I just found myself in a very interesting situation. I got a great deal on an ~1984 JCM800 2203. Plugged it in - sounded ok - a little flat and lacking punch/depth so I went happily along replacing original filter caps, and checked the rest of the circuit - the usual. Looks like the amp had been retubed at one point - EL34s, but one had a Marshall logo on it, so I figured that one was original. In recapping, I did note a thing or two - the first that jumped out to me was the bias pot was a 10K, not the typical 22K, but I didn't really give it much thought at the time, it was just more of a passing thought of "hmm, it's definitely original, I guess it's just one of those anomalies in this silly amp world". (Spoiler alert, I should have looked closer at the rest of the bias circuit, it all would have been obvious if I did that.) When I got the caps in and put in a brand new set of tubes, the amp was better but still lacking "fullness" so FINALLY, I decided to check output tube current and at the very hottest end of the adjustment, the tubes were each drawing less than 10ma on a plate voltage of 468V - and BANG!!! it hit me, this amp was designed for 6550s or KT88s!!! In finally taking a closer look at the bias circuit, it's clear. Below is a snapshot of a typical EL34 circuit where I have pencilled in the bias circuit values in this amp. MY QUESTION: Is there a simple way to determine if this amp should use 6550 or KT88 tubes? Is an indication the bias feed resistors? The 150K in this amp make me think 6550 as a lot of KT88 amps will have 100K bias feed resistors... I don't see any difference anywhere else in the amp compared to the "standard" schematic. Or would either work? While 6550s require a higher negative bias than EL34s, but my gut tells me the bias at its hottest setting would still not allow quite enough current draw. I'm thinking the bias is actually high enough for the even higher negative bias requirements of a KT88. The thing is, I don't have quads of either laying around to test the theory. The negative bias sweep is about -53.4V down to 48.6V which is pretty narrow, but that's what it is. More than likely after I figure out which is the best tubes, I may modify the bias to give a wider range.