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@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 > <larry41...@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@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 >> <larry41...@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 <larry41...@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@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@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@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@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@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
