[ARC5] Receiver Tube Substitution

[Kenneth G. Gordon]

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

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