V914 valve uses?

[Pete_kaye]

I have acquired a Mazda V914, and wonder how it was used. It is described as a Mains set Double Diode with a 4V heater and not suitable for HT. It is unusual shape.

[ms660]

http://www.r-type.org/exhib/aaa1005.htm

Lawrence.

[Herald1360]

Sounds pretty much like an ancestor of the 6H6 and EB91.

[Leon Crampin]

The V914 unfortunately has a single cathode. This precludes its use in some applications such as more sophisticated phase discriminators for AFC correction voltage generation, which require isolated diodes.

There are 2 of these valves in my Murphy A28C - one as detector/AGC rectifier and the other as AFC discriminator in a simple Round-Travis circuit, which uses 2 L-C resonant elements tuned above and below the IF. When correctly tuned at the IF centre frequency, the rectified voltages from the two AFC coils are equal and opposite, and therefore cancel.

The later A52 uses a DD41 double diode with separate cathodes and is a more sophisticated phase detector to generate the AFC. All good stuff for 1938 - and it works well.

Leon.

[FERNSEH]

There is a Mazda V914 in the Murphy A36 I'm currently restoring.
The V*** type code was generally used by Ediswan for development types.

For example I believe the 6F12/EF91 was coded as V888.

DFWB.

[ukcol]

The Murphy A40C uses 3 (there must have been a sale on )

V4 is the detector, V9 is used for ATC (AFC), and V11 for AVC (AGC).

You will see that the V914 in the detector stage has the anodes strapped together but the one used for AVC uses only one anode, the other being connect to cathode. Exactly why this is done is a mystery to me.

Note: the delay voltage for AVC is taken from the cathode of the first audio valve.

[Leon Crampin]

I suspect the detector benefits from a low impedance diode in simple terms of rectification efficiency.

AGC rectification is in some respects, more complex. Due to the delay voltage, the AGC rectifier is cut off until a sufficient signal level allows the diode to conduct. When this happens, there is a de-tuning effect on the resonant circuit feeding the diode as extra capacity is effectively connected to it. I suspect that it was to minimise this effect that Murphy used a single section of the V914.

I have always disliked the de-tuning effect of a delayed AGC system, despite its advantages over simple AGC in most other respects. It's very evident when aligning a receiver and allowing the signal input to become large enough to enable AGC action to occur - something normally to avoid. Later on, Ekco minimised this problem by using a very low value of AGC coupling capacitor in some sets - sometimes as low as 5 pF.

Of course, the A40C is an almost "perfect" design anyway in that it has a separate side-chain IF amplifier system from which the AGC is derived. De-tuning with AGC action would still have been undesirable though.

Murphy had some excellent design engineers.

Leon.

[ukcol]

Hi Leon

That is an aspect of delayed AGC that I hadn't considered before. Its interesting that when you think you understand a subject well there is always something else to learn.