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Old 1st Nov 2006, 9:00 am   #2
Join Date: Oct 2003
Location: Alton, Hampshire, UK.
Posts: 137
Default Re: KT66 as substitute for KT61

You can make almost any output pentode or beam tetrode work, provided that you change the cathode bias resistor to suit. Unless you actually measure the power output at full drive, you will probably not notice a degree of mismatch from the O/P transformer. However, you need to be careful not to overload either the transformer primary or the HT supply by drawing excessive anode and screen current. The easiest way to ensure this is to set the cathode bias resistor to give the same nominal HT current as the valve you are replacing. I would guess that you need to drop the KT66 anode current to about 40 mA. I think I had a U/S KT61 in one of those sets and just plugged in a spare 6V6. There was some loss of audio gain due, I presume, to a lower mutual condictance. Why use a KT66, when there are so many cheaper alternatives or when you can find a good used KT61 from our usual suppliers?

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