UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 120W PP EL34 amplifier re-born.

bikerhifinut · 26th Sep 2018, 10:51 pm

Andy,
daft idea maybe but have you tried feeding the preamp and LTP valves with 6.3VAC? i.e. use a separate filament transformer as I guess you are using a 9V or similar AC supply into the rectifier and regulator for that supply? Or even feed a 6.3V DC supply to the heaters from a bench PSU. That would eliminate the regulator/heater PSU if the fault still manifests.
Then I'd be thinking maybe the 317 could be faulty/damaged, these things can happen and they are pennies to substitute, or maybe a faulty transformer winding? again these days I prefer something like a LD1086 or similar as I feel they are a bit superior to the 317 but that may just be my bias.
Personally my feelings these days are DC heater supplies are overkill on circuits running at line level signals which on a power amp in my opinion sould be of the 1V mark or even a bit higher assuming an input from preamp/source of around 2V.
At those sort of sensitivities good wiring practice should eliminate heater induced hum.
Just my take on things, nowt personal!
Andy.

26th Sep 2018, 10:51 pm	#15
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: 120W PP EL34 amplifier re-born. Andy, daft idea maybe but have you tried feeding the preamp and LTP valves with 6.3VAC? i.e. use a separate filament transformer as I guess you are using a 9V or similar AC supply into the rectifier and regulator for that supply? Or even feed a 6.3V DC supply to the heaters from a bench PSU. That would eliminate the regulator/heater PSU if the fault still manifests. Then I'd be thinking maybe the 317 could be faulty/damaged, these things can happen and they are pennies to substitute, or maybe a faulty transformer winding? again these days I prefer something like a LD1086 or similar as I feel they are a bit superior to the 317 but that may just be my bias. Personally my feelings these days are DC heater supplies are overkill on circuits running at line level signals which on a power amp in my opinion sould be of the 1V mark or even a bit higher assuming an input from preamp/source of around 2V. At those sort of sensitivities good wiring practice should eliminate heater induced hum. Just my take on things, nowt personal! Andy.