[ARC5] ARC-5 receiver mixer mod attached as PDF

[Richard Knoppow]

    When I got my BC-779 it was modified to circuits shown in a CQ 
magazine I actually had. I have forgotten the name of the fellow who 
wrote it but he wrote a number of surplus conversion articles.  The mod 
was to replace the RF and mixer stages with twin triodes. I think he 
used 6SL7s.  The set worked but seemed to have low gain. I had no way to 
measure it at the time. I decided to restore the original circuit. Not 
difficult as it turned out. Much hotter. I experimented later with 
various mixers and RF amps and discovered that the mods cause loading of 
the RF circuits. The oscillator tracking was affected and I think also 
RF tracking. I wound up with the original circuit despite its being 
noisier. There are two worthwhile mods to the old Super Pros, one is to 
add a voltage regulator to the oscillator. change the plate resistor 
from 12K to about 8K and put a VR-150 across the plate. This will stop 
oscillator pulling when the RF gain is varied. Second mod is to add a TC 
cap across the tuning capacitor. This was done on production SP-400 
receivers and is shown in late schematics. I don't remember the value 
but about 4 or 5 pF and N750 or higher if you can find one. This will 
substantially reduce warm up drift. These are very fine receivers.
     The Pullen mixer is a combination of two triodes with a common 
cathode, one operated as a cathode follower and the other as a grounded 
grid amplifier. The versions I've seen and used worked with a separate 
oscillator.  This may be a combination of a triode oscillator and triode 
mixer with common cathode injection. There are very many variations on 
this theme. A common arrangement is to use a combination triode pentode, 
like the 6U8, with the triode connected as an oscillator and the pentode 
connected as a pentode mixer with signal to the grid and oscillator to 
cathode with the two cathodes connected together. This has reasonably 
good conversion Gm and fairly good oscillator isolation.

On 1/3/2017 10:53 AM, Kenneth G. Gordon wrote:
> On 3 Jan 2017 at 10:51, Roy Morgan wrote:
>
>>
>> Ken,
>>
>> Perhaps it's the Pullen Mixer you refer to.
>
> No. This is a single-page of information written up by my successor in a rather strange but
> interesting job I had with a psychology prof in Missoula, Montana back in the 1970s. The
> info is dated 1974 by a Jim Ince.
>
>>   In any case, I'd be glad to get a copy of the article, to
>> see if it is one I may not have.
>
> I can 100% guarantee you don't have this one unless I sent it to you. :-)
>
>> I have a BC-779 that is scheculed for overhaul. It  might have such a  modification in it already.
>
> That would have to be a completely different one. Anyway, I have attached the info to this
> reply. It won't make it through the forum, but it will come directly to you.
>
> Ken W7EKB
>
>
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>    ---- File information -----------
>      File:  ARC5MixerMod.pdf
>      Date:  4 Jun 2005, 7:32
>      Size:  46367 bytes.
>      Type:  Acrobat
>
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL