[R-390] 5AR4 Clarification

[Jim Whartenby]

Perry
No, your were quite clear about the available transformer's high voltage rating.  When I looked at the schematic, that distinction quickly left the mind.  My apologies.

Yes, I too have a growing stable of 51Js.  Got about half at one time at the Orlando Hamfest two years ago.  Too cheap to pass by!  Two were $50 each, a third was free if I repaired another for a buddy and the last I don't remember where I got it.  Three are now repaired and working but in need of alignment.  The last one on the bench was modified for SSB so I am pondering whether to go back to the stock circuit or do the SSB mod correctly.  I have found several broken ferrite slugs along the way so perhaps one 51J will be an organ donor?  In the meantime, I'm off on other tangents!

Not sure that the top cover is a good idea, I wonder about the effects of long term heat on reliability.  But they are often called dust covers so it makes sense to keep the one or two that I have.  I have been pondering making bottom covers to protect everything under the chassis.  Again because of heat concerns, I plan to used perforated sheet metal.

As an aside, I have found that the original electrolytic filter caps have all reformed.  Not all of the 51Js still had them installed but those that did are doing OK.  As for the rectifier, I went the solid state route; less heat.

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

    On Wednesday, October 23, 2024 at 03:33:14 PM CDT, Perry Sandeen via R-390 <r-390 at mailman.qth.net> wrote:   

 List,
I probably wasn't clear enough about my R388 rectifier tube placement. 
Jim Whartenby  was confused by my remarks.  I didn't mean to imply that there had been a location change of the rectifier replacement.
Jim disagreed with my technical belief of heat damage.  I stand corrected that the 5V4 has a cathode sleeve.  My Boo Boo.
>From personal experience I've purchased a R388 that had a melted and failed power transformer for a very small price that included a replacement transformer.
I now have 3 R388's which two are for rehabbing.  The one top cover I have schematic is totally unreadable as the whole inside top  cover is almost totally black from overheating but it doesn't seem to be from the power transformer that has partially melted on to the chassis.
even if the Xformer is still good, removing the tar requires removing the power transformer-no easy task and then removing the chassis tar. Another difficult problem.
I found from experience that the funcky top cover does work as a RF shield.  So I've got two to fabricate.
My preference for the 5AR4 is that it is smaller thus making the r388 chassis cooler.  As it's and indirect heated cathode, the rise in B+ is slower making for less stress on the filter capacitors.
Which on that point, the modern "con brothers", Nichi, Ruby, Chem, and others make light years superior filter caps.  Some electrolytics are rated for long life at 105C.
As far as  surge protection being useful, almost every PS circuit that one sees relates to some type of thermistor used in the A/C power line. 
But everyone is entitled to their personal opinions. 
Thanks for your comments Jim. I hope I've answered your question.
Perry KM6FQV

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