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Tubes Matching on perstuntumalta arvattuna "bias balance". Kannattaa kyllä mitata lepovirrat, ohjeita löytyy netistä.
 
EDIT: Joo, arvio meni oikein.
 
Pitkä lainaus, kun alkup. linkki ei näköjään enää toimi ja sivu löytyi Googlen cachesta:
 
When adjusting my amp's hum balance control, should I have my guitar plugged in or not?
 
The hum balance control is used for cancelling out hum that is generated INSIDE the amp, not for hum picked up from the OUTSIDE
 
(i.e. via a guitar's pickup or wiring).
 
The basic idea:
Have NOTHING plugged into any of the inputs.
Turn up the bass and volume to make the internally generated hum easier to hear.
Adjust the hum balance pot for minimum hum.
Any other hum problems you notice when you plug in your guitar, have to be addressed and fixed in the guitar (or its cord). Trying to tune out the guitar's hum contribution using the hum balance pot, will be like trying to use a surveyor's level from a life raft.
 
- Chris Mohrbacher
 
Lord Valve wrote the following about Fenders, so just ignore the part about setting the bias balance control if you don't have one of those.
 
The "OUTPUT TUBES - MATCHING" control is a bias-balance adjustment. This is easily set by anyone who owns a screwdriver. First, you will need to adjust the other control on the back panel, the "HUM BALANCE" control. Adjustment of these two controls is very similar; they do completely different things inside the amp, however. To adjust the HUM BALANCE control, plug your guitar into the amp, turn all the controls on the amp and the guitar to the settings you normally use when playing, and then turn the GUITAR'S volume control all the way down, leaving it plugged into the amp's input. Insert a flat-blade screwdriver into the adjustment slot in the HUM BALANCE control and "tune" it back and forth...you'll find a place where the hum you can hear in the speakers is the softest; that's where to leave the control set. Note that it's a good idea to do this in a *very* quiet room, say at three in the morning. If you can't stay up that late, disconnect your speaker cabinet and plug a set of headphones into the speaker output jack; since the headphones are stereo and the amp is mono, you'll only hear sound in one earcup. BE VERY CAREFUL not to play any notes on your axe while you are wearing the headphones, or you will blow your brains out through your nose. (LOUD!!) Once you have the HUM BALANCE control set for the lowest possible hum, you can move on to the OUTPUT TUBES-MATCHING control. For this one, turn ALL controls on the front of the amp to ZERO, and adjust the OUTPUT TUBES-MATCHING control for minimum hum, just like you did with the HUM BALANCE control. You may want to take another shot at adjusting the HUM BALANCE control, because the hum you just nulled with the OUTPUT TUBES-MATCHING control may have been masking some of the hum when you adjusted the HUM BALANCE control. You may need to go back and forth between both controls a few times before you arrive at the quietest setting. Note that you must pay attention to the instructions (above) regarding settings of the front panel controls and guitar controls EACH TIME you adjust the rear controls...all zero for the OUTPUT TUBES-MATCHING control, and all in normal playing position for the HUM BALANCE control. If you find that you cannot null all (or most) of the hum from the output, you may need a new set of power tubes. Consult a technician for this; you may also have a fried HUM BALANCE control, and that's a sign that one or more of your power tube sockets have arced. The HUM BALANCE control is a 100-ohm pot which is connected across the 6.3 VAC filament supply, with the wiper grounded. It helps null 60 Hz hum out of the preamp circuitry. The OUTPUT TUBES-MATCHING control helps balance the DC current levels in the output transformer primary, and aids in removing 120 Hz power supply ripple from the output stage.
 
-Lord Valve
 
Actually, the reason for having the guitar cable plugged in is because in some amps, there is a switching contact in the jack which engages another tube stage, or turns something on (or whatever) that wouldn't be operating without a plug in the jack. If the guitar's turned all the way down, the tip is shorted to the sleeve anyway, so the cable is effectively not there as far as the amp's input stage is concerned. Just the physical presence of the plug in the jack is what I'm aiming for.
 
-Lord Valve
 
You might try it with and without the guitar plugged in. Also, if you use the headphones as LV suggested, (a) heed the instructions not to touch the guitar while playing if you value your ears and (b) for tube amps these should be low-Z headphones (4 or 8 ohms).
 
-Miles