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push pull tubes with mica inserts?

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Author Topic: push pull tubes with mica inserts?  (Read 364 times)
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Posts: 647

« on: December 27, 2021, 02:03:31 PM »

I have a number of these push pull tubes, like the 829B, with and without the MICA INSERTS surrounding the element structures at the top of the glass envelope.

My questions: Mica purpose? HV flashover? Date of manufacture, latter or earlier?

Trying to sort thru the stash and choose one or the other for a build.

I have looked thru the RCA and National Union archives as well GE and no info found.

Any ideas? Great and Thanks!

* 829B_mica.JPG (184.35 KB, 349x580 - viewed 41 times.)
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« Reply #1 on: December 27, 2021, 05:22:11 PM »

The one in the 4D32 is a screen grid bypass.

73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!

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Posts: 647

« Reply #2 on: December 27, 2021, 06:26:19 PM »

Excellent Tom.

So am I to assume that the series of 829B as well 832A and others that have no MICA inserts are also absent this screen grid 2 bypass?

The so called neutralized vs non - neutralized version of these tube series is distinguished by the mica insert? The construction of the tube with the internal connection of the cathode to one side of the capacitor suggests that this is the case.
Patrick J. / KD5OEI
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« Reply #3 on: December 27, 2021, 08:21:37 PM »

I see white ceramic ferrules on the bottom mica insulator of the tube in the image, and 'holes' in the top mica insulator. Those big holes could as well be there because the plate connections in the glass supply enough rigidity without the mica holes having to be tight to grip the plate support rods.

RMA/JEDEC release 347: The 829 is rated 500V on the plate.

RMA/JEDEC release 352: The 829B is rated 750V on the plate.
The max bulb temp for telegraphy and telephone is increased.
On both, the internal screen bypass C is 65pF.

Check if the 829 has no special insulators, and if the 829B has the ceramic insulators. Or, for some other difference in the build that could justify the higher voltage rating.

Similar but scaled differences may exist on the 832/832A if anyone has one handy to examine?.
The 832 and 832A both include the bypass capacitor.

The ceramic ferrules are @ the plate and would not visibly or logically be an electrical part of the screen bypass capacitor. That is probably in the base.. No one looks for it but I guess someone should solve this mystery.

Radio Candelstein - Flagship Station of the NRK Radio Network.

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Posts: 647

« Reply #4 on: December 28, 2021, 11:58:03 AM »

Hi Patrick,

Thanks for the detailed info.

I took a look at the series of 832's as their is more variety in the stash.
Close looks reveals quite a variation.

National Union 832: Mica Insert, No Ceramic FT
RCA 832: Mica Insert, No Ceramic FT
National Union 832A: Mica Insert, Ceramic FT via the Mica Insert at top
RCA 832A: Mica Insert, Ceramic FT via a Metal PLATE on Bottom
RCA 832A JAN: No Mica Insert,  Ceramic FT via a Metal PLATE on Bottom

So the common conditions for this tube are:


832A ALL HAVE CERAMIC FT, some thru the TOP, some thru the bottom
ALL 832A have MICA INSERTS except the RCA JAN unit.

In all cases the cathode pin connects to a bottom metal plate, so I believe
if the bottom plate is one part of the Cneut... all seem to use that connection.

Since the Ceramic FT is always present in the A, I suspect, this along with the
mica insert may be to increase voltage breakdown.
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