[AdrianH]

Well to date the tester has been a complete flop!

I tried to test a small selection of valves and was mightily confused with the responses I got, the first vale trying the triode side of a 30FL1 it seemed OK, moving on the the Tetrode side it gave OK readings until I started to back off the grid bias, in which case it went hard on.

Checked wiring and all seemed OK, try another 30FL1, still the same. EL84 when backing off the grid bias from say -12 Volts strange things happened, last valve to try 6925 same again. OK got out the diode probe and sure enough at a certain point the valves are oscillating.

I have tried 1nF caps down to chassis on the 9 pin valve style connector and 4n7F caps at the end of the wire chain on the octal vale, makes no difference, run the grid wire in a piece of co-ax to shield it, no difference. large smoothing caps on the DC still the same.10K resister as a grid blocker.

I can see why the rectifier type tester are easier to do.

I have two things left to try, rewire all the sockets try to get them a bit neater, rather then flexible wire and some small loops so I could get in to solder, try solid wire short as possible. Or the switch mode power supply which can oscillate on it's own when it wants to apparently, so would need to use a transformer, but that means a HV regulator.

Perhaps with a new day I will try again.

Adrian

Edit just tried replacing the HT with the one from my milliwatter audio amp which is transformer based around 160 Volts DC. and it still oscillates, so redo the wiring!