[ARC5] Quick sensistivity comparison

[Richard Knoppow]

----- Original Message ----- 
From: "Geoff" <geoffrey at jeremy.mv.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; "ARC-5 Mail 
List" <arc5 at mailman.qth.net>
Sent: Thursday, June 13, 2013 6:29 PM
Subject: Re: [ARC5] Quick sensistivity comparison


>
>
>>
>>>
>>>
>>>
>>> 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
>
>
>
    Thank you very much for the detailed description.  I 
might try something like that on a SP-600  The couple I have 
meet the published noise spec but are no better.


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