03-11-2024, 12:00 PM
Go here to determine the output transformer:
http://www.radioremembered.org/outimp.htm
Caps #48 & #51 should be replaced with a type that has a known Dv/Dt rating. When static is received by the radio, the plate current is abruptly cutoff. That causes a collapse in the magnetic field of the OPT and a high voltage pulse appears across the windings. That pulse can puncture the paper layers and arc. The transformer may not open on the first "hit" but acids are created that eat away at the copper until the winding goes open. Avoid the common metalized cap without a rating as these cannot withstand a pulse, it is NOT that the film will puncture, it is the metallizing, so thin it separates and looses capacity, therefore the cap can no longer protect the transformer. These same cap(s) provide a filter for high frequencies, part of the tone... Increasing the voltage rating will not help, as it is the metalizing is too thin. A cap designed for pulse service has thicker metalizing. Use the same voltage value as the OEM paper cap...
The OEM speaker has a hum buck coil in addition to the voice coil, that should bring the impedance up for a modern output transformer. It is BEST to re-cone such a speaker. It is tedious but not difficult.
The 6F6 is notorious for grid poisoning, if either of the output tubes has this problem plate current with dramatically increase and the plate may blush, punishing the OPT... Do be sure all the components in the passive phase converter are correct, that too can effect the 6F6's as WILL the grid voltage.
GL
Chas
http://www.radioremembered.org/outimp.htm
Caps #48 & #51 should be replaced with a type that has a known Dv/Dt rating. When static is received by the radio, the plate current is abruptly cutoff. That causes a collapse in the magnetic field of the OPT and a high voltage pulse appears across the windings. That pulse can puncture the paper layers and arc. The transformer may not open on the first "hit" but acids are created that eat away at the copper until the winding goes open. Avoid the common metalized cap without a rating as these cannot withstand a pulse, it is NOT that the film will puncture, it is the metallizing, so thin it separates and looses capacity, therefore the cap can no longer protect the transformer. These same cap(s) provide a filter for high frequencies, part of the tone... Increasing the voltage rating will not help, as it is the metalizing is too thin. A cap designed for pulse service has thicker metalizing. Use the same voltage value as the OEM paper cap...
The OEM speaker has a hum buck coil in addition to the voice coil, that should bring the impedance up for a modern output transformer. It is BEST to re-cone such a speaker. It is tedious but not difficult.
The 6F6 is notorious for grid poisoning, if either of the output tubes has this problem plate current with dramatically increase and the plate may blush, punishing the OPT... Do be sure all the components in the passive phase converter are correct, that too can effect the 6F6's as WILL the grid voltage.
GL
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
