Anybody have a lead on a grid bias control for a EICO 666 tube tester or know of a good replacement part? I read one article that says you cant get them but I'm sure there has to be some new part that can be used. I was given this unit years ago and finally cleaned it and fired it up yesterday. All other parts are good but when I opened up the grid control to clean it the fine wire inside was broken and all over the place. Thanks. J
Hmmm, I've read that too (http://www.fourwater.com/files/restr666.txt maybe?), but it's hard to believe. R16 in the schematic is a 5k pot with switch (single pole). Judging by the size of the thing, it can handle a few Watts (a rheostat?). I don't see any reason why a similar part shouldn't work, except for the scale maybe. I'd say a single-turn linear 5k 5W wirewound would fit, if you can't find one with a switch just add a dedicated switch to short out R17 below 7V of grid bias (because that's all the pot's switch does). PM me if you need a schematic.
I finally found the whole assembly manual with schematic and parts list. I had guessed 5k as when I measured the pot it is around 4k from the 1st lug to where the wire broke and there is just under 1k of wire from the break to the 3rd lug. In the replacement parts list the Pot is listed as a 5k - 4 watt pot with spst. I will probably have to follow your suggestion. It will be a drag having to remember to flip the switch and drilling a hole in the panel. Any suggestions as to the best way to do this?
The switch is supposed to be "off" when the tube chart calls for 0 on the grid control which would put the knob all the way CCW. Typically for rectifiers. So it isn't hard to remember to turn it on. With the original pot, when you turned the switch "on" (CW) the knob was at 7-100.
As to where to put it. If you use a miniature switch it should fit relatively close to the grid control.
As to where to put it. If you use a miniature switch it should fit relatively close to the grid control.
Did some investigating and DIGIKEY has the CTS 026 series pots which are 5watt wirewound and they have a spst switch option. The CTS part number would be like this: 026T1253411 Waiting on a quote from DIGIKEY. The standard 026 pot is only $3.61!! Let's see what they come up with! Keeping my fingers crossed.
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