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

Tue Aug 11 14:56:17 EDT 2026

Jim-
one section diode, the other reg triode is a compromise as the triode 
still functions just fine under the specific application.
the triode connected diodes all benefit especially noise/distortion 
aspect especially the limiters. I do not think the AGC or the AM 
detector cares. however there may be something else I have forgotten.... 
AC fil keeps nagging me.
re the calibrator- definitely not a critical item.....I am sure a 
compromise.
When I made my R725s  only diodes are run at -10% ( 4 ohm in that fil), 
yes I went through and rewired the fil circuits.
R

On 8/11/26 11:13 AM, Jim Whartenby via R-390 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.
>   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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