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

Tue Aug 11 14:13:00 EDT 2026

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. 
 In the R-390/URR only one section of each 12AU7 is at reduced current while in the R-390A/URR both sections of the 12AU7 are operated at a reduced heater voltage of about 5.6 volts.   So in the R-390/URR, only one diode connected 12AU7 is affected by reduced heater power while the other section is operated with a normal heater voltage and current.   In the R-390/URR, V507 section 2 is the Detector while section 1 is the negative peak limiter.  V510 section 1 is the AGC rectifier while section 2 is positive peak limiter.  So which function benefits from the reduced heater power?

I am just trying to understand what was the difference between the two designs other then the change from a heater supply of 25 volts to one of 6.3 volts.  Cathode poisoning was solved in receiving tubes long ago, it was due to a contamination of the cathode sleave material.  Noticed mainly in tube based computers when no plate current flowed for extended periods of time.   If this is what is happening in the R-390/URR then the calibration oscillator tube sees this condition all of the time but it has a normal heater supply voltage.
Regards,
Jim 
Logic is a systematic method of coming to the wrong conclusion with confidence.  Murphy

On Tuesday, August 11, 2026 at 12:32:11 PM CDT, k6fsb at arrl.net <k6fsb at arrl.net> wrote:

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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