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Old 18th Jun 2024, 2:15 pm   #1
Helder Crespo
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Default Wide envelope PL38?

I recently acquired two used PL38 valves. One of them is clearly a PL38 (valve on the right in the photos below), but the second one (on the left) has partially erased markings (only the "PL" is clearly visible) and its glass envelope is much wider (approximately 55 mm in diameter) compared to the valve on the right, as well as to all other PL38s I've encountered so far (45 mm max).

I could not find any information on this variant, and I cannot identify any other valve it could be besides a PL38 with a wider envelope. I don't have a tester, so any quick tests would involve replacing the valve in a TV. However, I am hesitant to test an unknown line output valve directly in one of my TV22s.

Does anyone have any information on this?

Many thanks,
Helder
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Old 19th Jun 2024, 1:37 pm   #2
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Default Re: Wide envelope PL38?

Am sure I remember there being some 'special' versions of the EL/PL series of valves, intended for use in things like EHT generators, ultrasonic cleaners and similar. The nature of the specialness escapes me though.

Are there any obvious differences apart from the bigger envelope? I am thinking about more robust micas or potentially thicker anode plates to give higher dissipation.
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Old 19th Jun 2024, 1:53 pm   #3
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Default Re: Wide envelope PL38?

Thanks G6Tanuki. The insides seem to be the same as those of a standard PL38 in terms of mica. Can’t really judge on anode thickness but seems very similar.
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Old 19th Jun 2024, 2:50 pm   #4
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Default Re: Wide envelope PL38?

It could just be an early one. I agree the innards look identical.
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Old 19th Jun 2024, 3:59 pm   #5
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Default Re: Wide envelope PL38?

Have you tried breathing on the wording and then turning it carefully in the light? This can often identify otherwise 'missing' legends.
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Old 19th Jun 2024, 4:31 pm   #6
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Default Re: Wide envelope PL38?

The voltage required for the heater to use 0.3A can sometimes be a pointer. If that is identical it is a strong case they are the same valve.

The size is quite an important specification, so if it is more than 45mm would point to a another variant.
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Old 19th Jun 2024, 9:54 pm   #7
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Default Re: Wide envelope PL38?

Many thanks for the various comments and suggestions.
The valve is definitely a PL38. After confirming heater voltage and current, I tested it in my TV22 Mk2 and it worked perfectly.
All the technical information from Mullard I found online (I don't have paper catalogues or data sheets) points to the diameter being a maximum of 45.5 mm at least from 1951 on. Since the PL38 was introduced in 1948, this particular valve could be from 1948, 1949 or 1950. But it would be interesting if someone could confirm this - I imagine the larger dimension must be given in a data sheet from those years.
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Old 19th Jun 2024, 10:37 pm   #8
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Default Re: Wide envelope PL38?

Hello,

This advert from 1948 for the EL38 (the 6.3V heater version of the PL38) shows a valve that looks similar to your wider valve.

http://r-type.org/adverts/adv-0217.htm

Yours, Richard
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Old 20th Jun 2024, 9:15 am   #9
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Default Re: Wide envelope PL38?

If it was a stamp or a coin it would be worth a small fortune!
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Old 20th Jun 2024, 10:43 am   #10
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Default Re: Wide envelope PL38?

Quote:
Originally Posted by Mr Moose View Post
Hello,

This advert from 1948 for the EL38 (the 6.3V heater version of the PL38) shows a valve that looks similar to your wider valve.

http://r-type.org/adverts/adv-0217.htm

Yours, Richard
It does look similar. Thanks.
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Old 21st Jun 2024, 11:15 am   #11
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Default Re: Wide envelope PL38?

Quote:
Originally Posted by Richardgr View Post
If it was a stamp or a coin it would be worth a small fortune!
Or this, the very rare reversed yellow print
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Old 22nd Jun 2024, 9:12 am   #12
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Default Re: Wide envelope PL38?

Quote:
Originally Posted by stevehertz View Post
Have you tried breathing on the wording and then turning it carefully in the light? This can often identify otherwise 'missing' legends.
Thanks for the suggestion. I tried this very carefully, but unfortunately, there’s really nothing left to see. The numbers have been wiped clean, and there are no remaining residues that could create any difference from plain glass. Nevertheless, the valve has been positively identified as a PL38.
- Helder
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Old 22nd Jun 2024, 9:21 am   #13
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Default Re: Wide envelope PL38?

Since I believe I have narrowed down the production year of the wide envelope PL38 to 1948-1950, perhaps I should start a new thread specifically requesting information from a data sheet from this period. Could the moderators please advise? Thank you.
- Helder
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Old 22nd Jun 2024, 9:33 am   #14
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Default Re: Wide envelope PL38?

What more is it that you are looking to find out? That the spec is different for that manufacturing period?
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Old 22nd Jun 2024, 10:44 am   #15
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Smile Re: Wide envelope PL38?

Hi,
Sorry to hi-jack this thread, but I used to have a PL81 with a wide body, and often wondered if that was special in any way?
I had no means of testing way back then, but it worked OK in my Bush TV43.
Cheers, Pete.
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Old 22nd Jun 2024, 1:21 pm   #16
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Default Re: Wide envelope PL38?

Quote:
Originally Posted by Helder Crespo View Post
Since I believe I have narrowed down the production year of the wide envelope PL38 to 1948-1950, perhaps I should start a new thread specifically requesting information from a data sheet from this period. Could the moderators please advise? Thank you.
- Helder
Thread moved to "Service Manuals, Circuit Diagrams and Data Sheets Wanted" section.
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Old 22nd Jun 2024, 2:07 pm   #17
Helder Crespo
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Default Re: Wide envelope PL38?

Quote:
Originally Posted by stevehertz View Post
What more is it that you are looking to find out? That the spec is different for that manufacturing period?
Hi. Not more. The same as before, i.e., is 55 mm a standard PL38 diameter in 1948-1950? From 1951 on it’s clearly 45.5 mm maximum.
Thanks

Last edited by Helder Crespo; 22nd Jun 2024 at 2:12 pm. Reason: Missing preposition
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