[R-390] Fw: R-390A L-601 swing choke question

[Jim Whartenby]

Jacques
I seem to remember a similar email exchange with you many years ago.

It appears that the 26Z5 has a lower voltage drop which is about half that of the 12BW4.  The heater current of the 12BW4 is a bit more then twice that of the 26Z5 but since the two heaters are in series, the total current demand from the power transformer is almost a wash.  Because of the larger voltage drop, do you think that the 12BW4s run a bit hotter then the 26Z5s?

I am still more concerned with waste heat then a bit higher B+ before the HV regulator.  I still favor silicon diodes as a 26Z5 replacement and would not bother with dropping some 20 volts to match the voltage drop of the 26Z5.  20v / 285v X 100 is an increase in voltage of 7%.  Since the R-390/URR was designed to operate over a line voltage range from 98 volts to 132 volts or 115 vac +/- 15% (per the R-390 final engineering report, page 14) the 7% increase doesn't seem to be all that much of an issue.  I know that I am bucking the trend here but heat is more of a reliability issue then voltage, IMHO.
Regards,
Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Tuesday, July 1, 2025 at 04:18:21 PM CDT, Jacques Fortin <jacques.f at videotron.ca> wrote:   

 Hello Jim,
If I am right, the 1A plate current rating of the 26Z5W is the maximum repetitive peak current charging an input capacitor filter (when used).
Compare that with the 12BW4 if you like, which is rated at 2A per plate.
Because there was only one maker of the 26Z5W (Tung-Sol) this can explain why they are hard to find and command a high price now.
But a possible substitution exists: it is called the 12BW4 solution.
My R-390 runs with a pair of 12BW4 and I also have a "spare" R-390A supply "converted" to the 12BW4s.
That requires to use lower profile sockets on the R-390A transformer module, but otherwise no problems were experienced.
Many manufacturers produced the 12BW4: Sylvania, Tung-Sol and GE, plus Rogers in Canada.
There is still a lot that can be found around at lower prices than the 26Z5W...

73, Jacques, VE2JFE in Montreal