Good day gents
I hope someone will find this helpful. About 12 years ago I built myself a set of Mark 3 monoblocks with parts supplied by Roy at Tubes4hifi, both with the VTA driver board (the original one with 12AT7 driver tubes). One amp was totally quiet and performed flawlessly, while the other had a a very slight 60hz hum - but seeing as it was inaudible about 2 or 3 feet away and otherwise was identical sonically I thought, oh well I guess I must have done something differently, maybe twisted the internal leads etc not as tightly or something.
About 2 weeks ago the humming suddenly started to get progressively louder, to a point where by last weekend I knew something was going VERY wrong and started looking for the issue. I was convinced that the power transformer on that amp is failing as the humming seemed to originate from it. Using a tip I picked up at another DIY site and used a long screwdriver as a "stethoscope" of sorts, and touching the tip to all various components above chassis level (trannies, tubes etc) I nailed it down to the GZ34 rectifier and NOT the power tranny. I "borrowed" the rectifier tube from my ST-70 (same brand - Mesa Engineering) and lo and behold, all of a sudden both my Mark 3 amps were dead quiet, for the first time in 12 years! I can hear a hum maybe 6 inches to maybe a foot away but my amps are basically as quiet as a good sand amp now (that "low noise floor" some audio magazines like to call it). Heck, at one point I suspected a bad spot weld on one of the cages (yes, I use cages) and was thinking to have one cage re-welded or glued even as it seemed to "rattle" when tapped.
I'm currently waiting for an order of two new JJ 5AR4 rectifiers from TubeDepot, I kinda think it is high time that I start replacing it (just can't bring myself to part with the cash needed for the higher-end / NOS rectifiers). Oh and FWIW it's the left-channel monoblock that was causing the issue as seen in the pic series: https://dynacotubeaudio.forumotion.com/t514-dynaco-mark-iii-build
Best wishes
-- JJ
I hope someone will find this helpful. About 12 years ago I built myself a set of Mark 3 monoblocks with parts supplied by Roy at Tubes4hifi, both with the VTA driver board (the original one with 12AT7 driver tubes). One amp was totally quiet and performed flawlessly, while the other had a a very slight 60hz hum - but seeing as it was inaudible about 2 or 3 feet away and otherwise was identical sonically I thought, oh well I guess I must have done something differently, maybe twisted the internal leads etc not as tightly or something.
About 2 weeks ago the humming suddenly started to get progressively louder, to a point where by last weekend I knew something was going VERY wrong and started looking for the issue. I was convinced that the power transformer on that amp is failing as the humming seemed to originate from it. Using a tip I picked up at another DIY site and used a long screwdriver as a "stethoscope" of sorts, and touching the tip to all various components above chassis level (trannies, tubes etc) I nailed it down to the GZ34 rectifier and NOT the power tranny. I "borrowed" the rectifier tube from my ST-70 (same brand - Mesa Engineering) and lo and behold, all of a sudden both my Mark 3 amps were dead quiet, for the first time in 12 years! I can hear a hum maybe 6 inches to maybe a foot away but my amps are basically as quiet as a good sand amp now (that "low noise floor" some audio magazines like to call it). Heck, at one point I suspected a bad spot weld on one of the cages (yes, I use cages) and was thinking to have one cage re-welded or glued even as it seemed to "rattle" when tapped.
I'm currently waiting for an order of two new JJ 5AR4 rectifiers from TubeDepot, I kinda think it is high time that I start replacing it (just can't bring myself to part with the cash needed for the higher-end / NOS rectifiers). Oh and FWIW it's the left-channel monoblock that was causing the issue as seen in the pic series: https://dynacotubeaudio.forumotion.com/t514-dynaco-mark-iii-build
Best wishes
-- JJ