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Old 27th Oct 2020, 4:04 pm   #35
Rich400
Tetrode
 
Join Date: Oct 2020
Location: Lowestoft, Suffolk, UK.
Posts: 71
Default Re: Adding a selectable line input to valve radiogram

Quote:
Originally Posted by Richardgr View Post
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.
Hi Richard
I have just scanned through your post and I am very much on board with what you are saying. I will look into it deeper shortly.
I am sure this will be contentious but it appears to fit in with my preference for electrical safety.
My main intention is restore this gram and make it acceptable and safe to operate in my household, as opposed to providing another layer of safety when fault finding. (Providing you have one hand in your pocket in a safe environment etc etc).
It is my opinion that Isolation transformers mainly Provide a recognised method of protection for a trained audio engineer when the chassis is exposed and live fault finding is underway.
Once this unit is up to spec the next time the chassis will be exposed is if it develops a fault, in which case it will be removed and passed into the hands of a competent person to repair in a suitable environment of their choice, along with attached details of potential mods.

I was thinking of locating a safety isolating transformer and fitting it, And exchanging metal pot spindles for ABS versions to mitigate exposure to mains voltage in the event the knobs fell off!

Now in reality it makes much more sense To provide a modern earth trip on board and earth the exposed metalwork. There are of course caveats to this but if your circuit modification is viable I am happy to modify it and provide additional Cabinet internal shielding with warnings of live conductors and enclose details of these circuit mods onboard.
I do not accept the rationale that earth trips should not be used as they may fail and only operate in the event of a shock so should not be fitted. It enables metalwork to be earthed and provides a better level of protection in normal use. In the event of An on board earth trip failure, My house (and I’d suggest many other houses) have another earth trip at the consumer unit albeit with a slightly longer disconnection time due to the slight increase in cabling. So redundancy there.
I am sure there will very good audio engineers with different views, and I look forward to hearing any pitfalls I have not considered in case my rationale is incorrect. I would only ask that safety schemes are considered as protection for the user rather than a repair engineer. An engineer of course would be armed with the mod information with the chassis and makes his own arrangements for safety.
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