[Hammarlund] HQ-129 and 717A mod.

[jeremy-ca]

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. 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. 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.

There was an article by Bill Orr that changed the NC-240D 6SA7 mixer to a 
6SB7Y and he noted a big improvement. 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.

If you want to be ultra sneaky then go to FET's mounted inside metal tube 
shells.

Carl
KM1H




----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: "Hammarlund List" <hammarlund at mailman.qth.net>
Sent: Tuesday, November 06, 2007 9:13 PM
Subject: Re: [Hammarlund] HQ-129 and 717A mod.


>I just did some reviewing of the 717A. Turns out that the
> 713A is an identical tube with a bakelite base.
>
> Also, the 717A/713A has a transconductance of around
> 4000 micromohs, while the 6SS7 has a transconductance of
> less than 2000.
>
> However, the 6SS7 is a remote-cutoff type and will work
> better with AGC, if you want to use that.
>
> I know my old Elmer always took his RF amps off the AGC
> line and used manual control for them.
>
> In any case, the much higher transconductance of the 717A
> WILL help with better sensitivity on 20 meters and above.
>
> I wonder if there might be a tube with the same basing as
> the 6SS7 or 717A, which has even higher yet
> transconductance?
>
> I'd be interested in trying it.
>
> I might take a closer look at my BC-779s too...
>
> Ken W7EKB
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