A few months ago I completed a full rebuild of my buddy's MkIIIs using the VTA boards. After 20+ hours of break-in their sound was like a resting giant - definitely big and powerful sounding but rather sleepy. Some background - I have built / rebuilt several MKIIIs using original iron but with either a driver of my own design or the Triode Electronics driver board. Also, I have rebuilt my Dynaco ST-70 using the VTA board of the same circuit design as the MKIII board. All of these amps have sounded great - fast, big open sound stage, etc., but not my buddy's. I have tried different tubes, rechecked the circuit and voltages and measured the turns ratio of the transformers. All is what it should be. 10kHz square wave is respectable.
One thing that I noticed however which could be the issue is the low amount of global NFB. From my measurement and calculation it's only 4.4dB or basically nothing which may not work with the KT88s running UL mode. I wrote Roy about this in December but didn't receive any reply. Comments would be appreciated. Thanks, Jack
One thing that I noticed however which could be the issue is the low amount of global NFB. From my measurement and calculation it's only 4.4dB or basically nothing which may not work with the KT88s running UL mode. I wrote Roy about this in December but didn't receive any reply. Comments would be appreciated. Thanks, Jack
