Metropoulos Forum

How far apart do valves have to be before they are considered a non matched pair? I have a pair of KT66 that are 12mA apart. Is that too far for a JTM45. Also how do you bias valves this far apart.

Bias after the hottest tube. It might work

You want to aim for no more than 5ma, but I'm using a set of GEC KT66 that I think are 7ma apart and it sounds great to me. Keep in mind the dudes back in the day didn't match shit for the most part and just threw them in there.

See how it sounds...bias to the hot tube and play it. Personally I think a cold biased JTM sounds raspy and shrill but YMMV.

Thank you for your responses. I don’t think the subject of what constitutes a matched set of valves and the importance of a matched set has been covered enough. It is something that sellers put a lot of store in, but not enough is known, certainly by me.
I would like to know what is considered acceptable with regards to a matched set of valves. I would guess that taste would come in to it. I would also think that the type of tube has something to do with it. But in the industry of valve operated equipment I would still like to know where the line is, if there is one. Also what is tested and what would be relevant in the hunt for good tone .
I have a set of G.E.C. KT66 that are 12 mA apart. I also have a set of Sylvania 5881 that are the same. What would be the best way to bias them? Also how does a miss matched pair of valves affect the sound and can any adverse affects be countered? I have done as coldengray suggested and got good results from a set of 5881, but not so good from my KT66.
Is there a circuit that can be put in to a JTM45 that can individually bias each valve? If so does anyone have a diagram or schematic of that circuit? Pictures of an amp with that modification in it would also be useful.
I would also like to know what would be considered a matched set when buying. How far apart would the tubes have to be before the tubes were no longer a matched set? What is the industry standard for a matched set?

Within 5mA is considered matched, but some sellers have enough stock to really tightly match them.

Hi guys,

5 ma is a sort of standard that is kicked around. There is no reason to live with that unless you want a certain brand of tube that the seller does not have enough to match better.

Without buying tubes in the hundreds there is not much possibility of close matching.

I can buy JJ's KT66 matched within 2 or 3 ma any time I need them from Eurotubes. The reason is simple, Eurotubes deals directly with the factory and buys large amounts of tubes.

There is no magic to amp biasing. The 70% number you hear about is just a number and is a good enough point to start with. The bottom line is not bias so cold that you have crossover distortion and not so hot as to burn up the tubes. After meeting those two things, it is about what YOU like the sound of.

Cheers,

Billy

If you go to SDMs temporary files on google drive, go to mods and you will see a neat independent bias mod.
It will fit on the end of a turret board if you use small 150v rated caps and add some extra turrets.