[Hammarlund] HQ129X Distortion

[jeremy-ca]

There were several mixer mods in the ham rags thru the 60's. A popular one 
used a 6X8 (6U8A/6EA8). It could also have been an adaption of any number of 
other circuits with a 12AT7, 7360, etc.
In recent years the Pullen mixer has had a renaissance with the 6ES8 being 
an excellent choice.

A less complicated swap for an original 6K8 is the 6SB7Y which gives a nice 
improvement but not earthshaking. BUT then change the RF amp to a 717A and 
the old girl will wake right up and compare(or excel)to the 140X and similar 
miniature tube offerings.

Carl
KM1H



----- Original Message ----- 
From: "Skip Frolik" <frolik at gulftel.com>
To: <Hammarlund at mailman.qth.net>
Sent: Sunday, April 06, 2008 10:03 PM
Subject: RE: [Hammarlund] HQ129X Distortion


> 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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