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Thread: First Homebrew HF power amp

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  1. #111

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    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.

  2. #112

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    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..

    Thanks,
    -Vern

  3. #113

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    Quote Originally Posted by TF3CY View Post
    I think it depends.

    With ground driven tetrode - you will have two control voltages to worry about - grid/screen - The tetrode board from GM3SEK is a regulated supply with trip-reset functions. You need to watch out that the grid of the tetrode is only allowed to dissipate few watts at most. It does the bias switching HV control (cut out HV when tripped) PTT control/sequencing.

    With the triode - they seem to be easyer to control, just a simple bias switch to cut off the triode. But with a expensive piece like the 8877 - (that I'we heard is fragile) I would like to have good control. There are two solutions available that I know of, Paul WD7S with the Triode board, and Ian GM3SEK with Triode and Tetrode boards.

    Only think I don't like about the GM3SEK board is the relays used for bias switching - I would welcome a SSR or a transistor switch of some sort.

    Read up on GM3SEK control boars, there is a lot of good theory of operation in each of the manuals. (http://www.ifwtech.co.uk/g3sek/)

    73, Benni TF3CY
    Not sure what "ground driven" tetrode means; if that's cathode driven, or grid driven. However....

    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..

  4. #114
    Join Date
    Aug 2001
    Location
    Fairhope, AL
    Posts
    1,696

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    Quote Originally Posted by N7GTB View Post
    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..
    Thanks,
    -Vern
    Bummer!!! Vern, I wish I could help - but I'm just not that technical. Did you say putting bias on "grid"? Is this a grid driven setup rather than grounded grid? Of course, grid bias may well be how it's done - as I said, I'm not that technical<

    73 de Ken H>

  5. #115

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    Quote Originally Posted by WA9SVD
    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..
    Semi- and Super-Cathode Driven Amplifiers
    William I. Orr, W6SAI and William H. Sayer, WA6BAN
    July 1967 QST magazine
    http://mikea.ath.cx/eimac/eimac08.pdf

    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.
    Nullius in verba

  6. #116

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    Quote Originally Posted by K1ZJH View Post
    You might be able ton fire them up on a jig in the same manner as is done for 3-500Z tubes.

    About 1000 volts on the plate, and run the grids positive until the plates show color.
    You will need a fan to keep the filament and other seal temps within spec. I think the recommended grid voltage was
    in the area of 20 to 30 volts positive. Although I haven't tried it, I'd guess the plate voltage could be unfiltered DC
    or even raw AC. I'd wait for some other input since I've never tried cooking my 4-400 tubes, which have been sitting
    in the boxes since the late 1960s

    Pete
    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...

    -V.

  7. #117

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    Thanks Ken. No worries tho It'll all get figured out eventually, or end up in a different amp...

  8. #118

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    Quote Originally Posted by W9GB View Post
    Semi- and Super-Cathode Driven Amplifiers
    William I. Orr, W6SAI and William H. Sayer, WA6BAN
    July 1967 QST magazine
    http://mikea.ath.cx/eimac/eimac08.pdf

    Both authors had this article: The Cathode Driven Ampliier in the June 1967 QST issue.
    Thanks much for posting the link! Reading through it now...

  9. #119

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    I found the following thread here at qrz that has a very brief description of how to 'getter' a 3-500Z tube, by w8ji:

    http://forums.qrz.com/showthread.php...63#post1991563

    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...

    Thanks,
    -Vern

  10. #120

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    Replaced the LM317T in my bench supply. The old one suffered horribly, but perhaps it was a quick death...

    failed_lm317t_1KV_small.jpg

    ...after replacement, the 'new' one failed as well. It was my last one too. Looks like more damage was done than I had hoped.

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