[Collins] 75A4 Roofing Filter ?

[Carl]

The 7360 conversion and lately the Pullen is the best of the tube 
versions.

The Pullen can use a 6ES8 for best performance but even a 12AT7 is 
decent.

Then change the RF to a 6GM6 and a 75A4 will run circles around most 
riceboxes for MDS and IMD.

If your stuck with a 6BE6 radio then a 6J6 in a Pullen will really make 
you sit up and listen.

And a 6SN7/6SC7 for you octal guys.

This is for those who really want to use their radios and not just get 
on a SSB net for an hour.

Carl
KM1H



----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
To: <collins at mailman.qth.net>
Sent: Friday, January 23, 2009 10:47 AM
Subject: Re: [Collins] 75A4 Roofing Filter ?


> On Thu, 2009-01-22 at 23:49 -0800, Adam Farson wrote:
>> Hi John,
>>
>> The architecture of the 75A-4 is such that a "roofing filter" in the
>> modern sense is an invalid concept.
>>
>> The 1st mixer down-converts the RF signal to a 1.5 - 2.5 MHz variable
>> 1st IF. The 1st LO is crystal-controlled; the bandswitch selects the
>> appropriate crystal. The 2nd mixer (excited by a PTO tuning 1955 -
>> 2955 kHz) down-converts the variable 1st IF to a fixed 455 kHz IF. 
>> The
>> 2nd IF filter is a 455 kHz Collins mechanical filter; this is the
>> selectivity filter, not a roofing filter.
>>
>> http://www.collinsmuseum.com/75a4.html
>>
>> I hope my explanation is reasonably accurate, and helpful (I am not a
>> Collins owner, but would love to have an HF9500 in my shack!)
>>
>>
>> Cheers for now, 73,
>> Adam VA7OJ/AB4OJ
>>
>>
> That's all correct. What I was going to say when I got around to it.
> With different frequencies the explanation also applies to all the 
> 75A,
> 75S, 51J, 51S, and R390() receivers. The broadband exposed mixers are
> the weak points in all these receivers, the multigrid tubes make noisy
> mixers and have limited dynamic range. Though the RF bandpass filters
> that are often tracked with the LO are not nearly as wide as DC to 30
> MHz often seen in the recent receiver designs that upconvert to a
> roofing filter at 45 MHz. The tuned RF stage definitely helps keep out
> strong signals from other band. It doesn't help keep out the KW 
> station
> down the block in the same pile up.
>
> It is possible, and there are numerous articles about, to work over 
> the
> mixers in these receivers to improve dynamic range. W0MLY (now SK) 
> used
> to sell a solid state mixer for the 75S receiver, though he and I
> disagreed on which mixer was most critical. One published mod for the
> 'A4 replaced the first mixer with a 12AT7 long tail mixer circuit that
> was lower noise, but also lower gain.
> -- 
> 73, Jerry, K0CQ, Technical Advisor to the CRA
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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