[Hammarlund] HQ-129 and 717A mod.

[Kenneth G. Gordon]

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