[Boatanchors] Tube reciever design banter

[Carl]

Good point on the 6J6 Drew.

Im a little skeptical about the extra buffering required for the Pullen. IMO 
it depends upon the LO regulation and the signal voltage available as well 
as the quality of the LO sinewave or lack of.

A 6SN7 makes a good Pullen in a WW2 SuperPro with no buffering at least up 
to its 20mc tuning limit. I plan to see how that works in a SX-28 which is 
known to be LO level shy on the top band. I picked up a $25 SP-400 at an 
October hamfest and will give it a shot there also and see if it can handle 
10M without a bit of help.

You will notice I suggested a 12AT7 as xtal osc and buffer in my first post. 
That is based on the low xtal LO signal due to the low B+ used. OTOH the 
Heath LMO used in the next conversion seems to have sufficient level and 
stability but thats still open to a change if I find a problem with it or 
birdies.

I used a freebie S-40B as a test bed for several ideas, some bozo had 
painted the exterior with white latex but the interior was almost pristine. 
It has plenty of chassis space for extra holes and is wide open underneath.

Carl
KM1H



----- Original Message ----- 
From: "Drew P." <drewrailleur807 at yahoo.com>
To: <boatanchors at mailman.qth.net>
Sent: Tuesday, December 22, 2009 12:55 AM
Subject: Re: [Boatanchors] Tube reciever design banter


>
>>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
>
>
>
>
>
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