[ARC5] Receiver Tube Substitution

[Richard Knoppow]

     I think you may be right about Lee not being influenced by Pullen 
but I am not sure when Pullen began to publish. The Lee mods were 
c.1948, I think but maybe later. I think he was trying to get the noise 
down but I wonder if he had the necessities to make valid noise 
measurements. Lee wrote a lot of articles about modifying many different 
pieces of equipment. I don't know if he was a graduate engineer and I 
don't know how many details he left out of his designs. I mean stuff 
like the effect of tube loading on the Q and selectivity of the RF coils 
and attendant problems with spurious responses, etc. The Super-Pro as it 
stands is almost free of spurs and birdies. It was about the best of 
contemporary receivers with the possible exception of the RCA AR-88, 
which was not generally available (but was probably known to Lee). 
There was another mod for changing the amplifier to a pentode connected 
one with considerably more power output. It turns out that for cathode 
bias the required plate to plate load is the same for both with about 
double the power output for the pentode connection. All out of the RCA 
tube handbook. I am not sure any more but don't think mine was modified 
this way, just the RF stages.
     All this was hot rodding as in it goes fast but doesn't steer so well.

On 12/19/2016 12:23 PM, Kenneth G. Gordon wrote:
> On 19 Dec 2016 at 11:54, Richard Knoppow wrote:
>
>>     What you describe is what was done to the set I saw for sale at a
>> local swap meet. I didn't know that extreme mod had been in QST.
>
> Yes. I have PDF copies of all those articles.
>
>>      My BC-779 was also modified when I got it but a much less extreme
>> one. I think this was also a Lee mod because I had seen it in CQ. The RF
>> and mixer stages were changed to cathode coupled triodes using 6SL7s.
>
> I remember that one: I never could figure out the advantage to that. I think my hacked up
> one has those in it too. I haven't looked that closely at it yet.
>
> One of my three BC-779s is stock and complete with original power supply. One is that
> hacked-up Lee mod, and the other one is in really terrible, but stock, condition. I think the
> last one is restorable, but it is going to take a lot of work. I keep the Lee-mod mainly to see
> what, if any, sort of "improvement" those actually made. I doubt if they were really much
> help...as if the Super Pro needed any "help"...other than for short-term stability...
>
>> It worked but the triodes loaded the tuned circuits too much.
>
> I shouldn't wonder.
>
>> I restored it to the original circuit and found it much better.
>
> Undoubtedly...
>
>> I think Lee was impressed by some early articles by Keats Pullen.
>
> I am not so sure about that, but in any case, IMHO, Lee misinterpreted what Keats was
> trying to accomplish.
>
>> The common cathode approach works fine if the whole circuit is designed
>> for it.
>
> I wouldn't use the cathode-coupled circuit in an HF frequency RF amp stage, but in the
> mixer, especially when the circuit is arranged so that the transconductances of the two
> triodes are properly proportioned as Keats originally intended, it is a really good circuit. I
> think that the "Pullen" mixer is so little used because it was not very well understood. Also, I
> have seen very few instances in which it was implemented as Keats intended. About the
> only one I am familiar with which is done properly is one done for the 75A4 using a 6ES8, as
> I remember it. ENR for that one is 160 ohms. There may be mixer circuits with a lower ENR,
> but I don't know of any...
>
> The "Pullen" mixer also makes an excellent product detector, BTW.
>
> The "Pullen" doesn't "like" wide variations in input impedances either, which is why it
> doesn't work well in a receiver with no RF amp stage.
>
> Ken W7EKB
>
> ---
>
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL