10-12-2024, 02:18 PM
Separate filament windings are required for the 71a and the 80.
The PT-156 look carefully at the two green wires the one closest to the core is the start and will be the A- or bias connection the green closest to the outside of the core is the grid connection.
Or. measure the two windings green to black and black to green the winding that has the least resistance will be the A- side therefor that associated green would go to bias. The winding black to green that has more resistance because it has greater length, that associated green goes to the grid. Use a D'Arsonval meter to measure the transformer.
The P-T156 may not have sufficient plate impedance for the 26 tube so there may be distortion. Try adjusting the bias. If that does not work then another choice for the inter-stage transformer. This replacement transformer is bobbin wound, there are no layers of glassine between the windings...
The 71a has a relatively high plate impedance look for a 5k impedance transformer.
See: http://www.radioremembered.org/outimp.htm
Much has been written and published in the Antique Radio Forum on the refurbishing of the 100a speaker. These too are very plentiful so finding a rebuild-able specimen is not hard.. The audio coupler fails or a coil winding. The speaker does much better with a magnet re-charge or the use of modern helper magnet.
The 1.4 volt filament supply must provide enough current, the 26 emission falls off rapidly below 1.25 volts.. You will need to provide for center-tapped hum pots on the '26's and the 71a.
Beware that early 27's are prone to heater cathode leakage, use a very late edition of the 27 for the detector.
Your building of a power supply is commendable but there are many 18's out there that have supplies that can be salvaged... Often the caps are still viable after nearly 95 years...
Chas
The PT-156 look carefully at the two green wires the one closest to the core is the start and will be the A- or bias connection the green closest to the outside of the core is the grid connection.
Or. measure the two windings green to black and black to green the winding that has the least resistance will be the A- side therefor that associated green would go to bias. The winding black to green that has more resistance because it has greater length, that associated green goes to the grid. Use a D'Arsonval meter to measure the transformer.
The P-T156 may not have sufficient plate impedance for the 26 tube so there may be distortion. Try adjusting the bias. If that does not work then another choice for the inter-stage transformer. This replacement transformer is bobbin wound, there are no layers of glassine between the windings...
The 71a has a relatively high plate impedance look for a 5k impedance transformer.
See: http://www.radioremembered.org/outimp.htm
Much has been written and published in the Antique Radio Forum on the refurbishing of the 100a speaker. These too are very plentiful so finding a rebuild-able specimen is not hard.. The audio coupler fails or a coil winding. The speaker does much better with a magnet re-charge or the use of modern helper magnet.
The 1.4 volt filament supply must provide enough current, the 26 emission falls off rapidly below 1.25 volts.. You will need to provide for center-tapped hum pots on the '26's and the 71a.
Beware that early 27's are prone to heater cathode leakage, use a very late edition of the 27 for the detector.
Your building of a power supply is commendable but there are many 18's out there that have supplies that can be salvaged... Often the caps are still viable after nearly 95 years...
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
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