TUBE BIAS, socket to me!

Elliott, check the paperwork that came with your Cayin and see if it states that it has "automatic biasing" or something to that effect. If not, there should be a procedure on how to do it. Are there any test points on the top or back of the chassis?
If it must be adjusted and the measurements are internal flip the amp on it's back and use the spring loaded hook type test leads with the power off. Make sure there is a load (speakers) on the outputs unless stated otherwise. You want nothing connected on the inputs. They should have a setting (usually in millivolts) that you adjust a pot to. That will set the current flowing through the tube. Some amps have an individual pot for each tube and some share a pot between both tubes in a push pull type amp. If I recall, yours is a push pull design.
I suspect that it's auto biasing or more than likely you would have red plated a tube if your swapping tubes. Uneven current matching on the tubes can lead to a runaway current situation that results in red plating.
Don't rely on your tube tester to match tubes, they don't test the current at full operating voltages.
There is a Fisher forum on AK, lots of information over there on adjusting those amps.

BillWojo

Don’t rely on your tube tester to match tubes ..

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What would you test them with, or rely on?

A TUBE TESTER.... That’s right.. you have to maintain them too. LOL

I guess you need a tester for the tube tester?

Don’t rely on a "TUBE TESTER" to test tubes.. Just hit my funny bone..:-) One poster said.. "Monkey Bone"...

Oh and flip the switch back to normal position on your amp after the Bias check and set... New valves recheck a couple of times over the next couple weeks of playing... Keep an eye out for valve failure.. they do.. New more so..

Regards

what model cayin, elliott?

I suggest you buy an elementary treatise on how vacuum tubes operate, and then read it. In the accepted model for a vacuum tube, electrons are emitted from the cathode when it is heated. The cathode accumulates electrons and hence is thought of as having a negative charge. Because the anode is positively charged, or relatively deficient in electrons, they flow cathode to anode. If this is all that happened then vacuum tubes would be like light bulbs; the cathode would give up electrons in an uncontrolled manner, and they would flow to the anode. When this process reaches a state of equilibrium, the tube would be burned out and that would be the end of it. But instead there is a "grid". The grid is situated in the path of the electrons on their way to the anode. In a functioning tube, most of the time, the grid carries a net negative charge with respect to the cathode, and thus retards the rate at which electrons get all the way to the anode. That is, when no music signal is present, the grid has a DC voltage on it that is negative with respect to the DC voltage on the cathode. When a music signal is present, think of that as an AC voltage, a voltage that varies with frequency and in magnitude and represents music. So, music enters via the grid which always has a constant DC bias voltage on it, in simplest terms. The music signal modulates the grid bias voltage, and in another story, that’s how we get amplification of the signal across a resistor that connects the anode to the PS voltage. When amplifiers talk about "bias", they are usually referring to this steady state DC voltage on the grid. That grid bias voltage also results in a steady state current (electrons) passing from cathode to anode when there is no music signal. This DC current is referred to as "bias current". The grid bias voltage, the "plate resistance" of the tube, and the power supply voltage between anode and cathode, also called the "plate voltage" together determine the bias current. (Ohm’s Law.) For any power tube, or any tube at all, you can usually find a complex graph depicting "plate curves". This single graph will show current on the Y axis and plate voltage on the X axis, and a set of curves depicting what that tube will do at different acceptable grid bias voltages for different changes in plate voltage or current. And....