[R-390] R-390/URR heater circuits.

[Jim Whartenby]

Jim S.
I also think that this is the answer.  The lower heater power reduced 60 cycle hum transmission in high impedance circuits, how novel!   Those crafty Collins design engineers!  <grin>

>From "getting the most out of VACUUM TUBES" by Robert В. Тоmer, 1960, page 30:
"It has been demonstrated that the development of leakage is much slower when the heater is positive, rather than negative, with respect to the cathode. Apparently, there is a semiconductor characteristic to this insulation material, causing it to behave very much like a rather inefficient diode. This is borne out when an AC potential is applied between the heater and cathode. Rectification is quite nonlinear, resulting in high-order harmonics of the fundamental appearing on the cathode, if there is considerable resistance in series to ground."

Who would have thought that the 12AU7 pins 1, 2 & 3 are triode section 2 and pins 6, 7 & 8 are triode section 1?  Now to take a close look at the base of a 12AU7 to see which heater stack is associated with which triode section.
Thank You,
Jim W.

Logic is a systematic method of coming to the wrong conclusion with confidence.  Murphy

On Tuesday, August 11, 2026 at 01:34:44 PM CDT, Jim Shorney <jimnu0c at gmail.com> wrote:


Maybe this is relevant.

https://www.wb4hfn.com/DRAKE/DrakeArticles/Why_8BN8-2B.htm

On Tue, 11 Aug 2026 18:13:00 +0000 (UTC)
Jim Whartenby via R-390 <r-390 at mailman.qth.net> wrote:

> Fred and Renée Well yes and no WRT answering my question.   It is now my understanding that the heater power reduction was done to improve some aspect of using a triode as a diode.  I am not sure what circuit function is actually benefiting from this.  The noise aspect is a reasonable explanation for reduced heater power.
-- 

73

-Jim
NU0C
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