[Hammarlund] Old Hammarlunds

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: <Hammarlund at mailman.qth.net>
Sent: Tuesday, April 12, 2011 5:20 PM
Subject: Re: [Hammarlund] Old Hammarlunds


> On 12 Apr 2011 at 14:15, Richard Knoppow wrote:
>
>>     I used a BC-779 as my station receiver for some 
>> years. I
>> used several experimental LO and RF circuits in it at one
>> time or another.
>
> Could you describe those for us, in general?

     When I got the RX it has been modified in accordance 
with an article in CQ magazine of some years earlier. The 
mod used 6SL7s as common cathode pairs and both RF stages 
and the mixer stage were so modified. The gain was far down 
from what it should have been and I suspect the loading on 
the tuned circuits was increased, not sure of that. I 
restored it to original and then made adaptor sockets for 
the RF and mixer stages so that I could experiment. I 
modified the LO to an ECO design copied from a General Radio 
frequency meter. Very stable but I think now the output was 
much lower than the original. Of course, being in my teens I 
had no instruments. I tried several variations of RF stages 
and mixers including cascode types for the first RF with 
manual gain on it to prevent overload. I wound up with 
6BA6's for the RF and a 6BE6 for the mixer, very 
conventional but I found that the dynamic range and overload 
resistance was better than the more exotic circuits. I 
eventually restored it to original including the oscillator. 
I then discovered how much the oscillator varied with B+ 
changed caused by the RF gain control and AVC. I had used a 
VR tube for the ECO and went back to it. The arrangement of 
the B+ to the tube must be changed a bit to maintain its 
output so that the conversion gain of the mixer stays up.
    While the 6BA6 tubes undoubtedly have lower noise than 
the original 6K7's I think there is a problem with loading 
due to the length of leads etc. The ECO definitely affected 
the dial calibration and probably RF tracking, which is 
normally very good on these RX. So, I think the only mod I 
would recommend is adding the VR tube to the LO. Ideally, 
the screens of the mixer should also be regulated  but that 
gets more complicated.
    It should not be difficult to add a product detector and 
slow AVC which would make the RX a very good SSB receiver. I 
used it for SSB using manual gain control and it was quite 
satisfactory.
    One feature of the Super-Pro is that the BFO is 
completely isolated from the AVC allowing rather high 
injection levels without desensitizing the RX.
    I suspect that if it had not been for the war time 
contracts Hammarlund would have refined the design. As it is 
the SP-400 has some decided improvements but I think by that 
time the SP-600 was already in the works so not much effort 
went into updating the older products.
    I think Collins rather surprized everyone else with 
their 75A and 51J receivers, which were very innovative at 
the time and essentially obsoleted conventional design. I 
don't think National, Hammarlund, or Hallicrafters ever 
really recovered from that and competition to Collins 
eventually came from smaller companies like Drake and Racal.


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