are compatible, i.e. sufficiently short to fit in the Ekco A22- it seems that most of those available have been converted from side-contact EBL1s, the give-aways being the added-on (and incompatibly high) International Octal base and 3/8" rather than 1/4" top-cap. It struck me that the EBL21 could be used in these sets with a socket change, and with the bonus of reduced heater current/internal heat. Not strict originality, I guess it depends on whether having a going concern or concours example is the priority.

I believe that the ECH21 was originally designed as a B9G type, but was then decanted into the 8-pin base, hence the cathode-to-spigot connection quirk. Maybe it was felt that it was better to have a valve whose socket was compatible with existing International Octal chassis punching for production convenience- the B9G format may have been an advance over existing types in terms of short internal connections and glass base, but being of even larger diameter than the already large International Octal seems curiously retrograde.
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