What is meant when tubes are sold as "matched"?

[Brisso57]

Newbie here.

I'm gearing up to build my first tube amp - an RH84.

I see on fleaBay and elsewhere tubes (both new and used) advertised as matched pairs or matched quads ...

What is meant by this term?
What properties are measured to enable tubes to be rated as matched?
What % tolerance is generally permitted within matched sets?

Thanks

Doug

[SY]

It means whatever the seller decides it means. There are no standards and about a million ways to do it that all give different results. Caveat emptor!

[kavermei]

Ideally it SHOULD mean that the tubes draw equal plate current and have equal transconductance/mu/rp at the plate voltage and grid voltage at which you intend to use them.

But I think 99% of the sellers on eBay will not be able to back this up when asked. Most will just have sticked the tubes in a tube tester and seen that the needle points more or less in the same region...

[Colt45]

Usually: nothing.

[richwalters]

Originally Posted by SY

There are no standards and about a million ways to do it that all give different results. Caveat emptor!

Yup, sloppy heater wiring will put physics charateristics haywire. The idea of "matched" isn't bad because someone has put the stuff on test or soak bed, so the chances of one receiving a duff tube have been reduced but not diminished. A good vendor will colour spot the tube and if goes foul will replace with a sim.

richy