[ARC5] Receiver Tube Substitution

[Kenneth G. Gordon]

On 18 Dec 2016 at 16:39, Richard Knoppow wrote:

>     I had assumed this was mistyping. I also said 6K8 but it is a 6L7, 
> another noisy mixer.

Yes. I made about the same mistake. I was operating from memory, which is always 
dangerous...

>     I have seen some seriously hacked Super-Pros. One had half the audio 
> amp removed and a power supply tacked on the main chassis. Not 
> restorable.

Yes. I have one of those: that was a series of very involved modifications under the title 
"The Ultimate Super Pro" by Lee and published in QST. The only modification in that entire 
series which had any value at all as far as I was concerned was the addition of an antenna 
trimmer, and even that was "iffy".

>  I did some experimenting with the RF stages on mine but all 
> the modifications were done on plug in modules and I eventually restored 
> the original circuit. I tried several different types of mixers and RF 
> amps but found they made little difference, mostly for the worse. One 
> mod that is worth wile is to add a 150V regulator to feed the plate of 
> the oscillator and adding a TC cap across the oscillator section of the 
> tuning cap. The TC cap was added by Hammarlund in the last model so its 
> not really a mod.

I agree.

> I used my BC-779 for SSB and did not have problems 
> with low BFO injection, however, you can't run the RF gain at maximum.

I think, in retrospect, MY issues with severely low BFO injection was a bad component in the 
circuit somewhere and was not inherent in the design.

I didn't know any better at the time, though, thus my PD modification, which turned out to be 
fortuitous.

As far as mixers are concerned, in my opinion, the so-called "Pullen" mixer has great 
potential, and is about the best there is, but it needs a good RF amp ahead of it. 

Keats used a triode HFO followed by a cathode-follower to reduce or eliminate any pulling, 
then his cathode-coupled mixer stage worked very well.

I think that setup results in the lowest possible noise from a mixer stage.

Ken W7EKB

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus