Chuck Harris wrote:
in rec.radio.amateur.boatanchors40283c62$0$3200$61fe ,

: Hi Ross,
:
: It certainly could be that they are very similar inside.
: I have used the 829B, but I have never had my hands on a 3E29.

Ditto. (Well, I haven't actually *used* them yet - they're still NIB! :-)

: Purely speculation on my part, but I would guess that the 3E29 has
: fewer sharp edges inside to help prevent arc over and other funny
: stuff at the 6x increase in plate voltage. I would guess that the
: cathode is made of much sterner stuff, and the vacuum is purer.

I have seen mention of harder vacuum and tolerances in pulse-rated tubes.

Ned, in a 1997 r.a.t post, compares a pulse-rated 4PR65A to a 4-65A, itself
already having a higher vacuum than your average receiving tube, and notes
that the latter wouldn't sub for the former. [http://tinyurl.com/3c5wx]
Elsewhere he mentions 4-400A and 4PR400A. Eimac numbers, I think.

: They make a special point of saying that the 3E29's mu will always
: be within 10% of the spec'd value. That tends to indicate that more
: care was taken with internal tolerances, particularily cathode to
: control grid spacing.

Quite possibly. I hadn't heard the 10%. Top of the bell curve selection?

: The part I find odd is usually when RCA is listing a similar tube
: they will say something like plate curves match 6L6, or some such.
: There is a total absence of mention of the 829B in the 3E29 spec
: sheet.

Exactly my point with the 6146 equivalent examples, all from TT5.
All listed as sharing 6146 data except for that very alike pulse tube 6293.

Yet, again, the ARRL Handbook data section has 3E29/829B sharing.

That German audio retailer with the Chinese amps has them using 829B, 3E29,
or GU-29, implying AF similiarity. There's a clear picture of the GU-29 at:
http://ly1dq.hypermart.net/gu-29.htm (and a simple 2M TX example at
http://ly1dq.hypermart.net/exam/gu-29_01.gif , to stay slightly on topic:-)

: What do you want to bet that the 3E29 also cost 3x the 829B?

(:=}) Quite possible, since military or other radar the likely application.
Plus it likely wouldn't do to say "Just use our 829B, it'll work fine" ;-)

They must have at least been "selected" or perhaps built to better specs.

It wouldn't make sense to be building an essentially identical tube too
differently. A subtly different cathode coating, perhaps, for the different
pulse current requirements? (like starting batteries vs storage) (total
speculation on my part!) and a longer time on the vacuum pump would be easy
to arrange on the same production line, with the build otherwise identical.
Perhaps harder vacuum plus statistical selection alone would be enough?

Ah! I've been scrolling through my files, and found some text on 829B/3E29
I'd saved from the Joenet archives (someone building an amp with one or two)
in which Gary Longrie says "Same tube, but the 3E29 is pulse rated, can hold
off 5kv, being built with ceramic parts."

I don't know how true this is ...

I notice that the UK mil CV2666 has a completely extra top mica disc above
the plates, extending all the way to to the glass, with ceramic inserts, as
well as the bottom metal disk with ceramic inserts like the RCA 829B.

It just occurred to me to do an image search on 3E29. Sure enough,
http://www5b.biglobe.ne.jp/~tritium/rca3e29-1.jpg
http://www5b.biglobe.ne.jp/~tritium/rca3e29-2.jpg
http://www5b.biglobe.ne.jp/~tritium/rca3e29-3.jpg

http://home.snafu.de/appelt/tubes/3E29.jpg

It looks the same as the RCA 829B that I have here, as far as I can see.

Hmm. http://hereford.ampr.org/cgi-bin/tube?tube=3E29
-
"Maximum Ratings (Design Center Values)

Plate Voltage ................................. 750 Volts
Grid No. 2 Voltage ............................ 240 Volts
Plate Dissipation ............................. 40 Watts
Grid No. 2 Dissipation ........................ 7 Watts

For other characteristics and typical operation, see 829B"
_

http://www.r-type.org/exhib/aaa0174.htm shows 829B (& "=" 3E29)

Commentary mentions the superior European double tetrodes, which

"made the 829B and 832 obsolete, and offered greater output power.
This valve type would not have been the valve of choice by 1950.
Currently, designs exist to use all of the VHF double tetrodes as audio
amplifiers, these are physically beautiful devices and deserve to be seen."

: Anybody have a 1950 price sheet?
:
: -Chuck Harris

That would be interesting, too.

RdM

[obviously with too much time on his hands right now, but likes his 829B!]