Russian 6C33C Tube Testing and Matching

Check with TC Tubes. He offers a tube testing service using the Amplitrex AT1000, one of the best, if not the best tube tester available. This will test the 6C33C. A good friend bought some KT-88's from him, and was very pleased.

Good advice. But understand he must charge you something for this service - the Amplitrex AT1000 costs more than usd 2500.

From the specs of the AT1000
Heater/filament power from 1.0 to 12.6V, up to 3.5A

The 6C33 tube is extremely hard to test. The reason is its highly pronounced positive thermal feedback, turning any fixed bias testing into a test of your nerves. In reality I only recommend testing it using a servo-controlled bias circuit.

Barring that, your best bet is, in my view, to buy the tubes from the manufacturer, or to ask them to test your tubes, is they agree to do that.

If I'm not mistaken the Amplitrex with adapter allows only up to 150mA for a 6C33C. For example, BAT use slightly more than 200. So I would consult manufacturer as a first step.

I know that TrueSound in Northern California has tested and matched 6C33C tubes for a customer, so you might try the other, Nick Gowan.