[ARC5] Receivers and mixer noise.

[Richard Knoppow]

----- Original Message ----- 
From: "john rose" <brokenthumb at live.com>
To: "arc5" <arc5 at mailman.qth.net>
Sent: Tuesday, October 28, 2014 12:05 AM
Subject: Re: [ARC5] Receivers and mixer noise.


> 1935 was a watershed year in tube development. I remember 
> reading the League Handbooks from that era. The 6L7 mixer 
> came out and you would have thought from the way they 
> gushed about it, this was the ‘second coming’. A year or 
> two later the 6K8 (also introduced in 1935) was the 
> darling of the Handbooks probably because it did not 
> require another tube, the LO. Right on the heels of this, 
> the 6SA7 (also introduced in 1935) became the darling of 
> the Handbook. The converter war raged, with advocates of 
> pentagrid converters siting lower noise without having to 
> worry about someone moving the grid cap wire and the 
> triode/hexode citing more gain. The 6BA6 put an end to 
> that.
>
> I don’t have a Army/Navy Preferred Types list prior to 
> 1945. It may have come down to the 6K8 was the A/N choice.
>
> A sub for the 6SK7 would be the 6SS7 if you are willing to 
> deal with the filament current issue, there never was a 
> 12SS7. It has twice the gain, half the noise and half the 
> heater draw.
>
>
     I think you are confusing the 6SS7 with something else. 
It does have half the heater current of the 6SK7 but 
otherwise has very similar characteristics.  With 250V on 
the plate and 100V on the screens the 6SS7 has a 
transconductance of about 1800 uMho and the 6SK7 about 2000 
uMho. Plate resistance for the 6SK7 is about 800K and for 
the 6SS7 about one meg. Grid voltages for maximum gain and 
minimum gain are the same for both. The 6SS7 seems to have 
been designed where minimizing filament power was desirable.
     The quietest of the old pentodes is probably the 6SG7 
although some of the TV pentodes have higher 
transconductance they are not as suitable for HF use with 
automatic volume control.
     The 6SA7 is about the overall best of the old pentagrid 
converters although its a noisy tube. The 6SB7Y, designed 
for FM receivers is noticeably better and is a straight 
across plug in for the 6SA7 although for optimum performance 
its bias and voltages need some adjustment. The 6BA7 is a 
nine-pin miniature equivalent of the 6SB7Y and the 6BE6 and 
approximate seven pin miniature equivalent of the 6SA7.
    The 6BA6 is not quite a miniature equivalent of any of 
the above since it has a much wider grid control voltage 
range than the 6SG7. However, it has similar Gm and RP but 
is a semi-remote cut off tube requiring only about 20volts 
for minimum Gm where the 6BA6 takes about 50 volts.  The 
6SK7, 6SS7, and double ended versions(have grid caps) take 
about 35 volts for cut off but it depends on who's handbook 
you are looking at.  My data comes from the RCA HB-3.  The 
6SG7 while an older design than the 6BA6 is actually a 
slightly quieter tube.
    Triode mixers are much quieter than either pentodes or 
other multi-grid tubes but have other vices of their own. 
They began to become popular when Collins introduced cathode 
coupled triode mixers in their S line receivers.
    Choice of a mixer or converter depends on several 
factors; noise is one but one must also be concerned with 
the loading effect of the tube on the RF circuits, feed 
through of the LO and, in a converter, the performance of 
the oscillator section.
     For design information on the older type mixer tubes 
see the "Radiotron Designer's Handbook" forth edition. 
Unfortunately, the book predates the Pullen and other 
common-cathode mixer stages.
     A Google search for Keats A. Pullen will get a lot of 
hits including several of his articles. He had an 
interesting approach to electronic design.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com