[R-390] OT: HP-410B Tubes

[Shoppa, Tim]

5814's (or more often 5814A's) were widely used in lots of instrumentation as preferred tubes and usually swapped out at regular predefined maintenance intervals.

I'm sure there were good brands/batches of 12AU7's too but the 5814A's were often put in there by shops who used them everywhere else already, mostly based knowledge learned from the school of hard knocks.

This is backed up by my largest users of this base tube, my 390A's, where consumer-type 12AU7's in there, I have found to be noticeably less reliable than the industrial-rated 5814A's.

In other words I'd go for the 5814A's every time, never mind the slightly higher heater current.

The 410B manual makes it clear that balanced sections in the dual triodes are desirable, and there was probably some industrial-numbered (4 digit) 12AU7 variant that was graded for balance between sections.

Tim N3QE

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Barry
Sent: Tuesday, March 01, 2011 9:52 PM
To: r-390 at mailman.qth.net
Subject: [R-390] OT: HP-410B Tubes

I have an HP-410B that I have cleaned, recapped and trying to get working properly again.  The problem is with the tubes.

It came with 5814's in place of the 12AU7's.  These work great, but they draw extra heater current and I would like to get 12AU7's in there again.  I ordered a set of 12AU7A's but the meter will not work well at all with them.  It won't stabilize, zero, or do anything else correctly (well, the ON lamp does work, but...) with them in place.

The  pair of 12AU7A's check quite strong on my Hickok 752A (they check about the same uMho values as the 5814's).  Is this just a case of finding a pair of well-matched 12AU7's (non-A, whatever...) that work?  I'd rather not continue throwing money into tubes that don't work but at the same time, I don't like the extra stress the heaters in the 5814's put on the old transformer.

Any ideas?

Thanks,
Barry - N4BUQ

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