UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Adding a selectable line input to valve radiogram

Richardgr · 27th Oct 2020, 2:18 pm

Looking at the schematic, this is not an 'AC/DC circuit', where the tubes are in series with the mains wiring, but there is a transformer that is just not connected as we would want in this day and age.

If you wired the radiogram for 3 wires, create a chassis ground, then only supply the primary of the power transformer from the mains, breaking the connection of the neutral from the chassis, and take a amplifier ground from the last smoothing capacitor, then it would be no more treacherous than any other vintage tube amp.

I have read lots of comments about isolation transformers, but they are tools for professionals, and there is just the same opportunity to be shocked on the other side of the isolation transformer; their primary purpose is to ensure the equipment is galvanically isolated, so you can't kill yourself when you working on a piece of equipment and you hold the radiator, for example.

Whilst an isolation transformer would improve the safety of the device, it does not solve the chassis ground issue, which is their to protect you from the device if it has a failure that causes a breakdown in isolation.

27th Oct 2020, 2:18 pm	#33
Richardgr Heptode Join Date: Oct 2019 Location: Täby, Sweden Posts: 702	Re: Adding a selectable line input to valve radiogram Looking at the schematic, this is not an 'AC/DC circuit', where the tubes are in series with the mains wiring, but there is a transformer that is just not connected as we would want in this day and age. If you wired the radiogram for 3 wires, create a chassis ground, then only supply the primary of the power transformer from the mains, breaking the connection of the neutral from the chassis, and take a amplifier ground from the last smoothing capacitor, then it would be no more treacherous than any other vintage tube amp. I have read lots of comments about isolation transformers, but they are tools for professionals, and there is just the same opportunity to be shocked on the other side of the isolation transformer; their primary purpose is to ensure the equipment is galvanically isolated, so you can't kill yourself when you working on a piece of equipment and you hold the radiator, for example. Whilst an isolation transformer would improve the safety of the device, it does not solve the chassis ground issue, which is their to protect you from the device if it has a failure that causes a breakdown in isolation.