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RE: GRAAF GM 200

I would recommend that you match tubes by:
1) Doing a basic test with a tube tester to make sure that the tube is in working condition
2)Putting the tube into the amp itself at a set tube position in the amp when the amp has been set up at the correct bias and then measuring the voltage across the anode resistor of the new tube when the amp is on. Write down the voltage across the anode resistor for each tested tube and this can then be used to grade your tubes so that you can select similar tubes that have been tested under operational conditions.
If you put in a new tube in every time you switch on the amp to listen to it, and you listen once a day, you will have tested 30 tubes by the end of the month. I have had tubes measure almost 0 V across the anode resistor (i.e. tube is virtually not operating) and the amp works quite happily with the remaining 15 tubes in the channel spreading the load and showing 1mV extra across their anode resistor. The amp will actually happily work with one of the tubes being blown (red LED showing next to the tube).

While on this subject, I had the opportunity to visit the Graaf factory in Modena about a year ago and see Mr. Mariani fix a GM 200. Not something I would recommend but he happily disconnected the anode clip on a PL504 tube, unplugged the tube, plugged in a new tube and reconnected the anode clip, all WHILE THE AMP WAS ON without any problems (no speakers connected). As I believe there is about 150V on those anodes there is NO WAY I would do the same, but he certainly did, being careful not to touch anything else (comes with 40+ years of playing with tube amps, I suppose :-) ).


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