[Hammarlund] HQ-129 and 717A mod.

[jeremy-ca]

Ah yes, your long love affair with those subminiatures. I dont have any 
listings on those little beasts.

I wonder how they would hold up under strong signals?

For that matter why not a pair of 6CW4's or 7586's ? The latter especially 
are rather available.

Heathkit certainly had superb results with the 6HS8 but I appreciate your 
comment on individual triodes. Isolation between sections is important.

I think I already mentioned the 7F8 as used by Hallicrafters in the 
SX-42/SX-62 as a nice dual triode performer well into the VHF range. At 
least a loctal socket fits in an octal hole so no metalwork required.

All I need is a few 36 hour days to get some evaluations started much less 
completed!

Carl
KM1H


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: "jeremy-ca" <km1h at jeremy.mv.com>
Cc: "Hammarlund List" <hammarlund at mailman.qth.net>
Sent: Wednesday, November 07, 2007 11:51 AM
Subject: Re: [Hammarlund] HQ-129 and 717A mod.


> On 7 Nov 2007 at 8:35, jeremy-ca wrote:
>
>> A couple of things Ken.
>>
>> The 717A has the mu of the 6SG7 but its really a VHF tube and it
>> performs much better on the higher frequencies.
>
> I figured as much.
>
>> It still allows AVC
>> action but Pin 3 on the socket must be lifted off ground. The 717A has
>> 2 cathode pins and the suppressor grid is tied to the cathode
>> internally. The 6SG7 has the suppressor going to Pin 1 which is the
>> shield.
>
> Yes. I noticed that from the tube data I compared, thanks! :-)
>
>> In the 129X the 717A is operating slightly above the maximum
>> plate voltage but I dont expect it will be a problem. Its easy to add
>> a resistor to bring it down. However "Typical" operation has both
>> plate and screen at 120V so it might be interesting to see what level
>> gives the best S/N.
>
> As per our recent discussion on lower plate voltages in receivers! Yes.
>
>>
>> There was an article by Bill Orr that changed the NC-240D 6SA7 mixer
>> to a 6SB7Y and he noted a big improvement.
>
> I didn't know Bill did that. But then again, I sure was never able to find
> everything he ever wrote either. He was a very prolific writer.
>
> I did the 6SA7 (actually a 12SA7) to 6SB7Y swap in, of all things, a
> Hallicrafters S-41G, and it made a tremendous difference in that radio.
> The only real difference between the two tubes is that the 6SB7Y has a
> little more than twice the conversion transconductance as the 12/6SA7,
> and thus half the noise and twice the gain. It was well worth the effort.
>
>> It might be worthwhile to
>> look at if you dont want to change sockets for a 6X8. If I were going
>> thru the work of a socket swap Id go for the 6HS8 Pullen circuit and
>> be done with it, Google "pullen mixer" for several examples.
>
> Ha! I am one of the main proponents of the "Pullen Mixer"! Hee hee!
>
> I have quite a lot of information on that circuit up on the website that I
> take care of (sometimes) at http://www.mines.uidaho.edu/~glowbugs/
>
> I got into an argument with Chris Trask about it some time ago. He
> insists that I test it thoroughly to confirm its characteristics before he
> will believe anything about it, which is certainly a good idea, but I have
> not had the time to do it for several years now. It is another of my
> projects lined up for when I can retire.
>
> There are two problems with most of the implementations of the Pullen
> mixer which I have seen: 1) most people don't understand that Keats
> Pullen suggested use of TWO dual triodes for his circuit: one for the
> cathode-coupled mixer itself, and a second one for the HF oscillator
> with a cathode-follower output to the mixer part, which eliminates
> pulling on the oscillator, and 2) that in order to get the best
> performance out of the circuit, you must use tubes with very high
> transconductance.
>
> The tubes of choice are the 6ES8, which is a variable-mu dual-TRIODE
> of high transconductance, and the 7963 which is a subminiature dual
> triode with a transconductance of 13000 micromhos, and a 6.3 VAC
> filament. Two 7963s could be easily wired into an empty octal base and
> simply plugged into the socket for a 6SA7 or equivalent to try it out.
> There would even be enough room for resistors and disk-ceramic caps.
>
> The 6ES8 version was used in a modification for the 75A4. Total ENR
> is 160 ohms, which is incredibly low for a mixer, most of which have an
> ENR up in the 100,000 ohm range!
>
>> If you want to be ultra sneaky then go to FET's mounted inside metal
>> tube shells.
>
> Well... :-)
>
> Ken W7EKB
>