[Hammarlund] HQ-120X Restoration

[Richard Knoppow]

----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: <hammarlund at mailman.qth.net>; "Joe Connor" 
<joeconnor53 at yahoo.com>
Sent: Saturday, February 19, 2011 12:03 PM
Subject: Re: [Hammarlund] HQ-120X Restoration


> Basically, the HQ-120X had the old large pin tubes, then 
> morphed into the HQ-129X with octal tubes, then to the 
> HQ-140X with miniature tubes, and finally into the HQ-150 
> which added a "Q" multiplier.  The circuits in all of 
> those receivers are of the same design.
>
> Glen, K9STH
>
     Check the handbook for the HQ-120-X, they always had 
octal tubes, and metal ones to boot,  the design was too new 
to use the old large-pin tubes. The HQ-129-X replaced the 
6S7RF and IF tubes which had grid caps, with 6SS7 the 
single-ended version, which were developed after the 120 
went into production. It also has some other changes in tube 
types and circuitry but is still essentially the same.
     The tubes with grid caps were made that way to reduce 
grid interelecrode capacitance but at the price of increased 
capacitance and inductance from the leads. Some tubes had 
caps simply because there were not enough pins on the base, 
converter tubes being an example.
     Other than that the time line is correct.
     The first versions of the Super-Pro had large pin glass 
tubes with metal shields. I think the SP-200 series was the 
one that went to metal octal tubes.
     I don't know why Hammarlund chose the 6S7 and 6SS7, 
they were low filament current tubes with somewhat lower 
transconductance than the 6K7 and 6SK7, which makes them a 
bit noisier. Its possible to replace the RF tube with a more 
modern one, say a 6SG7, but they are designed to run at 
lower grid bias for the correct transconductance and may 
also require somewhat different plate load. I have not 
attempted to work this out from the characteristics. If 
plugged straight in I am not sure how much difference they 
would make.
    HF performance has to do with other things than tubes, 
for instance, the losses in the coils and insulation. 
Hammarlund used high-quality components for the time but 
getting good performance beyond about 20mhz was difficult 
then. Of course, some RX (Hallicrafters) went up to 40mhz 
but I think that was more wishful thinking than anything 
else.
    Note that in the versions of the Super-Pro that go 
beyond 20mhz the RF stages are different. They use shunt fed 
coils instead of the series fed ones in the standard models. 
That gives a slightly higher Q. The Q affects gain as well 
as bandwidth. The newer tubes, with much higher 
transconductance than the 6K7, 6S7, and their single-ended 
sisters, have lower noise and potentially higher gain but 
also tend to have higher interelectrode capacitances. Books 
like the famous Radiotron Designer's Handbook have long 
sections on all this stuff.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com