[Hammarlund] HQ-129-X filament current requirements

[Kenneth G. Gordon]

On 29 May 2019 at 14:50, Frank Barnes wrote:

> 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.

Yes, this is true, but the original designers worked very hard to maximize the S/N ratio. See 
more below

> 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.

Although I do agree that any receiver should have as high a S/N ratio as possible, 
atmospheric noise is the biggest source of noise at this time of horrid propagation 
conditions, and times when that noise is very low are now very rare. 

The 6AC7 is a sharp-cutoff pentode and by that very fact, AGC does not work well with it. In 
addition, the 6AC7 will exhibit much greater intermod from nearby signals, so you might 
wish to take that into account also.

I have an HQ-129X in the queue to be restored here. I plan to leave the 6SS7s in place.

However, in my opinion, the greatest source of noise in any of our older receivers is due to 
the "mixer" stage. The 6K8 in the HQ-129X is one of the noisest "converters" out there, with 
an ENR of something like 240,000 ohms. There are no direct plug-in replacements for that 
tube, base 8K, but you COULD build a plug-in adapter on an octal tube base using two 
7963 sub-mini dual-triodes in the Pullen configuration. ENR using that circuit varies around 
160 ohms in the 75A4, for instance.

> 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?

I would say no, since many people have made that sub and I have never heard of the power 
transformer complaining.

> I'm thinking of adding a small fan to relieve the internal heat...

That would be a good idea in any case.

> Or add
> another small filament xmfr.

That would also help.

> Or maybe a 12.5% increase is not worth worrying about.

I wouldn't.

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

Here are some links to the Pullen and its applications on my website.

http://www.w7ekb.com/glowbugs/rx/Pullen/PullenMixer.pdf

and

http://www.w7ekb.com/glowbugs/rx/Pullen/75A4Pullen.pdf

and

http://www.w7ekb.com/glowbugs/rx/Pullen/Pullen%20Mixer%20from%20RSGB.pdf

and

http://www.w7ekb.com/glowbugs/rx/Pullen/A%20Like-New%20Mixer%20Circuit.pdf

and there are others...

Ken W7EKB