[Hammarlund] HQ-129-X filament current requirements

[Richard Knoppow]

    The 6AC7 is not a straight across replacement for the 6SS7. 
Have a look at its grid bias characteristics and at its 
interelectrode  capacitance. It will load the RF transformer and 
broaden it meaning you will loose some image rejection, which is 
minimal to begin with. It is a sharp cut off tube meant to be 
used as a video amplifier. It is not a good  choice where AVC is 
used.
    I don't know why Hammarlund chose the 6SS7. Its advantage 
over the more common 6SK7 is somewhat lower filament current, 
meaningless in the HQ-129-X.  There are certainly better tubes 
but they will change the gain structure of the receiver and are 
not direct replacements.
    If you want to experiment you can try a 6SG7, but again, its 
bias requirements are rather different so something will have to 
be changed in the AVC distribution. You can also try various 
cascode arrangements, they are far quieter than any pentode but, 
again, will have more gain and other effects. This is not a 
trivial problem. For some guidance see the changes Hallicrafters 
made in some of their receivers of the time in going from a 6K7 
or 6SK7 to a 6SG7 as the front end. They did this with the S-40 
series when redesigning the S-20R, which is essentially the same 
receiver. I don't know what changes were made in the RF coils, 
there may be specs somewhere. I think the did the same thing with 
the SX-28.
    I did some experimenting long ago modifying a BC-779, a 
version of the Super-Pro and found the best combination for the 
RF stages was the 6BA6 for the RF stages and the 6BE6 for the 
mixer, a common combination. I would probably have tried the 6BA7 
as a mixer but didn't. I tried a cascode but found it had too 
much cross modulation. Also a Western Electric 717A in the first 
RF, same problem, its quiet but is a sharp cut-off tube. Same as 
the 6AK5.
     You can't do any damage by just plugging in tubes and may 
find something you like better than the original but be careful 
that a big change in gain doesn't fool you that the noise is any 
lower or that some other problem has happened like easy 
overloading. In any case a 6SG7 is a better choice or, if you 
want to make an adaptor for a 7pin miniature to octal base try a 
6BA6 which has AVC characteristics closer to the 6SS7 and 6SK7.
     One problem with replacing vacuum tube rectifiers with 
silicon diodes is that the voltage drop through them is much 
less. That raises the B+ which can increase heat dissipation. No 
magic answers.
    Check your line voltage, if as in many areas you have high 
line voltage you may get rid of some of the heat by using a 
bucking transformer on the line input. These receivers were 
designed for line voltages around 115VAC, often now its closer to 
120 or even 125V. If its really high a bucking transformer will 
make the receiver run cooler and last longer.

On 5/29/2019 11:50 AM, 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.
> 
> 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
> 
> 
> 

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL