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

[Jim Whartenby]

FredThanks!  This makes sense.  It seems that only one section of the 12AU7 is at reduced heater power so I will have to figure out which section of the tube is doing what function.  First guess is that pins 1 through 4 are one section and 5 through 8 are the second section with pin 9 the heater center tap.Regards,Jim
Logic is a systematic method of coming to the wrong conclusion with confidence.  Murphy

On Monday, August 10, 2026 at 07:52:19 PM CDT, Fred Moore <fred_moore at usa.net> wrote:

Jim, 

All four sections -- V507 A & B and V510 A & B -- are diode connected.  I
suspect that these sections are operated at a reduced filament voltage to
reduce internal noise.  (A diode with a current flowing is a noise generator.)
 Or it could be to affect the shape of the I - V characteristic.

I would have to breadboard this and experiment to sort it out better. 

The 0.1 capacitors C548 and C549 in the IF unit are to RF bypass lines that go
"off board."  These and other filament bypass capacitors are to prevent RF
from moving between stages via the filament wiring, where they might cause
"birdies." 

-Fred 


------ Original Message ------
Received: Mon, 10 Aug 2026 10:47:52 PM GMT
From: Fred Moore via R-390 <r-390 at mailman.qth.net>
To: Jim Whartenby <old_radio at aol.com>, reflector R-390
<r-390 at mailman.qth.net>
Subject: Re: [R-390] R-390/URR heater circuits.

Jim, 

Just yesterday I was working on this part of the IF unit.  I will give this
some thought.  There must be a reason.  Hang on.  

-Fred

------ Original Message ------
Received: Mon, 10 Aug 2026 09:17:01 PM GMT
From: Jim Whartenby via R-390 <r-390 at mailman.qth.net>
To: reflector R-390 <r-390 at mailman.qth.net>
Subject: [R-390] R-390/URR heater circuits.

The tube heater supply is 25 volts and change in the original R-390.  So
there are several series heater strings that add up to 25 volts or so. 
Interesting that the two 12AU7s, V507 and V510, have a 120 ohm resistor
connected from pin 9 to pin 4 while the other two 12AU7s, V601 and V602, do
not have this resistor but they do have a series 27 ohm resistor to drop the
heater voltage a tad.  

I have checked the tube manuals and did not see any reason given for this 120
ohm resistor.  Does anyone know why Collins added this resistor?  One
triode
then has about 150 mils flowing through the heater and 100 mils through the
other.  Why?

On the same heater string topic, I notice that some heaters in a string are
capacitive bypassed to ground while other strings are not.  I have the
feeling that the physical size of the capacitor dictated this decision.  I
would think that a cap across each heater would be most effective in removing
line noise since it would offer the lowest possible impedance to anything
much
greater then 60 cycles.

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