05-06-2012, 10:59 PM
Well, so much for effort. I finished setting up the blower unit to circulate air past the tube (from the bottom up), a filament transformer with the small variac to supply 5.0 VAC, and the plate supply on a separate variac to produce 1 kvdc. I blew the dust off of my little homemade 0-30 v regulated bench supply to place positive bias on the grids. So far so good, and things seem to be coming together...
I connected up a tube that I had hi-pot checked earlier, that indicated gas. I set it in the socket and placed the chimney around it. Initially the filament voltage was very unstable, but I finally got it to settle down, and adjusted the variac for 5 v. I then adjusted the small regulated DC bench supply to provide 10 volts. At this point I started ramping up the plate voltage...still no change in the plate color, so I continued to bring the voltage up. I hit 1001 volts DC, and all was well (sort of) but still no change in plate color.
So I went to turn up the grid bias and noticed the analog volt meter on my 'bias' supply was pegged. As soon as I touched the control knob to adjust the bias, there was a flash of light from inside the supply, followed by a loud bang. At that point the bias supply was instantly dead...except for the cooling fan, which happily puffed out a hint of burnt insulation...
Everything except the filament supply was tied to a common ground. The secondary of the filament transformer and the tube filament were allowed to 'float'. I don't know if this was correct, but that's what was done.
I haven't bothered to open up the bias supply to see what failed, but I'm guessing that the tube shorted between the grids and the plate, causing things to discharge through the bias supply (?) I built the little critter a long time ago, and while I think I remember including short protection diodes, I don't think there is any sort of high voltage protection.
Since I have not found schematics for any of this, and very little written guides on the web, I pretty much flew by the britches. It's Sunday afternoon, the wife is still out of town, so I think it's time for a beer...ok, maybe two.
05-07-2012, 04:53 PM
BTW, assuming anyone is still monitoring this thread, I could really use some technical assistance/advice with this. Preferably from those who have built a similar rig for, and successfully 'rejuvenated' power transmitting tubes like the 4-400, 3-500, etc..
05-07-2012, 11:20 PM
Not sure what "ground driven" tetrode means; if that's cathode driven, or grid driven. However....
Originally Posted by TF3CY
It depends upon the actual tube in use. Tetrodes such as the 4CX250 and similar tubes CAN be cathode driven, but to prevent rapid destruction of the tubes, the tubes STILL must be provided with proper screen and control grid voltages, they will NOT operate in "DC" grounded grid fashion without easy destruction of the tubes. Tubes such as the 4-400, 4-1000 CAN be run with control and screen grounded, and cathode driven. Bill Orr, W6SAI had a series of Application notes from EIMAC on that exact subject back in the 60's. I'll see if I can dig the note out scan it, and post it here..
05-07-2012, 11:45 PM
05-08-2012, 02:12 AM
Semi- and Super-Cathode Driven Amplifiers
Originally Posted by WA9SVD
William I. Orr, W6SAI and William H. Sayer, WA6BAN
July 1967 QST magazine
Both authors had this article: The Cathode Driven Ampliier in the June 1967 QST issue.
Last edited by W9GB; 05-08-2012 at 02:24 AM.
05-08-2012, 03:15 AM
This is the inspiration for what I was attempting to do. Although I approached it a bit backwards after re-reading this... I gave it grid voltage first, then plate. Perhaps that's where I went wrong...(?) I found other mentions in other forums (as well as here I think) of running the tubes with positive grid bias; just enough to cause the plates to glow and activate the getter, and thus absorb any gas... That's the theory at least. I have no clue if it's actually possible...
Originally Posted by K1ZJH
05-08-2012, 03:21 AM
Thanks Ken. No worries tho It'll all get figured out eventually, or end up in a different amp...
05-08-2012, 03:28 AM
05-08-2012, 03:44 PM
I found the following thread here at qrz that has a very brief description of how to 'getter' a 3-500Z tube, by w8ji:
Tom, if you're watching this thread, would you please comment on how this should be done with 4-400 tubes? I don't beleive that I've damaged the tube I was working with (any more than it may already be), but did manage to hammer my 'bias' supply...
05-10-2012, 01:17 AM
Replaced the LM317T in my bench supply. The old one suffered horribly, but perhaps it was a quick death...
...after replacement, the 'new' one failed as well. It was my last one too. Looks like more damage was done than I had hoped.