AVO Valve tester question.

[Phil Y]

Does someone know:

I have an AVO MK3 tester.
The highest current range for the meter is 100mA.
A few years ago I fixed a 200W Marshall guitar amp for a friend, but I hit a problem when it came to checking the output valves(4XKT88). The test data states an anode current of 130mA which is obviously off the scale. Is there a way of increasing the range of the current meter or is it just not possible for bigger valves to be tested on the MK3 ?

I meant to ask on here at the time but forgot all about it.

Regards, Phil.

[Phil Y]

Please ignore the above, I have gone back through the paperwork I got with the tester (loose photocopies)and found a second set of settings with lower anode and screen voltages that give a usable anode current.
One set must be for a different model.

Phil.

[Richard]

Phil,

I have characteristics of

-20V grid, 250V anode, 250V screen, 57mA IA and 8mA/V for KT88

I also use them for 6550 as the "book" specifies characteristics including 400V anode which my machine doesn't have but it's a very similar valve and tests fine with KT88 settings.

So it's worth thinking of a similar valve if you get stuck.

The other thing you can do is test a known good valve at settings that are available on your machine and as near to the recommended ones as the machine allows. The you can then use those settings for other valves to see how they compare with the good one. I do this for 300B as the "book" ones are too high neg grid volts for my machine so have found my own which suit the machine.

[Phil Y]

Hello Richard,
Those settings for KT88 are the same as the new ones I found but thanks for confirming them.
Both your suggestions for getting around testing problems are good ideas, I will remember them for future use.

Thanks, Phil.