Beginner here, working on putting together a TSE-II 300B with a target operating point of 385v B+ @ 60mA. Jack of Electraprint is suggesting a 65mA rating for the output transformers after I'd initially mentioned wanting a 100mA, saying "all tubes rise in current when driven, it is only temporary or 70% of the peak" (sic).
Is this correct? My reasons for second-guessing this are that, first, most of the OPTs for 300B applications I see have at least a 100mA rating, and I've seen multiple references to a 100mA rating for Electraprint OPTs in particular; and second, my understanding is that in an SE amp the current through the output tubes is relatively constant, varying little with applied signal. I'm not sure quite what to make of his explanation.
How likely am I to run into saturation with OPTs rated for 65mA? Am I guaranteed to run into saturation if I were to later decide to run the tubes at 80mA?
Is this correct? My reasons for second-guessing this are that, first, most of the OPTs for 300B applications I see have at least a 100mA rating, and I've seen multiple references to a 100mA rating for Electraprint OPTs in particular; and second, my understanding is that in an SE amp the current through the output tubes is relatively constant, varying little with applied signal. I'm not sure quite what to make of his explanation.
How likely am I to run into saturation with OPTs rated for 65mA? Am I guaranteed to run into saturation if I were to later decide to run the tubes at 80mA?
Single ended OPT's have a gap in the core do deal with the DC current in the primary and the constant magnetic field that it creates. These fields cancel in a push pull OPT, but will cause saturation in an SE OPT, so the core is gapped to prevent it.
The gap width is set by the transformer manufacturer. It will work over a range of primary currents, but is optimized for a single primary current value.
Confusion arises when some manufacturers state the maximum current, while others state the optimum current. I believe that Jack states the optimum current value, and 65 mA should work over the range of currents usually seen in 300B amplifiers.
My original TSE was built for 45's, but the Electra Print transformers were made for a current a bit higher than the usual 30 mA for a 45 tube. It was nearly 20 years ago so I don't remember the exact spec, probably around 50 mA. They work fine with 45's at 30 mA, and with 300B's at 70.
Setting an "optimum" gap at 100 mA would put the useful operating range above where most people would run a 300B tube.
The gap width is set by the transformer manufacturer. It will work over a range of primary currents, but is optimized for a single primary current value.
Confusion arises when some manufacturers state the maximum current, while others state the optimum current. I believe that Jack states the optimum current value, and 65 mA should work over the range of currents usually seen in 300B amplifiers.
My original TSE was built for 45's, but the Electra Print transformers were made for a current a bit higher than the usual 30 mA for a 45 tube. It was nearly 20 years ago so I don't remember the exact spec, probably around 50 mA. They work fine with 45's at 30 mA, and with 300B's at 70.
Setting an "optimum" gap at 100 mA would put the useful operating range above where most people would run a 300B tube.