[ARC5] Triode mixer mod

[Dennis Monticelli]

Ken,

I looked at the schematic of the Pullen mixer.  It is a common circuit
topology for early transistor mixers.  I have used it myself in the 70's.
We just called it the emitter-coupled mixer and gave no credit to Pullen.
I did not know its origin.

Dennis AE6C

On Fri, Oct 24, 2014 at 11:22 AM, Dennis Monticelli <
dennis.monticelli at gmail.com> wrote:

> Ken,
>
> I will check that out.  The plug-in feature is appealing.  No doubt some
> tracking calib would be in order.   I have found that different 12K8's do
> affect the tracking.
>
> The improvement in MDS is about what I would expect going from a early
> pentagrid to a modern set of triodes.
>
> Dennis AE6C
>
> On Fri, Oct 24, 2014 at 10:12 AM, Kenneth G. Gordon <
> kgordon2006 at frontier.com> wrote:
>
>> Gents:
>>
>> I have been in correspondence with a very competent ham in Europe who
>> contacted me about a modification to the 12K8 mixer, which had been
>> developed back in 1970 by someone in Missoula, Montana.
>>
>> This "mod" is a fairly simple, easily reversible, plug-in mod using a
>> 12AT7 on
>> an octal adapter. I have had it published on my website as a PDF for some
>> time if anyone is interested. It is simply a dual-triode mixer/HFO, and
>> not a
>> "Pullen" mixer.
>>
>> Here are his results:
>>
>> "My unmodified BC-455 has got an MDS of -113 dBm whereas the one with
>> the 12AT7 mixer modification has one of -121 dBm. Measured with R&S
>> (Rhode and Schwartz) sig gen and HP true RMS voltmeter."
>>
>> This is a substantial difference.
>>
>> As he pointed out to me, this would not really make a noticeable
>> difference in
>> normal operation at frequencies below 10 MHz due to normal band noise,
>> but it is certainly interesting, and IMHO, is applicable to other
>> receivers which
>> use very noisy mixers. The advantage in my opinion is that it is a
>> "plug-in"
>> mod.
>>
>> Ken W7EKB
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>