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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Sep 2019, 12:25 pm   #1
geeoboeh2s
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Default VR91 vs VR91A

This is a topic that has been discussed before;
https://www.vintage-radio.net/forum/...d.php?p=841949

However following discussion with M0FYA Andy I believe that I have found a fundamental difference between the two valves.
Most of the responses in the above link seem to be related to the spread in production characteristics either specially selected or relaxed figures.

Looking at the CV1578/VR91A specification there is one figure that I would suspect is a definite requirement and may well have required a change to the electrode structure. The maximum heater to cathode voltage has been increased from 100V to 140V.

What was the equipment that required this increased figure? I don't know, but it can explain why no samples of the CV1578 apparently exist. The VR91A would have been allocated the the number CV1578 with the change to CV specifications but the equipment may have been short lived and no valves ever issued under this number.

Comments appreciated.

Chris
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Old 16th Sep 2019, 2:24 pm   #2
M0FYA Andy
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Default Re: VR91 vs VR91A

Very interesting!

Without taking one apart, what provides the heater to cathode insulation in an EF50?
Is it spacing between the heater and cathode or an insulated coating? How is it improved without changing the geometry of the electrode structure and the important parameters of the valve?

Andy
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Old 16th Sep 2019, 3:17 pm   #3
geeoboeh2s
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Default Re: VR91 vs VR91A

Andy,

I don't know the answer to that.

Getting deeper into the mystery was the VR91A manufactured only by Sylvania or was Mullard involved.

A number of valves in the CV156x to CV 158x range show changes to the more common variant. Typical is the VR65A that would appear to be a more powerful version of the VR65 with the heater current increased to 0.95A and the anode dissipation to 4.5W, about a 50% increase in each case.
Other changes may have more significance involving a change to tyhe manufacturer. The VR99A/CV1581 an unmetalled ECH35 used in the R1155 DF section was also given a GEC experimental number E1341.

Chris
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Old 16th Sep 2019, 7:06 pm   #4
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Default Re: VR91 vs VR91A

I wonder if uprated H-K voltage capability is the sort of thing that would have started off as a "designated batch" thing, and then been applied across all production anyway to ease stockholding and guard against maintenance confusion, removing the need for designation differentiation? One can imagine that increasing use of blanking/gating/DC amplification/cathode-follower type duties that arose in radar-type equipment as the war progressed would create a need for increased H-K rating. An improvement in heater insulation might well be negligibly more expensive once mass-production was underway.

The VR65A changes sound rather like what would be needed for good wide-band amplification service, the sort of thing that the EF55 was intended for- perhaps this was another low-production interim thing until the B9g EF55 was established.
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Old 17th Sep 2019, 10:29 am   #5
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Default Re: VR91 vs VR91A

Some quick searching reveals the VR65a referred to in post#4 is,actually a VR65 with a 4volt heater, thus the greater heater current. Otherwise identical to the VR65.
I recall a conversaion between two local radio amateurs in a local W.D.surplus radio shop in about 1952 in which one claimed to have a VR91a, and was comparing it to the performance of the standard VR91 in his home-constructed T.V. .
This suggests that they were actually issued, maybe in very small quantities. Tony.
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Old 17th Sep 2019, 1:33 pm   #6
geeoboeh2s
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Default Re: VR91 vs VR91A

"Some quick searching reveals the VR65a referred to in post#4 is,actually a VR65 with a 4volt heater, thus the greater heater current. Otherwise identical to the VR65".

I agree on the heater voltage, my mistake.
However the maximum anode dissipation for the VR65 is given as 3W whereas the VR65A is increased to 4.5W. The anode grid capacitance for the latter is 0.033pF and for the VR65 0.005pF. This might suggest a larger anode for the VR65A and hence greater allowed dissipation.
I have an original data sheet for the VR65A that provides these figures although I note that the successive Wireless World data books show that the only difference is with the heater voltage and current.
All rather confusing.

Regarding the VR91A. This was certainly issued and is currently available as NOS from at least one one of the UK major suppliers. It would appear that although the CV number was registered no valves have been found bearing the number CV1574.

Chris
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Old 18th Sep 2019, 10:26 am   #7
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Default Re: VR91 vs VR91A

I have followed the paper trail of the VR65A that will hopefully clear the various issues.

Two points should be noted;
With the introduction of CV codification RAF valves bearing a VR code without a suffix were allocated a CV number by adding 1000 to the original number e.g. VR65 became CV1065. Those with a suffix were allocated a new CV number e.g. VR65A/CV1574.
In June 1941 the Inter-Service Technical Valve Committee issued a list of preferred valves i.e. valves recommended for new equipment. The VR65 was on this list, the VR65A was not.

Although no early specifications have been found it is likely that the VR65 and VR65A were originally commercial types SP61 and SP41 with additional testing to meet the service requirements.

At some later time probably due to service requirements the allowed anode and screen dissipation was increased. This is seen in issue 3 of the VR65A specification dated May 1943. This issue is over stamped CV1574.
As the electrode assembly, apart from the heater, was common to both VR65 and VR65A a logistic issue would have arisen for the VR65 as this was recommended for new design where full use may have been taken of the increased dissipation. To avoid this issue the requirement for the up rated valve was allocated a new number CV118.

In order to fully justify the above it would be necessary to have access to all issues of the VR65, VR65A, CV118 and CV1574 specifications. It is very unlikely that these exist.

Chris
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Old 18th Sep 2019, 3:25 pm   #8
M0FYA Andy
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Default Re: VR91 vs VR91A

The Virtual valve Museum says the "CV118 is the same valve specially aged for positive grid drive conditions", comparing it with the CV1065 and with a table showing the maximum anode dissipation as 3 watts.

Jeremy, are you following this thread?

The CV118 datasheet says it is 4.5 watts.


Andy
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File Type: pdf cv118.pdf (56.5 KB, 32 views)
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