Valves in commercial applications for which they were not specifically designed.

[Radio Wrangler]

Originally Posted by Synchrodyne

Well, the Philips datasheet for the ECH81 had the opening statement: [I]“Triode-heptode for various purposes in F.M., FM/AM, A.M. and television receivers”.

Active device manufacturers still make these sorts of statements in their advertising but it looks like they are written now not by engineering staff but by the wishful thinking section of the marketing department and have degenerated into simply being lists of every mass production consumer gizmo.

"This 500A GTO thyristor is suitable for applications in mobile phones, tablet computers, Digital televisions and milling machines."

Reading old adverts in WW etc reveals a delightful level of honesty and treating their readers as if they actually knew what they doing!

David

[Synchrodyne]

The Z77 (aka EF91, 8D3, 6F12, 6AM6) was widely used and probably in many applications outside its primary function, which was as a high-slope, sharp cutoff RF pentode. Video amplifier applications were mentioned in some datasheets, but I haven’t seen any mention of audio applications.

One may certainly see why the Z77 was used as an AF amplifier in communications receivers where it was already employed for other purposes. And the use of RF pentodes for AF work was hardly unknown. For example, the Philips literature covers the use of the EBF80 pentode in this manner. (The oddity there was that the pentode was of the remote cutoff type).

But the use of the Z77 as an AF voltage amplifier in high quality audio equipment is probably far enough away from original intent to justify its inclusion here. And it was used thus by Dynatron in the days when it was still aiming more towards the upper end of the market.

The Dynatron LF613 amplifier had a pair of triode-strapped Z77s, the first as the input voltage amplifier, and the second as the phase splitter. (The output was a pair of triode-strapped KT66s). I suspect that the preceding LF612 had a similar valve line-up. And the T139 tuner-control unit had a triode-strapped Z77 as its audio tone control stage (as probably did the earlier T99). So Dynatron must have considered the Z77 to be an acceptable valve for high quality audio work.

At the time, Dynatron was very triode-oriented, and the triode-strapped Z77 was used (from 1952 as best I can determine) in situations where the Mazda HL41 triode had previously been used. There may have been an element of necessity. Dynatron appears to have shifted from Mazda to Osram as valve supplier coincident with its adoption of miniature valves. (Perhaps it didn’t want to use Mazda Rimlocks?) Circa 1951-52, Osram may not have had a suitable AF valve, either an AF triode per se or an AF pentode that could be triode-strapped. At that time, its B309 (12AT7) and Z729 (EF86) were probably still in the future. And its L77 low-mu triode may not have fitted the requirement. So a triode-strapped Z77, which was high mu (75), may have been Dynatron’s best option. The Z77 datasheet did cover triode-strapped operation, but this may have been done with RF applications in mind. This valve could also be found as an oscillator, and as a grounded-grid RF amplifier, in which mode it was used in the Dynatron T139.

In one circuit variation of its T139 (of 1954), Dynatron did use a triode-strapped Z729 as gramophone input amplifier, nothing unusual there. But the same variation also had a DH77 used as a triple diode (demodulator, agc rectifier and agc delay). The triode grid was used as a diode anode, with the triode anode earthed. This use probably fell outside of anything found on the datasheet or application notes.

In its TCR16 hybrid car radio, Pye used the PCL83 pentode as a driver for a push-pull transistor output. This was probably a novel application and not one that was covered by the datasheet, although the use of smaller output valves to drive larger output valves was not unusual. In fact one GEC design had a DA100 pair driven by a KT66 pair, in turn driven by an N709 (EL84) pair.


Cheers,

[kalee20]

The same valve as you've just mentioned (EF91 / Z77 etc) is also used in the Soundmirror tape recorder, twice, one valve as the second-stage voltage amplifier on playback (and head driver for record), and the other valve as bias oscillator (unused on playback).

It's a versatile valve!

[Radio Wrangler]

In the sixties i made a couple of oscilloscopes with EF91/Z77/6AM6/CV138 valves in all stages. They did their jobs well. Perhaps it wasn't so much a matter of selection for purpose as having bought about half a teachest full of the things from Jim Fish.

A very versatile valve.

David