[Hammarlund] HQ-129-X Mixer Noise

[Richard Knoppow]

     Even different remote cut off tubes may have quite different 
characteristics. For instance bot the ancient 6J7 and 6SJ7 
require about -40 volts to cut off while the 6SG7, a much quieter 
tube, requires only about -20 volts. The 6BE6, which has similar 
performance to the 6SG7 also requires about -40V for cut off. 
Again, the 6SB7Y is a much quieter mixer than the 6SA7 but has a 
lower bias requirement. In a straight across change in my RCA 
AR-88, which uses the 6SA7 a 6SB7Y has just about the same gain. 
It may be quieter but not noticeably without measurements. Its a 
very good mixer designed originally for FM receivers but was 
displaced by the 6BE6 and the 6BA7. The 6BA7 is one of the best 
of the multigrid mixers but is a 9 pin miniature and requires a 
different bias than a string of 6BA6 tubes. For various reasons 
one seldom finds the 6BA7 in receivers where the 6BE6 is 
extremely common although one of the noisiest.
     Quite a long time ago I experimented with various RF and 
mixer stages in a BC-779 (1940s Super Pro). I tried a Pullen 
mixer, all sorts of RF tubes for the first RF including the 717 
and a couple of cascode stages. Eventually I went to the 6BA6 and 
6BE6 because they gave me the best cross talk performance.
    I also found after restoring the original circuit that the 
old fashioned tubes evidently had less load on the RF 
transformers and gave me somewhat better RF selectivity. None of 
this was formal but based on performance in use.
    The effective loading of the circuits by some high Gm tubes 
is often not taken into account. It seems that straight across 
substitution is not as simple as it may seem.
     I am reminded BTW, that I have some HQ-129 questions but 
will post them under a separate subject heading.

On 5/22/2019 9:46 AM, Kenneth G. Gordon wrote:
> On 22 May 2019 at 9:27, `Richard Knoppow wrote:
> 
>>      If the noise disappears with no antenna its coming from
>> outside the receiver.
> 
> Correct.
> 
>> While the mixer is a noisy one the RF gain
>> should overcome it.
> 
> And in the HQ-129X, it does just that.
> 
>>      The RF tube is relatively noisy but replacing it will require
>> re-designing the AVC. The 717 was an octal base prototype of the
>> 6AK7. Its a sharp cut-off tube that does not work well in AVC
>> controlled stages.
> 
> And this is another reason to be wary of using the 6AC7: it is also a sharp-cutoff type.
> 
>> Note that below about 20Mhz the atmospheric noise will be
>> greater than receiver noise.
> 
> Yes, but even there, there are times when the atmospheric noise is at a very low level.
> Which is why I have always worked for as low receiver noise as possible.
> 
>> You need a calibrated signal generator and known dummy load
>> to check the receiver sensitivity. That is, how much output it
>> has for a given signal strength. From that you can figure out the
>> amount of noise.
> 
> Agreed, and if you don't own such gear, someone near you surely does. Ask them to help
> you with this.
> 
> Ken W7EKB
> 

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL