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

Tue Aug 11 13:32:11 EDT 2026

Fred, Jim and all,  - A more complete answer-
reducing the fil voltage (lowering the cathode temp) increases 
stability, decreases cathode poisoning, assists in sensitivity by 
decreasing noise and distortion.a 10 to 15% reduction is abut right ie 
6.3 to ~5.5 V each tube has an optimum V for the application.
re Regens and In grid leak detectors a neg bias can be produced and 
which is partially mitigated by lowering fil voltage-, its a balancing 
act of g leak resistor/fil voltage, hence a rheostat to adjust the fil V.
I think the stability , noise and poisoning were the main reasons in the 
390 and 390A. appears the engineers determined ~%10 for that 
tube/application where different mfg of tubes gave a good average 
optimization. remember a lot of compromise is at work many times when 
designing things.
Renée

On 8/11/26 7:56 AM, Fred Moore via R-390 wrote:
> Jim,  Yea, but Renee has given us the actual reason that the filament voltage
> reduction was done! -Fred
>
>
> ------ Original Message ------
> Received: Tue, 11 Aug 2026 12:35:26 AM GMT
> From: Jim Whartenby via R-390 <r-390 at mailman.qth.net>
> To: reflector R-390 <r-390 at mailman.qth.net>
> Subject: Re: [R-390] R-390/URR heater circuits.
>
> 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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