[Boatanchors] Tube reciever design banter

[Drew P.]

>From an exchange between "Match" and Carl:

Carl: "Dual triode mixer in a Pullen circuit. 6ES8 preferred as its a very
unique variable mu tube."

Match: I've not yet tried a Pullen... I have an SP-600 that came to me with a 6J6 in the 1st mixer position, which I suspect may be a Pullen mod, but I've not looked at it to see.

Carl: It could be a regular dual triode mixer also, the Pullen hasnt received much attention until recent years.

I for a while plugged in a 6J6 to replace a 6C4 in my R-390A, simply becoause I was too motivationally-challenged to go get a 6C4.  The correct pins were in the correct places to use just one triode section of the 6J6 and leave the other unused.  What can I say but that it worked...perhaps the 6J6 was substituted for the 6C4 in a similar fashion in the aforementioned SP-600.

It would appear that a drawback of the Pullen Mixer is the need to actually use 3 triodes - two in the original cathode-coupled mixer circuit, and a third as a cathode follower to avoid loading of the L.O., unless the L.O. is designed to work into the nearly unbiased grid of the mixer section, with its attendant low impedance.  Pullen described use of 4 triodes: 2 in the cathode coupled mixer, one as L.O., one as the aforementioned L.O. buffer. This arrangement would usually be implemented with a pair of dual triode tubes.

I have an SP-600 and at first, the Pullen seemed a good (not hacking up the metalwork) mod candidate, but it would not be so easy (without metalwork) to add the cathode follower L.O. buffer.  The subsequent least-metalwork plan I've come up with is to rewire the 7-pin 6BE6 mixer socket as a 6J6 Pullen Mixer, and replace the 7-pin socket of the original 6C4 L.O. with a 9-pin to accommodate a twin triode tube (cathodes not internally tied together as with 6J6) as L.O. and buffer. Haven't done it. SP-600 still apart in mid-recap.  Too many other more pressing 'Round Tuits, punctuated by an endless barrage of 'Tall Penzons.

Carl went on to say:

"Its been known for decades that reducing B+ to the low level stages has
minimal or beneficial effect on S/N, cuts down heat and tube noise in later
stages. In the late 60's I had a HRO-5 and both the 180V and 250V supplies.
I was amazed at how much better it worked at 180V even on 20M, so much of
the hiss was gone. James Millen made the suggestion in a 30's QST that if
you didnt need speaker volume it was better at 180V. It took manufacturers
until the 60's to wake up, most likely led by Racal."

I have a Heathkit SB-101 and have long thought that the 300V B+ used for most of the receiver stages (and most transmit stages also) seems a bit high for best tube longevity.  I've contemplated mods toward B+ reduction.  Ditto for the SP-600.

Drew