What were the tubes used in this model originally purposed for? My recollection was that the E810F in the H3000 was meant for telephony switching, and was not originally intended for audio, which is why it's an offbeat tube- not expensive, but not usually carried by the usual audio suppliers of NOS (though who knows what they have accumulated). wrm57 is correct as to my knowledge, David is retired, Albert's your man. (David was great to work with, but you have to appreciate Albert's sense of humor- a wonderful character).