How much difference between tubes is acceptable ?
I have an amp with somes differences in voltage between tubes.
45 Tube with:
Left 240v plate , and 249v right.
Left cathode -34v and right -34.8v. ( 1K resistor )
And 300B with:
Left 400v plate and right 410v.
Left cathode -68.1v and right -71v. ( 1K resistor )
It is acceptable ?
I have an amp with somes differences in voltage between tubes.
45 Tube with:
Left 240v plate , and 249v right.
Left cathode -34v and right -34.8v. ( 1K resistor )
And 300B with:
Left 400v plate and right 410v.
Left cathode -68.1v and right -71v. ( 1K resistor )
It is acceptable ?
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The variance between both cathode sets is < 5%. You define what acceptable is, but < 5% strikes me as being quite reasonable.
For Va 10v is ok to my mind, a volt or so for Vk, but this is the DC conditions, what more important and more revealing is the AC conditions.
Andy.
Andy.
Your variances would also be caused by the DCR of the output transformer. That can cause one tube to have lower plate voltage in a pushpull amplifier
One -static- operating point (if it not too far from operating point in datasheet) means that tube is operating, nothing more.
Try to measure -at least- gm in this point.
Try to measure -at least- gm in this point.
The plate voltage in a power amp is normally == the power supply voltage. Any real difference suggests a fault in the output transformer.
The cathode voltage in a self-biased power amp is significant, and we always allowed as much as 20% tolerance. 1950s tubes in self-bias usually did better, but the '45s are not 1950s tubes.
I have seen the two halves of a Russian 12AT7 match to 2% but I think I was lucky.
The cathode voltage in a self-biased power amp is significant, and we always allowed as much as 20% tolerance. 1950s tubes in self-bias usually did better, but the '45s are not 1950s tubes.
I have seen the two halves of a Russian 12AT7 match to 2% but I think I was lucky.
If it is for single tubes in single ended amplifiers, then consider that most ears are not matched within 5%, left to right.
5% voltage match is 0.5 dB match.
If it is for a push pull amplifier, it is an issue . . .
Most push pull output transformers do not operate as well when the quiescent push and pull DC currents do not match.
Saturation of Bass frequencies.
Early saturation and Intermodulation Distortion.
Just my opinion.
5% voltage match is 0.5 dB match.
If it is for a push pull amplifier, it is an issue . . .
Most push pull output transformers do not operate as well when the quiescent push and pull DC currents do not match.
Saturation of Bass frequencies.
Early saturation and Intermodulation Distortion.
Just my opinion.
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