[R-390] A better rectifier tube for many B/A receivers. The 5AR4

[Jim Whartenby]

Perry

I am not aware of any change to the 51J receiver mechanical design regarding the location of the rectifier and power transformer.  The 5V4 is located next to the power transformer in all of the 51J incarnations that I am aware.  The 5V4 does have a cathode sleave so it's cathode is not directly heated.  Perhaps I am misunderstanding what you mean by "cathode filament construction"?  I have worked on several 51J receivers and have never noticed any leakage from the power transformer but I have seen where the bathtub capacitors have often leaked.

Directly heated rectifier cathodes appear to have a larger internal voltage drop compared to the indirectly heated cathode rectifiers.  Not sure why this is but perhaps the indirectly heated cathodes have a lower work function so they have an easier time releasing electrons then the directly heated cathodes?  It appears that substituting a directly heated cathode rectifier with an indirectly heated cathode rectifier will raise B+ by the difference in the two voltage drops.  Not that it really matters much but some think that this is an issue, especially when solid stating the rectifier.

As for this continuing discussion about reducing the inrush current when first powering up the equipment, I am not convinced that this is a real problem.  The first full cycle of the AC waveform will have charged the filter capacitor to at most some 63% of it's final voltage.  That the filter capacitor is essentially a short circuit when first powered is the nature of the beast.  All capacitors are this way.  You would think that if this was a real issue, it would have been addressed long ago by using a ballast tube or some other such method of limiting the line current at startup.  The only equipment I have seen using a step start relay are in very high power equipment.  It is used to prevent the blowing of fuses or nuisance tripping of circuit breakers. 

Jim

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

    On Friday, October 18, 2024 at 03:18:18 PM CDT, Perry Sandeen via R-390 <r-390 at mailman.qth.net> wrote:   

 List,
It is easy to miss the detail that was put into the 26Z5 rectifier. It was designed with indirect heated cathodes.  This makes for an easier startup on the PS.
In most of the old B/A's they were equipped with 5V4's and similar large tubes having a cathode filament  cons5ruction.
Besides being big the y caused major problems with early R388's where the rectifier tube was  next to the power transformer causing the insulating tar to melt out and ruin the transformer.  Later Collins put a "Z" shaped shield or on some an added straight plate for heat protection.
Circa mid-50's or so Mullard in England developed what I believe they called IIRC the GZ53, It is and indirectly heated cathode tube about the size of a 5Y3 and capable of powering all B/A's using the old 5XX series.  I've had one in my 18W per channel stereo amp for years and years and it works like a champ.
Now the audiophools have repro's from a bunch of venders for astronomical prices.
The good news is a ham tube seller on line sells soviet reproductions 5AR4 for $10. The Russians make GOOD and inexpensive tubes.  The problem with the 6BA6 sized tubes is the pin configuration doesn't match what we have.
The smaller size allows for much better cooling as well as space saving.
Perry KM6FQV


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