[Hammarlund] HQ-129-X Mixer Noise

[Kenneth G. Gordon]

On 22 May 2019 at 11:57, Frank Barnes wrote:

> I'm new to the group and have an old subject which I'm sure has been
> discussed multiple times.   I've restored an HQ-129-X and I really like
> it.   It was a lot of work to bring it back!
> I really like the quality and love the 3-IF system.  I'll use it mostly on
> 40M CW and for some general SW listening.
> However it seems to have a lot of "noise" - sort of a background hash.
>  More than some other receivers I've used, including my Halli S-40B.
> Disconnecting the antenna eliminates the noise so I think that means I'm
> hearing noise radiated from power lines, appliances, etc.

First of all, if removing the antenna removes the noise, then the noise you are hearing is 
NOT from anything internal to the receiver.

I have no idea how often you have been operating on the various bands of late, but I have 
been very active, mostly traffic handling, and mostly by CW, and I can assure you that the 
bands have been particularly noisy of late, and propagation has been dismal, at best.

> Am I hearing mixer noise?

It does not sound like it to me.

>  I've read about replacing the 6K8 mixer with a
> very expensive 6SB7Y (requires some rewiring).

Well, while the 6SB7Y is a good upgrade from a 6SA7, for instance, since the 
base-connections to those tubes are identical (8R), and the 6K8 is possibly the noisiest 
"converter" tube ever manufactured (something like 240,000 ohms ENR), before you decide 
to make any substantive changes to the HQ-129X, you really should get some accurate data 
of what you have now. 

I would measure the actual sensistivity on all bands with an accurately calibrated and 
connected signal generator, detailing your actual signal-to-noise ratio in the process. Then 
after you have that data, read up on how to improve that, if you really need to.

> Does anyone have advice regarding quieting the mixer noise, other than tube
> replacement?

Well, if you really want to reduce mixer noise to the absolute minimum, you might consider 
building a plug-in adapter using two 7963 submini dual triodes in the Pullen-mixer 
configuration. Here is some data on the Pullen mixer on my website:

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

and

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

and

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

and

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

and

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

You could build such an adapter onto an octal tube base, including any resistors that would 
be needed, and then try it out. You may have to peak the following IF stage using this 
conversion.

Notice that the ENR (Equivalent Noise Resistance) for the Pullen conversion of a 75A4 is 
160 ohms vs the 240,000 ohms of the 6K8. This is a really vast improvement.

Or, you could build a simple adapter for the 6SB7Y using an octal tube base, and octal 
socket, which would preclude your having to rewire anything. The 6SB7Y has twice the 
conversion-transconductance as the 6SA7 for instance.

>From my experience, the 6SB7Y does improve mixer noise issues, but the tube is also 
somewhat more delicate than other metal octals. If you drop it on, say, a concrete floor, the 
filament will be destoryed. 
 
> I'm also interested in replacing the RF tube with a better one - 717A?
> 6AC7? 6BZ6 mini?  Any advice/experience?

This can be done, but you have to be very careful of which tube you use. The 6AC7, while 
exhibiting much lower noise than some other RF amp tubes, is also a sharp-cutoff pentode 
and can, by that very fact, cause far more intermod and overload than the remote-cutoff 
types. However, if you do not have any really strong signals in your vicinity, that CAN be a 
good sub.

You could try the minis by simply making an adapter. 

Carl KM1H has vast experience on tube subs and can give you about the best advice that 
way as anyone.

As I remember it, he uses the 717A in such places.

Again, as I remember it, the 717A is simply a 6AK5 in an octal base.

vy 73 for now,

Ken W7EKB