I've got a couple of questions about biasing the KT88:
First, the tube data sheets have what seems to be an odd footnote regarding the bias circuits for both cathode bias and fixed bias applications.
For cathode bias, the footnotes say that it is essential to use two separate cathode bias resistors.
For fixed bias, the footnotes say that it is essential to provide two separately adjustable bias sources.
I guess I must be missing something, as I've never seen this sort of footnote on a valve data sheet before. Can anyone explain it? Is this an artifact from the day when tubes were not sold in matched pairs, and the typical tubes were so poorly matched that bias had to be set individually?
I've been looking at some old HiFi schematics, and the cathode bias amps do all seem to use independent cathode bias resistors on each KT88. The fixed amps all use a separate bias control pot for each KT88, and sometimes they even have a 3rd pot to adjust the bias range of the other two pots.
In contrast, for other tubes cathode bias is typically accomplished with a common cathode resistor. Fixed bias is typically accomplished with a single bias range control, and if you're not planning on using matched tubes perhaps a balance pot is in the circuit. I'm wondering if there's something "different" about the KT88 that requires independent adjustment. I sure seem to be missing the point on this.
Second, I'm looking for a source for bias pots for fixed bias applications. If anyone can recommend what type of pots would be best in this role, I'd appreciate your help. If you could provide a Mouser part number, I'd be happier still!
TIA!
First, the tube data sheets have what seems to be an odd footnote regarding the bias circuits for both cathode bias and fixed bias applications.
For cathode bias, the footnotes say that it is essential to use two separate cathode bias resistors.
For fixed bias, the footnotes say that it is essential to provide two separately adjustable bias sources.
I guess I must be missing something, as I've never seen this sort of footnote on a valve data sheet before. Can anyone explain it? Is this an artifact from the day when tubes were not sold in matched pairs, and the typical tubes were so poorly matched that bias had to be set individually?
I've been looking at some old HiFi schematics, and the cathode bias amps do all seem to use independent cathode bias resistors on each KT88. The fixed amps all use a separate bias control pot for each KT88, and sometimes they even have a 3rd pot to adjust the bias range of the other two pots.
In contrast, for other tubes cathode bias is typically accomplished with a common cathode resistor. Fixed bias is typically accomplished with a single bias range control, and if you're not planning on using matched tubes perhaps a balance pot is in the circuit. I'm wondering if there's something "different" about the KT88 that requires independent adjustment. I sure seem to be missing the point on this.
Second, I'm looking for a source for bias pots for fixed bias applications. If anyone can recommend what type of pots would be best in this role, I'd appreciate your help. If you could provide a Mouser part number, I'd be happier still!
TIA!
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