Re: Kolster-Brandes MR10
It's very unlikely that the heater is short circuit. It's much more likely that it's open circuit.
It doesn't matter if you fit an ECC81 or 12AT7, they are alternative codes for the same valve. If you need to buy one, just buy on price. There are various special quality and military variants, all of which should be fine. I agree with Mike that you should buy from somebody with a track record rather than any random seller. You may well find that a forum member has something suitable.
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