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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 15th Sep 2017, 9:48 am   #21
Panrock
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Default Re: EHT warm-up delay in pre-war sets

Yes Hugo, I agree, it's a beautiful valve. Mustn't go off-topic but fortunately the X41C in the set I'm working on is in good condition.

Steve
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Old 15th Sep 2017, 1:49 pm   #22
merlinmaxwell
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Default Re: EHT warm-up delay in pre-war sets

Off the wall...
A photocell looking at a heater?
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Old 15th Sep 2017, 8:21 pm   #23
TonyDuell
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Default Re: EHT warm-up delay in pre-war sets

I remember a circuit in one of the magazines years ago (probably either Practical Electronics or Practical Wireless) for a similar delay timer. I think the intended application there was to keep the HT off you amateur transmitter PA valves until they had warmed up.

It used a PY800 (boost diode), chosen because the high heater-cathode voltage specification of that valve means there is a lot of insulation between the heater and the cathode so it is slow to warm up. I think the heater was under-run too. The anode current operated a relay, possibly via a transitor of some sort.
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Old 15th Sep 2017, 8:51 pm   #24
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Default Re: EHT warm-up delay in pre-war sets

Just fit a Mullard HVR2 indirectly heated EHT rectifier or a HVR2A. same B4 base, one has a 4v heater, the other a 2v. Can't remember which. Saves all the add on contraptions. John.
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Old 16th Sep 2017, 11:30 am   #25
Panrock
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Default Re: EHT warm-up delay in pre-war sets

Quote:
Originally Posted by Heatercathodeshort View Post
Just fit a Mullard HVR2 indirectly heated EHT rectifier or a HVR2A.
A 'top tip' if I may say so! The HVR2 has a 4v 0.65A heater - and should be able to cope with the ½mA from the bleeder chain too. And I have two in stock! Thanks. Why didn't I think of it?

What with that, and Argus25's switch-off pulse mod, it looks like I could be sorted.

Needless to say, I am grateful to all those who came up with practical alternative suggestions.

Steve
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Old 16th Sep 2017, 4:27 pm   #26
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Default Re: EHT warm-up delay in pre-war sets

Hi John,

I had not realized the differance between the HVR2 and the HVR2A.
I was going to say... Why on earth did HMV go for a 4V heated EHT rectifier in this set? After all, what was wrong with the U16?

Of course a pre-war 2V rectifier would need too much heater current to simply fit an inline resistor to drop the extra 2V. However, it could be done with the lower heater requirement of a post war EHT rectifier.
I've attached a photo of a Mazda U25 posing as a U16 which I made a few years ago when I was restoring an HMV 1804, but in a similar way had to wait for a replacement valve to turn up.
Maybe if I added a heater resistor to my frankenstine U16, I could make a U17!!

Cheers
Andy
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Old 16th Sep 2017, 4:56 pm   #27
Panrock
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Default Re: EHT warm-up delay in pre-war sets

I see the U17 is rated at a colossal (for an EHT rectifier) 30mA in Iliffe - unless this is a mistake. Whereas the HVR2 is 3mA, which is more than enough.

Steve
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Old 16th Sep 2017, 6:19 pm   #28
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Default Re: EHT warm-up delay in pre-war sets

Yes it is 30m/a. Just another case of MOV strange valve design especially the TV section.
The two pre war EHT rectifiers are attached. An early U16 and the rather strange U17. It's internals look more like a power triode. John.
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Old 23rd Sep 2017, 5:00 pm   #29
Panrock
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Default Re: EHT warm-up delay in pre-war sets

A postscript.

HKS's HVR2 substitution for the U17 proved just to be the ticket. That, and Argus25's switch-off suppression mod mean that no alteration of the contrast and brightness controls is now needed at switch off or on.

Two more photos are attached. This was not a total 're-build' like the amazing magic Argus25 worked on his set, but in this case a 'restoration' was possible, since the basics were mostly in restorable condition although they look quite a bit better now! Capacitors were re-stuffed or re-formed as appropriate.

This example still has its original Emiscope 3/1. The picture has an ion burn. However it's also pleasingly bright and very watchable.

Now, just to put it in the cabinet and organise some suitable 1939 viewing. Arthur Askey anyone?

Steve
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Old 24th Sep 2017, 10:36 am   #30
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Default Re: EHT warm-up delay in pre-war sets

Steve,

That is a great looking chassis and you are very lucky to have the 3/1 tube.

Can you post a picture of a test pattern on the CRT ?

Hugo.
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Old 24th Sep 2017, 2:15 pm   #31
Panrock
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Default Re: EHT warm-up delay in pre-war sets

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Can you post a picture of a test pattern on the CRT ?

Hugo.
I'll see what I can do in due course, and maybe start a new thread about it.

Steve
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