[Hammarlund] HQ-129-X filament current requirements

[Frank Barnes]

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