[R-390] Fw: R-390A L-601 swing choke question
David Wise
d44617665 at hotmail.com
Tue Jul 1 16:52:07 EDT 2025
A conducting thermionic diode has voltage drop therefore resistance.
________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Jim Whartenby via R-390 <r-390 at mailman.qth.net>
Sent: Tuesday, July 1, 2025 12:08 PM
Cc: R-390 Forum <r-390 at mailman.qth.net>
Subject: Re: [R-390] Fw: R-390A L-601 swing choke question
Dave
I am under the impression that perveance concerns the space charge and the spreading of an electron beam. A high perveance tube implies a strong space charge which increases the spacing of the electron emitted by the cathode. This is the opposite of what an electron gun produces.
AFAIK, there is no beam forming elements in the 26Z5 so it is a high perveance tube and it can handle high currents. The surge current in the 26Z5 is one amp which should qualify as a high current. The question is just how long can the 26Z5 sustain one amp of plate current? Isn't a condition like this the cause of cathode stripping?
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Tuesday, July 1, 2025 at 12:31:01 PM CDT, David Wise <d44617665 at hotmail.com> wrote:
The perveance of the 26Z5 itself supplies some resistance.
Dave Wise
> On Jul 1, 2025, at 9:36 AM, Jim Whartenby via R-390 <r-390 at mailman.qth.net> wrote:
>
> Larry
> Thanks for your power supply data, much appreciated!
>
> Since the Absolute Maximum rated current per plate is 55 mA and the supplied current is 50 mA per plate then the 26Z6 is working at 91% of it's capacity. That's harsh in my book. There is little reserve so it is not a surprise to me that the 26Z6 sells at a premium price.
>
> Curious that the HV winding of the power transformer has the same specs in both the R-390/URR and the R-390A/URR. The R-390/URR uses R801 thru R804 to force current sharing in each of the four rectifier plates but these resistors were deleted in the R-390A/URR. This voltage drop of about 2.5 volts is eliminated in the R-390A/URR. This lack of current sharing adds to the stress on the 26Z6.
> Regards,
> Jim
>
> Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
>
> On Tuesday, July 1, 2025 at 06:02:12 AM CDT, Larry Haney <larry41gm2 at gmail.com> wrote:
>
> Jim, I've measured the current at F102 on numerous 390As and it is
> around 180 ma (with the BFO and calibrator off). The 26Z5 datasheet
> says the plates can handle 55 ma each, so that is a total of 220 ma
> for both tubes. The BFO uses 9 ma and the calibrator about 20 ma, for
> a total of about 209 ma. The datasheet also says that the recommended
> max current is 50 ma per plate, for a total of 200 ma. So, since the
> calibrator is off most of the time, I don't think the 26Z5's are being
> overworked. This agrees with my experience that they do last a long
> time. Of course these measurements are taken at 115 vac in to the
> transformer. If you have a higher line voltage, these currents will
> be higher. A max of @ 117 vac is important here. I don't know what
> the currents are for 120 vac in.
>
> Regards, Larry
>
>> On Sun, Jun 29, 2025 at 9:47 PM Jim Whartenby via R-390
>> <r-390 at mailman.qth.net> wrote:
>>
>> Larry, thanks for the reply.
>> Two of the three chokes in the R-390A have a DC resistance of 137 ohms maximum (L601 & L602), the third has a DC resistance of 113 ohms maximum (L603). The DC resistance of the windings are specified in the drawings found in Nick England's site. At 200 mA L601 will drop about 25 volts. I don't know the current split through the other two inductors but I would guess 50 mA for the Audio and 150 mA for the RF & IF modules so another 20 volts for the RF & IF modules and 5 volts for the AF module.
>> It seems that the 26Z5 rectifiers are operated above their spec limit. The voltage drop across the rectifier is some 30 volts at 100mA per the graph for the choke filter. The graph has a 120 mA current limit so I guess the rectifiers don't last all that long. So all of this raises questions that are not addressed in any of the R-390A documents that I have seen on the web including the Pearls of Wisdom.
>> Jim
>> Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
>>
>> On Sunday, June 29, 2025 at 02:47:59 PM CDT, Larry Haney <larry41gm2 at gmail.com> wrote:
>>
>> Hi Jim, I forgot to include that I haven't seen any criteria anywhere
>> about the ESR of the swing choke. I'm not sure that at 60 cycles, it
>> is that important. It may be, but I haven't seen it.
>>
>> Regards, Larry
>>
>>> On Sun, Jun 29, 2025 at 12:03 PM Larry Haney <larry41gm2 at gmail.com> wrote:
>>>
>>> Hi Jim, Sorry for the delay on this post. You are right about the
>>> different methods to satisfy the spec, the 390/urr does it by
>>> regulating the B+ and thereby the screen voltages on sensitive tubes.
>>> The 390A/URR does it by only regulating the screen voltages (the +150
>>> supply). The plate voltage in most of the circuits is not critical
>>> for frequency stability. I used my variac to change the input volts
>>> from 105 to 125 vac with no frequency drift.
>>>
>>> Regards. Larry
>>>
>>>
>>> On Fri, Jun 20, 2025 at 11:54 AM Jim Whartenby via R-390
>>> <r-390 at mailman.qth.net> wrote:
>>>>
>>>> Has anyone measured B+ current and voltage in the R-390A/URR as the function switch is set to various positions? I'm curious as to what trade offs were made in the power supply during the cost reduction program. The differences between the two power supplies are substantial but the specifications with respect to line voltage specifications are quite similar, if not exactly the same.
>>>> The R-390/URR regulated power supply holds B+ variation to less then one volt for a line voltage variation of some 15%. The R-390A/URR seems to have a much greater variation in B+ for the same line voltage variation but it still meets the same spec. The papers on the reflector that look into the power supply seem to ignore the ESR in the inductors used in the R-390A/URR.
>>>> Does anyone have any data to share?Regards,Jim
>>>> Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
>>>> ______________________________________________________________
>>>> R-390 mailing list
>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>>> Help: http://mailman.qth.net/mmfaq.htm
>>>> Post: mailto:R-390 at mailman.qth.net
>>>>
>>>> This list hosted by: http://www.qsl.net
>>>> Please help support this email list: http://www.qsl.net/donate.html
>>
>> ______________________________________________________________
>> R-390 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:R-390 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
More information about the R-390
mailing list