07-10-2024, 05:57 PM
Hi DPorcaro,
+1 on all the other comments.
For some reason, maybe to keep hum down, all transformerless AA5 type sets had the power switch at the B- voltage level (chassis ground level)and "low" side of the heater string (1st AF Tube heater), but chassis ground to B- was almost always through a 0.2 - 0,5 uF cap. However, on some sets, the B- was directly to the chassis. This is dangerous because:
If B- was to the AC Neutral pin of the plug, the chassis was at earth ground potential with the switch on, but with the switch off, the chassis would be hot because with no current flowing through the tube heaters, their resistance is very low.
Many restorers will change the wiring so the switch is in the "hot" side, in other words, switch is in the line between the hot pin of the power plug and the rectifier plate and heater string high side (usually the rectifier heater) and install a polarized plug to ensure that the B- and chassis is at neutral level. This makes the set much safer.
The problem with this set (46-1203) and similar Philco units was that it used a voltage doubler circuit. Not just a half wave doubler, but a full wave doubler. This put about 175V of rather stable DC to the output transformer and allowed the 35L6 to put out almost 2 watts of audio instead of just 1 watt if run at 90-110V of B+. The RF Amp, Converter and IF Amp tubes were also run at higher plate voltages, increasing the range of the set. This gave the set great performance especially if it has the 8" speaker. However, with a full wave doubler and no transformer, B-, which is also signal ground for the audio, is ALWAYS hot compared to the chassis. Hence, the 0.2uF cap in the "ground" (signal return) side of pickup. Unfortunately, this signal return is grounded to the changer frame through another cap, C100.
For a lot of folks in a house with wood floors and either a basement or crawl space, there would be no "tickle" (or worse). However, in a house with a "slab" foundation (like where I spent my childhood getting blasted by AA5s), or an apartment, this is different. To make the unit completely safe, you maty have to obtain an isolation transformer.
Philco always found a cheap and efficient way to squeeze as much performance as possible out of their sets, and the voltage doubler was one such measure.
+1 on all the other comments.
For some reason, maybe to keep hum down, all transformerless AA5 type sets had the power switch at the B- voltage level (chassis ground level)and "low" side of the heater string (1st AF Tube heater), but chassis ground to B- was almost always through a 0.2 - 0,5 uF cap. However, on some sets, the B- was directly to the chassis. This is dangerous because:
If B- was to the AC Neutral pin of the plug, the chassis was at earth ground potential with the switch on, but with the switch off, the chassis would be hot because with no current flowing through the tube heaters, their resistance is very low.
Many restorers will change the wiring so the switch is in the "hot" side, in other words, switch is in the line between the hot pin of the power plug and the rectifier plate and heater string high side (usually the rectifier heater) and install a polarized plug to ensure that the B- and chassis is at neutral level. This makes the set much safer.
The problem with this set (46-1203) and similar Philco units was that it used a voltage doubler circuit. Not just a half wave doubler, but a full wave doubler. This put about 175V of rather stable DC to the output transformer and allowed the 35L6 to put out almost 2 watts of audio instead of just 1 watt if run at 90-110V of B+. The RF Amp, Converter and IF Amp tubes were also run at higher plate voltages, increasing the range of the set. This gave the set great performance especially if it has the 8" speaker. However, with a full wave doubler and no transformer, B-, which is also signal ground for the audio, is ALWAYS hot compared to the chassis. Hence, the 0.2uF cap in the "ground" (signal return) side of pickup. Unfortunately, this signal return is grounded to the changer frame through another cap, C100.
For a lot of folks in a house with wood floors and either a basement or crawl space, there would be no "tickle" (or worse). However, in a house with a "slab" foundation (like where I spent my childhood getting blasted by AA5s), or an apartment, this is different. To make the unit completely safe, you maty have to obtain an isolation transformer.
Philco always found a cheap and efficient way to squeeze as much performance as possible out of their sets, and the voltage doubler was one such measure.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
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Best Regards,
MrFixr55
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