Dpac996,
thank you for your very detailed answer.
I will try to answer all your points:
1. Yes, to +60 Volts measuring point was OK.
2. I did not check any of the pin 5 power tubes measuring points. When I switched on my amp to check it another resistor start smoking - 5.11 Kohms before Q4 transistor. I thing that must be something wrong with those Sylvanias. I would like to try another set. I am considering Elecro Harmonix 6922 Gold pins current production tubes but with balanced pairs option (offered in Europe by jacmusic store). I hope that this way I can get perfectly matched octet of input tubes.
3. I think that ordering new power tubes is a good option and I also prefer winged C Svetlanas.
4. I have used older version of ProGold contact enhancer and it if fact become shellac, but now Caig have improoved version with extended temperature range specially designed for tube pins - it is DeoxIt GX if I remember correctly.
I asked my questions to ARC and we will see what they say.
Tan you or your inut.
Miimetr
What Jacmusic say about EX balanced tubes:
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Balanced means, both triodes within one tube are identical within a pre-defined limit. Only like this both tube halves will produce the same sound. We do this matching both for plate current, and Transconductance, using official Telefunken operating points. The system balancing was an official part of the Telefunken tubes like ECC802S etc. However, for 6922 this officially was never done by manufacturers. Still we offer it for this tube, because specially with this tube it is more important than with any other audio tube! This is the number one tube where people report sound differences quickly. We use: Balancing delta = Max 1,5mA for the plate current (Static parameter) and 10% maximum difference for the transconductance (dynamic parameter).
So before a tube can be called balanced, first both halves must first have the same plate current, and after that have the same Transconductance It must pass both tests in sequence. This tube is a high gain (50x) AND a low voltage (100V) tube type at the same time. These two requirements are very hard to meet AT THE SAME TIME. So this makes it very difficult for the manufacturer to produce those tubes. The question is what will they do with the tubes that are not so good. Good NOS E88CC is almost impossible to buy now. It is normal that within one glass, you can have two quite different tubes, also with NOS. So very few tubes pass the balancing test.
Special ECC88/6922 note for technicians: I think we can say, this is one of the most difficult tubes, if you need a good replacement. If any discussion comes up about about too much noise, or bad sound, it is with this tube. Where does this come from? There are many amplifiers using this tube in so called SRPP circuits. As you know, with a circuit like this, both halves of one tube are put in series, so must draw the same plate current per definition. Problems will come with an unselected tube, where both tube halves by itself actually would have liked to draw a different current. The result of such an unbalanced tube in an SRPP circuit is, both tube halves are in a "battle" about what current they can draw. One of the two halves will try to dominate, and determine the final current for the circuit. The voltages in the circuit are now not correct any more. Also the actual current drawn can be too high or too low, depending if the "higher" or "lower" current tube is the winner. If a stronger mis-balance occurs, the circuit will produce more noise than normal and add too much distortion. Also tube wear out can be the result of this, and an acceptable result in the beginning will soon be less good, and the complaints starts from the beginning. To prevent any problems like this, for an SRPP circuit ONLY balanced tubes should be used.
Balancing a "GB" single tube means: only like this, both halves are guaranteed to have the same plate Transconductance, the same bias and same gain and by this produce the same sound. This is done on a calibrated tube tester, which measures the tube data with a 1600 Hz test tone. (So not one of those Hickock testers, and nobody knows what the tester is really testing)
