[ARC5] Quick sensistivity comparison

[Geoff]

>
>>
>>
>>
>> Since you didnt specify mode or receiver bandwith those statements are 
>> rather ambiguous.
>> When dealing with vintage receivers that have only fair to poor IF 
>> filtering I use the AM test for a 10dB SNR at 30 MHz at maximum bandwidth 
>> for the models mentioned and even .25 to .5uV is a very rare occurance. 
>> For CW I crank in whatever maximum selectivity is available and of course 
>> sets with selectable filters will have different results. And then 
>> measure MDS which is described as 3dB above the noise but many, myself 
>> included, can hear well below that.
>>
>> There is also minimal shielding inside most vintage receivers, and 
>> impedance mismatches that add to the generator and cable issues.
>> In fact using the lowest range of a generators attenuator is not 
>> considered good practice and using an external lab quality step 
>> attenuator and a much higher generator signal results in the expected 
>> accuracy with all other things considered.
>>
>> A HQ-140X fully overhauled but unmodified will have a hard time reaching 
>> 1 uV on AM while a slightly modified HRO-60 is an easy .25uV. Radios such 
>> as the 51J and R-390 families are rather poor at 30 MHz.
>> I rate my minimally hopped up HRO-60 as the most sensitive on 10M AM of 
>> the over 100 tube radios I own and a fully overhauled NC-300 with a 
>> selected 6BZ6 and 6BA7 a very close second. I do a lot of 10M AM 
>> operating.
>>
>> I disagree about SS having lower sensitivity at HF and it has been proven 
>> many times by many people. The HRO-500 and others without PLL phase noise 
>> are extremely sensitive with very low internal noise. Those with a PLL 
>> vary considerably but most are very sensitive on a test bench. It is when 
>> the wide open untuned front ends meet hundreds/thousands of strong 
>> signals that the PLL becomes a noise generator.
>> You can hear the noise during a contest by monitoring outside of the ham 
>> band and the backround noise will rise at the contest start and fall at 
>> the end.
>>
>> Carl
>>
>     I agree with all this. I am curious what the minimal modification to 
> the HRO-60 is.  The stock receiver uses 6BA6 RF amplifiers feeding a 6BE6 
> mixer.  This is the same arrangement used on the SP-600-JX and many other 
> receivers. Whatever you have done has lowed the noise very considerably 
> from what I get on these other receivers.
>     30 mhz performance has a lot to do with losses in the RF stages. 
> Evidently the HRO has very low loss coils an insulators, etc.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com

After a full rebuild and alignment with selected 6BA6 and 6BE6's to set the 
benchmark I wound up with a 6GM6 at the first RF and 6BY6's for both mixers. 
A 6GM6 at the second RF was overkill so I went back to the 6BA6.
I suppose a pair of 6BZ6's would have done as well but the second RF stage 
gain would have to be reduced even more from stock, that stage is just there 
to add RF selectivity and overcome circuit losses.

A few resistor values were changed to get the 6GM6 withing tube manual 
typical voltage specs at 150V and a pair of resistors were used to stop 
oscillations likely due to the long lead to the coil box. A 22 Ohm at the 
grid and 100 at the screen bypassed at both sides with .01's standard taming 
procedures.

Several have done similar with SP-600's and just swapped tubes and report a 
big improvement. The 6BY6 is a better tube for high signal levels and it has 
much less noise. Developed for early FM radios and soon surprassed with 
better choices.

Carl
KM1H