[Hammarlund] HQ-129-X filament current requirements

[Joel Hallas]

Frank,

Yes, that was a rather remarkable design decision by the folks at
Hammarlund, and I have always respected it. While a different tube could
provide improved noise figure, I submit that in a high noise (or most any)
environment, the external atmospheric plus local noise will surely dominate
the receive S/N and any improvement will only be noticeable at the high end
of the 129's coverage range -- say above 20 meters (where it doesn't work
well in any case). 

My recommendation would be to stick with the 6SS7s in all designed
locations. The cost and difficulty of finding a replacement power
transformer would be way more than any benefit of a quieter receiver --
especially since it would just be noticeable with the antenna disconnected.

I doubt that it would hurt to try it for a few minutes while listening to a
weak station -- my guess is that there will be no noticeable improvement.

Enjoy the HQ-129X -- imho it is one of the best two-dial receivers of the
period. Spreading the range over more bands makes such an improvement in
usability that I'm amazed others didn't do the same -- well I guess Halli
did in the SX-28 and later the SX-88, and try to find one!

BTW, due to declining health, both my HQ-129X and a nice HQ-110A-VHF (double
conversion, ham bands only 160-2 meters, with internal 2 meter nuvistor
converter) are available for inspection and purchase 50 miles NE NYC, e-mail
direct for more details. 

Regards, Joel Hallas, W1ZR
Westport, CT


-----Original Message-----
From: hammarlund-bounces at mailman.qth.net
[mailto:hammarlund-bounces at mailman.qth.net] On Behalf Of Frank Barnes
Sent: Wednesday, May 29, 2019 14:50
To: Glowbugs; hammarlund at mailman.qth.net
Subject: [Hammarlund] HQ-129-X filament current requirements

I've restored an old and now very enjoyable HQ-129-X and now it is time to
do some upgrades.  The first is to replace the 6SS7 RF tube with a 6AC7.
 The 6SS7 has a Gm of about 1850 and an Equivalent Noise Resistance of
10,800.  So it does not hear as well as some other tubes and it is noisy.

The 6AC7 is much "hotter" with a Gm of 9,000 and an ENR of less than
1,000.  So it hears much better, is very quiet and is a plug-in swap.  I
have one so that's what I plan to use; I know there are others (e.g., 6GM6
with Gm=13,000)..maybe later.   I have a very high noise level here so any
signal to noise ratio improvement I can achieve will help.

BUT, the 6AC7 filament current draw is 300ma MORE than the 6SS7.   The
total 6.3V filament requirement of a stock rcvr is 2,400ma.   An additional
300ma increases the filament requirement by 12.5%.

My question to those knowing much more than I do, is will a 12.5% increase
be a harmful thing for the power transformer?   Hammarlund built
conservatively but who knows?

I'm thinking of adding a small fan to relieve the internal heat...that
might help the xfmr a bit...or not.   I could replace the 5V rectifier tube
with solid state to help the xfmr a little more, heat-wise.   Or add
another small filament xmfr.

Or maybe a 12.5% increase is not worth worrying about.  I know that many
have changed tubes and increased the filament draw and have heard of no
failures.

Comments?

The mixer tube will be next; after the RF tube is changed, measured and
stable.

Frank
W4NPN



-- 
Frank Barnes
W4NPN
Chapel Hill, NC
Grid Square FM05
Cell 919.260.7955
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