Valve matching with a KiT6550

[Richard]

Hi John, yes a valve tester is nice but quite expensive unless you think you'll be doing more valves in the future. Can you find someone locally who can check them? I can do them if you want to risk putting them on carriers, drop me a PM. It's time I had a sort out of my valve draw like Bob...

[jdd308]

Thank you Richard,

I am looking in to the less expensive testers that can still do the slope as well, as it will be reassuring for me to be able to match remaining valves.

Plus you and Bob are lucky to have a 'valve drawer' - maybe I went overboard with valve rolling in the early days and ended up with a valve cupboard!

John

[Thenice]

Hello Richard
I was reading this with interest
As lve just seen a GE 6550A on ebay used for &70 it looks ok
With avo 163 tested on KT88 setting is 63 IA 7.5 av/slope
What do you think? Are the readings sound for the amp [allowing for the demon murphy]
Thanks
Frank

[John Caswell]

Hi Frank,
This will of course depend on what the other valves left in your 6550 amp read.
As I said earlier on, ideally pairs should be matched but in practice this doesn't matter too much as long as they are close i.e. ±10%. Without access to a valve tester a good check is the cathode voltage. Even if the amp is fixed bias or a mix, as a lot of the WAD amps are, there as to be some method of adjustment/check and this usually relies on a cathode voltage measurement of some sort.
In the past I have repaired a good few amp that use 6C33C-b valves, which cannot be checked on an AVO MkIV, so generally use the cathode voltage method.

John

[Richard]

Hi both, yes absolutely as John says.

The main drawback to testing in the amp (rather than with a tester) is that you need to fit the valves into your amp to be able to check and compare their current draw at idle – which would mean buying the valve first to check it.

With regard to the one on sale I also test 6550s using KT88 settings. Most are in the 50-60mA range, 70 is high and 40 is low. So the one on sale looks good but if your amp is Kit6550 it does run them quite hard.

If you get it check idle current (cathode voltage pin8 to ground, divided by cathode resistor ohms) and check for red plates in a dark room.

If all is well keep an ear out for any extra or rising level of hum during your first listening session as this is the audible sound of a valve going into runaway if it is badly matched with its pair

[Thenice]

Thanks everyone for replying and answering my question
Frank