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Swapping 4-250 for 4-400 tubes

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Author Topic: Swapping 4-250 for 4-400 tubes  (Read 773 times)
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« on: November 14, 2017, 08:53:47 PM »

Hello, I have a homebrew transmitter with a pair of 4-250s in it modded by 810,s . I crapped out a 4-250 the other day-filament, and was just going to replace with 4-400,s Anybody ever do this? Thanks, John

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« Reply #1 on: November 15, 2017, 06:54:03 AM »

Before you switch, make sure the dark tube is really bad. The filament pins can get quite hot, and one or both pins may have gotten hot enough to melt the solder leading to a bad connection.

As far as switching to 4-400's, sure why not. My experience is limited, but with a Globe King 500, the 4-250 seemed to be more efficient. That said, my 4-400's were broadcast pulls gotten from W1GAC. Those could have just been tired tubes.


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« Reply #2 on: November 15, 2017, 08:48:50 AM »

     My KW-1 had an intermittent 4-250 and I replaced them with good 4-400A
tubes.  Direct swap, no circuit or voltage changes. Been running it that way for 20
years with the same set of tubes, no issues.

     Do check the solder on the 4-250 filament pins. Had a friend who have that problem a a Johnson 500. He put a small blower at the tube socket under the chassis to cool the pins. Worked.

  Bob  Ohio

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« Reply #3 on: November 15, 2017, 01:30:01 PM »


The swap should be a plug & play. Although when I've replaced large TX tube, always checked the neutralization afterwards.

Trivia......Junkston "500's" left the factory with a fan under the chassis/tube socket for the 4-400A. Just the same, check the pins & solder.


PS: I've never worked where 4-400A pulled tubes were available. 4-250A's (new in box) have been found at fests for about $10-$20 US dollarettes.


Ham radio is now like the surprise in a box of "Cracker-Jacks". There is a new source of RFI every day.
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