Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Sat 07, 2019 9:28 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 6 posts ] 
Author Message
 Post subject: 4CX250 Question - answered Thank You
PostPosted: Jun Wed 05, 2019 5:16 am 
Member

Joined: Jul Sat 23, 2011 9:33 pm
Posts: 935
Location: Long Beach Ms. USA 39560
I found an old two-meter amp using the 4CX250 tubes and plumbing pieces for the plate tuning. It was extremely well made but the screen bypass in both the sockets showed leakage so I replaced them.
I have some tubes which sat up for a few years.
I understand that the internal getter material will release gas atoms if tubes sit cold too long.
I do not want to test the tubes by arcing them internally at full voltage.
I also have a PL-172 with the same condition.
The Penta rep on the phone said to run them at about 40 miliamps until the blue inside the glass went away -
but the 4CX250s are ceramic so I would not be able to see the glow.
Does anyone have experience testing for gas - or degassing - this tube family?
Thank you,
Pat

_________________
Pat W5THT
Unhappy tubes blush while unhappy power FETs scatter plastic


Last edited by pehamel on Jun Thu 13, 2019 5:53 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: 4CX250 Question
PostPosted: Jun Wed 05, 2019 11:55 am 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 6807
Pat,

I am not sure about all of the manufacturers but Eimac (and presumably other manufacturers of this 4CX250 family of CX tubes) have the getter as part of the cathode so running it filament only without high voltage (or other voltages applied) for 12-24 hours should take care of the gas issue as long as the seals are intact. Forced air cooling MUST be applied during this time to keep the filament seal within spec and the requirement to supply forced cooling whenever the filament is "lit" is true of pretty much any power tube requiring forced air cooling.

If you have a high pot tester, testing leakage from cathode to plate will generally provide a good indication as to whether there is gas or other problems which will show as elevated leakage and/or a low breakdown voltage.

Most of the ceramic type external anode tubes (includes most Russian designs) are pretty easy to condition because the getter is part of the cathode structure. Most of the big glass envelope tubes have the getter either deposited on the plate or the plate material itself provides the getter function and with those tubes the conditioning process is more complex because you have to bias the tube so that the anode will achieve gettering temperature with voltage low enough to avoid flashover.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: 4CX250 Question
PostPosted: Jun Thu 06, 2019 2:25 am 
Member

Joined: Jul Sat 23, 2011 9:33 pm
Posts: 935
Location: Long Beach Ms. USA 39560
Thank you

_________________
Pat W5THT
Unhappy tubes blush while unhappy power FETs scatter plastic


Top
 Profile  
 
 Post subject: Re: 4CX250 Question
PostPosted: Jun Thu 06, 2019 2:22 pm 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23190
Location: Somers, CT
Hi... what did you replace the screen grid bypass caps with? Eimac made a special low inductance screen bypass ring that was designed to go with the Eimac 4CX250 socket. The better design provided some shielding between the screen contact ring and plate tank. I used 4CX250 tubes for years and never had a problem with gas. If you are going to run SSB with the amp, the screen voltage regulation (and all grid currents) are very critical for iMD and tube life.

Pete

_________________
Just because it can be done doesn't mean it should be done.


Top
 Profile  
 
 Post subject: Re: 4CX250 Question
PostPosted: Jun Fri 07, 2019 6:20 am 
Member

Joined: Jul Sat 23, 2011 9:33 pm
Posts: 935
Location: Long Beach Ms. USA 39560
Pete,
Thanks for the info.
I had a pair of sockets that I was able to clean up and install in the chassis. The metalwork was all done.
Many years ago I opened one of the same type sockets with a bad screen bypass and took it out to use the socket in a linear for an 80 meter net. For two-meter EME that is not an option.
I have the two tubes that came with the chassis and two more that I was looking to use with my original sockets and have a pair of 4CX150s if those four are no good.
I will be using the power supply chassis from my 600 meter rig ( four 811s) and adding an interlocked screen supply. I am still debating using big resistors from my junque box and zeners - or a second transformer and electronic regulation. I will be regulating and metering both grids of each tube.
Back in 1957 I got this bug to do EME, after my wife died in 2017 I finally started putting things together and have made four contacts with accumulated junk when the moon was in the best position. Then I saw the old RF deck at a hamfest and decided to try to get more than 100 watts of JT65.
3000+ dollars for a solid-state KW (for a few contacts before I go back to the 630 meter band) is out of reach.
I found some articles on this amp, but don't have the tuning procedures.
If you have the ability to fix the leaky-screen-capacitor sockets, I'll put them into the mail.
Pat

_________________
Pat W5THT
Unhappy tubes blush while unhappy power FETs scatter plastic


Top
 Profile  
 
 Post subject: Re: 4CX250 Question
PostPosted: Jun Fri 07, 2019 2:27 pm 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23190
Location: Somers, CT
Thanks, but sadly my days of playing with those tubes are long passed. Getting too old, and mainly concentrating on HF at this point. I put a few 4CX250 amps in the local landfill when none of the locals wanted them. Good luck with the amplifier! I wonder why they became leaky? You think it may have been moisture getting into the mica due to poor storage?

Pete

_________________
Just because it can be done doesn't mean it should be done.


Top
 Profile  
 
Post New Topic Post Reply  [ 6 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 3 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB