03-22-2010, 03:46 PM
The 24s or the 27 may have heater to cathode leakage causing residual hum. I have seen many 27s that develop H-K leakage.
Even on a good day putting your ear next to an EM speaker will result is some very slight hum, unless the field coil is powered with pure D.C. Since the field coil is usually the power supply filter choke, there's really no way around a slight residual hum real close up.
Also be sure no autio tube grid wires were moved or re-routed during restoration. Grid wires anywhere near an filament AC wire will pick it up too.
Chuck
Even on a good day putting your ear next to an EM speaker will result is some very slight hum, unless the field coil is powered with pure D.C. Since the field coil is usually the power supply filter choke, there's really no way around a slight residual hum real close up.
Also be sure no autio tube grid wires were moved or re-routed during restoration. Grid wires anywhere near an filament AC wire will pick it up too.
Chuck
Chuck Schwark,
The Philco Repair Bench
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