[Hammarlund] HQ129X Distortion

[Skip Frolik]

Interesting,
I had never seen or heard of an HQ129X receiver till a few
years ago. So when I was given one back then I looked it over
and stuck it in storage. Since then I've read a lot about them
and learned they are apparently a pretty good receiver. So I'm
looking forward to eventually getting it cleaned up and using it.
Now that said since I didn't know what I was looking at I just
assumed the mixer was a 9 pin tube type. Cause this unit has
some type of 9 pin miniature tube in that position. And I mean
it looks factory. I know now it's not and I guess even then
I should have wondered about it since all the rest of the tubes
are octal but I didn't. So was there a well known modification
to the mixer on this radio since most everyone seems to say this
is a weak point or do I have a one off mad scientist modification
to this radio ???? I mean it's been a while since I looked at it
but I swear it looked factory so someone must have done a fine job.
Skip
WB4GMQ 

-----Original Message-----
From: hammarlund-bounces at mailman.qth.net
[mailto:hammarlund-bounces at mailman.qth.net] On Behalf Of Kenneth G. Gordon
Sent: Sunday, April 06, 2008 3:02 PM
To: Hammarlund at mailman.qth.net
Subject: Re: [Hammarlund] HQ129X Distortion

I might add that the PRIMARY source of noise in the HQ-129X is that 
stupid 6K8 mixer.

The equivalent noise resistance (ENR) of that piece of junk is at least 
240,000 ohms. From my reading, that !@#$%^&*() thing is the noisiest 
possible mixer ever produced. And from my experience with it, my 
reading is correct.

(The 6BE6 is about as bad.)

This is the sole reason so many HQ-129X users replaced the 6SS7s 
with a "hotter" tube. However, they attacked the wrong enemy. 
Increasing gain AFTER the mixer simply increases the noise from the 
mixer along with the signal so the S/N ratio doesn't improve. 

Then, by doing this, distortion is increased by overloading the detector.

Although they COULD have overcome some of the mixer noise by using 
a hotter RF amp tube, like the 6AC7 for instance, all that extra gain in 
the RF stage results in OTHER hard-to-tame problems. Like reduced 
dynamic range, oscillation at RF, birdies, mixer overload (yet MORE 
noise), etc.

In contrast, the dual-triode 6ES8 mixer used by some 75A4 users has 
an ENR of 160 ohms, and causes NONE of the above-mentioned "side-
effects".

Ask Carl about that one some time. He uses it. 

Mixer noise is also the reason Collins used a triode, the 6C4, as a mixer 
in the R-390(*) receivers.

I SUPPOSE one COULD attempt to replace the 6K8 with a 6SB7Y, 
which has almost 3 times the "conversion-transconductance" as the 
6K8, but it would require doing a bit of socket rewiring: moving three 
wires. Or you could build a plug-in adapter.

Using that tube should drop the noise by a factor three, and noticably 
improve the S/N ratio.

Oh! BTW, this procedure would probably help ANY receiver which uses 
a 6K8 or 6BE6, or any other pentagrid mixer or triode-heptode mixer.

Ken W7EKB
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