Thanks very much, Jet. Could you give us a rough idea of the magnitude of these voltage changes that you see in your particular amp?
I did a few minutes of research trying to find specs on the resistances of filament windings of transformers that would be used in these kinds of applications, and came up pretty much empty. It is surprising that such specs aren't commonly provided, which makes it hard to pin down the issues being discussed in any kind of quantitative manner. Which opens the door to speculation and differences of opinion.
Not to mention that it seems highly unlikely that data would be available for the particular tubes being discussed, or most other tubes for that matter, which would quantitatively indicate how their MTBF (mean time between failure) varies as a function of filament current, among other application dependent variables.
In fact the only conceivably relevant data I was able to find was for some military grade 60 Hz filament transformers specified in MIL-PRF-27 and its associated documents. For transformers intended for applications that are roughly similar in terms of voltage and current those documents generally indicated filament resistances in the area of 0.1 to 0.25 ohms. Which seems reasonably small **IF** the transformer and its turns ratio are chosen by the designer based on a loading assumption that is at least roughly in the general ballpark of the actual load.
Best regards,
-- Al
I did a few minutes of research trying to find specs on the resistances of filament windings of transformers that would be used in these kinds of applications, and came up pretty much empty. It is surprising that such specs aren't commonly provided, which makes it hard to pin down the issues being discussed in any kind of quantitative manner. Which opens the door to speculation and differences of opinion.
Not to mention that it seems highly unlikely that data would be available for the particular tubes being discussed, or most other tubes for that matter, which would quantitatively indicate how their MTBF (mean time between failure) varies as a function of filament current, among other application dependent variables.
In fact the only conceivably relevant data I was able to find was for some military grade 60 Hz filament transformers specified in MIL-PRF-27 and its associated documents. For transformers intended for applications that are roughly similar in terms of voltage and current those documents generally indicated filament resistances in the area of 0.1 to 0.25 ohms. Which seems reasonably small **IF** the transformer and its turns ratio are chosen by the designer based on a loading assumption that is at least roughly in the general ballpark of the actual load.
Best regards,
-- Al