Nuts and Volts - September 2013

I do remember the output tubes (KT88s) being sold in matched pairs. They were supplied to us by Dynaco already "matched." There were no test sheets included; the boxes were glued together indicating they were a matched pair.

Maybe the fact that they were being sent to a service center and would more than likely be used in a customer repair instead of being sold over the counter to the owner directly had something to do with it.

As a service tech, I soon realized that matching the tubes could become compromised if certain components in the circuit associated with the output stage were off value or not exactly matching — especially in the MKVIs when comparing the left and right channels. No matter if the tubes were "matched" or not, components that were off value were more of an issue to performance quality and tube life.

In my opinion, the best way to make sure that matching the tubes is maximized is to verify the components — especially any carbon resistors and capacitors in the associated circuitry. Adjust the bias properly and always use eight ohm load resistors paralleled to the inputs of a dual trace scope with the input signal coming from a sine/square wave generator to see if — all combined — the circuitry end product is truly matched. I use this technique when servicing Marshall and Fender heads to this day, and it always gives me good results. How did the tube supplier "match" the tubes? I’m not really too sure, but I would assume that they used some sort of jig setup where they measured certain parameters of each tube from a large batch of tubes and then put them together as pairs as close as they came to having similar values/readings. It would be interesting if any of the readers could shed some light on this matter, and maybe describe the kind of setup that tube manufacturers actually use for matching vacuum tubes. David Asselin

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