Anyone know how often KT88 will drift over their life?
I picked up a used Xindak V30 and suspected the tubes needed adjustment. After searching around for a while I found this post in their discussion forum:
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The image points out the relevant resistors and trim pots to adjust, and seems to mention something about 0.5V, and 50mA, which is consistent with the 10 ohm value of these resistors based on ohms law.
After opening it up, they measured between 0.38 and 0.43V so I brought them back in spec.
Considering the hassle of opening and flipping over a heavy amp, is it worth making these points adjustable from the outside? How often might they drift?
By the way the amp sounds wonderful!
I picked up a used Xindak V30 and suspected the tubes needed adjustment. After searching around for a while I found this post in their discussion forum:
»¶Ó*·ÃÎÊе¿ËÍøÕ¾
The image points out the relevant resistors and trim pots to adjust, and seems to mention something about 0.5V, and 50mA, which is consistent with the 10 ohm value of these resistors based on ohms law.
After opening it up, they measured between 0.38 and 0.43V so I brought them back in spec.
Considering the hassle of opening and flipping over a heavy amp, is it worth making these points adjustable from the outside? How often might they drift?
By the way the amp sounds wonderful!
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This is a tough call. Do you know the age of the KT88's? If they were originally biased at 50mA and they were now reading 38 to 43mA then they are probably getting near end of life. I would say check them after a month and see if they have drifted. If they still measure 50mA after a month...I'd give them 6 months before I checked again.
If they were brand new KT88's I'd bias them then check them after a few days. If they were still balls on accurate I'd leave them for a year. Remember your voltage from your outlet can vary too which will change the bias reading so you might want to look at "are all the bias readings still equal" instead of are they all still 50mA. If they are all still equal and plus or minus a few mA I would leave them alone. Or take note of your outlet voltage and use that as a guide as to weather or not the bias is actually drifting on the tubes.
Bottom line is the more stable they appear to be the longer I would leave them....up to a year if they are new and stable.
Just my experience.
If they were brand new KT88's I'd bias them then check them after a few days. If they were still balls on accurate I'd leave them for a year. Remember your voltage from your outlet can vary too which will change the bias reading so you might want to look at "are all the bias readings still equal" instead of are they all still 50mA. If they are all still equal and plus or minus a few mA I would leave them alone. Or take note of your outlet voltage and use that as a guide as to weather or not the bias is actually drifting on the tubes.
Bottom line is the more stable they appear to be the longer I would leave them....up to a year if they are new and stable.
Just my experience.
Thanks, that's a good suggestion. I measured the B+ voltage at 471 so we'll see if that varies too next time I check.
I think based on the 50mA bias recommended, the tube dissipation works out to 23.5W, or 55% of 42 watts, so a bit on the cold side, but I'd imagine the tubes would last a few thousand in that case...
I think based on the 50mA bias recommended, the tube dissipation works out to 23.5W, or 55% of 42 watts, so a bit on the cold side, but I'd imagine the tubes would last a few thousand in that case...