Problem 4 4CX250B's amplifier blowing screen resistors

[JustinDePolis2]

Ok, for the first time I am posting a question on here, hoping that someone will have some insight. Lately, I've been working on a TN walker amplifier that uses 4 - 4CX250B tubes. When the amplifier was brought in, there was one shorted tube, let's call it tube # 3. The 47 ohm resistor going to the screen was burned, and the 47 ohm resistor going to the grid for the negative grid bias was toast. I replaced all of the resistors, and installed a new set of tubes, checked for shorts. I also, just for safe measure checked and replaced the diodes in the screen supply and the negative grid supply. After doing all of this, I fired up the amplifier, and upon keydown, with NO RF drive applied, which energizes the negative grid, and positive screen supply, within about 20 seconds, the screen resistor on the same tube socket as the shorted tube before blew. I replaced it, and checked voltages, approximately +220 on the screens, and approximately -72 volts on the grid. Tried it again, this time with no B+ voltage on the plates, and the same thing happened again. I have checked the screen with the tube in the socket to the chassis for shorts, and with the screen supply disconnected, it reads open circuit. My question is... what could be happenening? It's driving me crazy, I don't think it's a parasitic, as it happens quickly. I thought the ceramic bypass capacitor on the socket may have been shorted, but it isn't.



ANY IDEAS???


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[nomadradio]

Hi Justin,

I don't recommend keying a screen-grid tube with the anode supply unhooked. This will cause the screen grid to think that it's an anode, and draw all the screen current that it can. Don't know how the screen current is limited in a Tennesee Walker. Probably isn't, except for that 47 ohm.



If this is the same socket, blowing the resistor with a new tube, I'd say pull the tubes and key it. If there is a socket problem, this will prove it. Checking with an ohmmeter won't reveal a lowered breakdown voltage that goes 'poof' under bias. This would be a simpler test than hooking up a HV breakdown tester to the socket or cap. If one of your new tubes is bad, the odds are against it landing in the same socket as the previous failure. Just the same, a tube that has popped the screen resistor may sustain damage inside, even if it started out okay.



Aside from the socket, did you look closely at the resistor(s) on the parasitic suppressor for that tube? If it broke in half without making a scorch mark, this can make the tube try to oscillate. Wouldn't be apparent until you look closely and molest it a little.



Aside from a socket with an open connection to the control grid, that's about all that comes to mind.



73

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[JustinDePolis2]

Yeah I know it wasn't good to disconnect the supply from the anode... the screen supply on the amplifier is dropped down through a 47 ohm, and then two 5K ohm wirewounds wired in series from the pin on the socket to ground. Possibly a bypass capacitor breaking down?


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[JustinDePolis2]

Anyone, anyone?


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[JustinDePolis2]

That was obvious the moment I took the bottom metal plate off, that looks like it came from a scrap yard, after of course remving all of the wood screws.


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[JustinDePolis2]

Thanks, Bubble!! <img src=http://users.joplin.com/dutch64804/posticon25.gif ALT=":25"> I talked to a guy tonight with the help of AM Power on Pal-Talk, and he was stumped on the same issues that I was, as the situation doesn't make sense. However, he was able to give me the number of the guy that actually built the amplifier, so I will give him a call tomarrow. I will also contact the guy at the e-mail address that you gave to me and see what happens. Thanks everyone for the help.<img src=http://users.joplin.com/dutch64804/posticon58.gif ALT="58">


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[JustinDePolis2]

Well, everyone that I've talked to so far agrees that the amplifier from the get-go has a bozo-the-clown design. Rather than trying to repair the killer-clown's design, I think I would rather just design a new screen supply, and negative bias supply and be done with it, using real, high quality parts. I'll have it done by the end of the week, and post the results.


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[JustinDePolis2]

Took out the large paper, and drew up and new schematic for a new plate, screen, and negative grid bias supply circuit, we'll try it tomarrow.


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[JustinDePolis2]

Well after a few hours of reconstruction, the amplifier works. I decided to just remove their screen supply, and their negative grid supply, as they were bozo-the-clownish built on little terminal strips, poor regulation, no limiting, and poor filtering. I drew up a schematic last night and applied it today. The amplifier works perfectly, has adjustable bias, and just all around like the way it looks on the underside of the chassis now. Everything is wiretied, and the layout is neat. I even went as far as redoing the plate supply, those little diodes, and filter capacitors hot glue-gunned to the underside of the chassis wasn't going to cut it. Thanks everyone for your help!



Justin


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[JustinDePolis2]

Unfortunately I didn't take photos, although I probably should have, I still have the amplifier, and could take a photo of that, although I already sealed the amplifier's underside neatly with silicone to make sure there's no air leakage, and don't really want to have to scrape it all off, and re-do it all. I have a schematic of what I did that I could scan in???


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[JustinDePolis2]

It's all good, I wish I had taken pictures, it would have been cool to share the experience, if you would have seen how this guy throws together these TN Walker amplifiers you would probably be afraid to ever turn one on!!!! The "after" effect would have been cool to "show off". And people actually pay money for these, I was looking at a price list and the guy sells the model that I repaired for 2800.00!!! So let's see........



2800.00 for the amplifier with 4 tubes 4cx250b's

211.00 shipping is what he paid

820.00 for me to rebuild, and all new tubes, as the filaments were open from the filament transformer shorting out.



The total cost this guy has in it now is: $3831.00



GO BUY A HENRY!!!<img src=http://home.comcast.net/~shockwav/large-smiley-042.gif ALT=":ideasmly">








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[4CX400000watercooled]

Hey Justin, Im curious to know, how's that hunka junka amp workin now? My good friend has gotten the same model, I think, as the one you had. It should be a pretty powerful amplifier, of course after we re-work all the, hmmm, what can we say here,,,,,,backyard engineering that went into making it!! Got lucky on this one, he picked it up, for $300, as it only has 2 tubes left in it. Can you say 8321 tube conversion?? Haha, can get them really cheap now from Russia....









" NO HEAT LIKE TUBE HEAT!"


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