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07-02-2024, 10:10 AM
Hi Guys
Yes, half of the bias balance pot value should be a portion of the bias circuit impedance.
Since we are considering the amplifier in its operating state, the first approximation has the diode shorted to get a rough DCR equivalent for the supply resistance.
Grid circuit resistance limits listed on the tube data sheet is often ignored, as we see when we look at OEM products. Someone in the past decided to use a certain value and everyone has parroted it ever since, generally because of ignorance - not knowing how the value is chosen; not wanting to risk losing some "magic".
A good and bad example is the SVT power amp, which TUT3 states is an inherently unstable circuit. One of the bandaids to enforce stability is the use of a high-value grid-stop for each output tube. These are 50k, which is the maximum grid-circuit resistance listed for 6550. All three grid-stops are driven by the low-z output of a cathode follower. The grid circuit resistance for each tube is then roughly 50k. Normally, at least 90% of the grid circuit resistance would be the grid-leak resistor, but not here. One can see that the data sheet has been respected although in an unusual manner.
In some silver face era Twin Reverb and Dual Showman versions, the usual 220k grid-leak for the parallel output tubes is reduced to 68k. Neither value reflects the fixed-bias limit of 100k for a single tube.
During normal operation with tubes that have a good vacuum, grid current is essentially zero. If a tube draws grid current due to some internal failure, having a lower grid resistance likely won't make a difference to the tube's demise. However, since the bias voltage of the output tube is the most important voltage in the chassis it seems appropriate to attempt to design-in reliability wherever we can.
Yes, half of the bias balance pot value should be a portion of the bias circuit impedance.
Since we are considering the amplifier in its operating state, the first approximation has the diode shorted to get a rough DCR equivalent for the supply resistance.
Grid circuit resistance limits listed on the tube data sheet is often ignored, as we see when we look at OEM products. Someone in the past decided to use a certain value and everyone has parroted it ever since, generally because of ignorance - not knowing how the value is chosen; not wanting to risk losing some "magic".
A good and bad example is the SVT power amp, which TUT3 states is an inherently unstable circuit. One of the bandaids to enforce stability is the use of a high-value grid-stop for each output tube. These are 50k, which is the maximum grid-circuit resistance listed for 6550. All three grid-stops are driven by the low-z output of a cathode follower. The grid circuit resistance for each tube is then roughly 50k. Normally, at least 90% of the grid circuit resistance would be the grid-leak resistor, but not here. One can see that the data sheet has been respected although in an unusual manner.
In some silver face era Twin Reverb and Dual Showman versions, the usual 220k grid-leak for the parallel output tubes is reduced to 68k. Neither value reflects the fixed-bias limit of 100k for a single tube.
During normal operation with tubes that have a good vacuum, grid current is essentially zero. If a tube draws grid current due to some internal failure, having a lower grid resistance likely won't make a difference to the tube's demise. However, since the bias voltage of the output tube is the most important voltage in the chassis it seems appropriate to attempt to design-in reliability wherever we can.
