B+ ramping up in a Superlead build
Posted: Tue Feb 20, 2018 6:21 pm
Lately I've been feeling a major degradation of my Superlead's tone - it seems that over the past couple of months the amp started sounding stiff, brittle, one dimensional, non-musical and muddy.
I decided to check bias and found out an outrageous 560VDC on pin 3 of the power tubes.
Bias current was 51mA, measured on pin 8 of the hottest of the four output tubes.
Once I set the bias to an optimal 31mA range I played for another 30 minutes, measured some minor fluctuations, rebiased and got a reading of 579VDC on pin 3 of each EL34.
Soundwise the amp pretty much regained (no pun intended) its former glory but I can't figure out what's up with the crazy voltages and I'm not sure the tubes would last this way (that is if they themselves are not the culprit).
The amp is pretty much a Metro 1969 kit which I built in 2015 with '68 filtering. It sports Classic Tone iron, power tubes are Mullard reissues which I've had since completing the amp some three years ago. I biased the amp some 7-8 months ago and measured a friendly 460-ish VDC B+ back then.
What could cause this upsurge of B+? bad diodes? filter caps? tubes? PT secondaries?
Any help shedding some light on what is the cause for the ramped up voltages will be highly appreciated!
I decided to check bias and found out an outrageous 560VDC on pin 3 of the power tubes.
Bias current was 51mA, measured on pin 8 of the hottest of the four output tubes.
Once I set the bias to an optimal 31mA range I played for another 30 minutes, measured some minor fluctuations, rebiased and got a reading of 579VDC on pin 3 of each EL34.
Soundwise the amp pretty much regained (no pun intended) its former glory but I can't figure out what's up with the crazy voltages and I'm not sure the tubes would last this way (that is if they themselves are not the culprit).
The amp is pretty much a Metro 1969 kit which I built in 2015 with '68 filtering. It sports Classic Tone iron, power tubes are Mullard reissues which I've had since completing the amp some three years ago. I biased the amp some 7-8 months ago and measured a friendly 460-ish VDC B+ back then.
What could cause this upsurge of B+? bad diodes? filter caps? tubes? PT secondaries?
Any help shedding some light on what is the cause for the ramped up voltages will be highly appreciated!