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Old 11th April 2021, 05:17 PM
MGCRead MGCRead is offline
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Join Date: Apr 2021
Location: Malmesbury, Wilts, UK
Posts: 2
Default Building a 300B PSE

Hello, I have just joined this forum as I am sure it will prove useful as I am considering building the 300B PSE having bought the transformers about 20 years ago !

I am currently studying the build instructions and circuit diagrams. The earthing puzzles me. I would expect the case to be connected to the mains earth but the parts list states R24 ( 10 ohm, 2W ) connects the mains earth to the earth post. As the earth post is on the case it will be grounded through R24, is this safe? The wiring diagram on page 9 show the mains earth directly connected to the case which is what I would expect. Can anyone clarify this for me please. I don't want to start building until I understand the circuit better.

The transformers are isolated from the case, as are the signal terminals, but the choke is not.

Any help will be much appreciated.

Thank you, Malcolm
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Old 12th April 2021, 07:24 AM
bob orbell bob orbell is online now
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Join Date: Nov 2009
Location: kettering northants.
Posts: 2,255
Default Re: Building a 300B PSE

Hello Malcolm and welcome, the mains earth should be connected directly to the chassis and the 10 R resistor is connected from chassis to the 0 volts, it is just an earth lift, the choke winding will be and should be isolated from the frame, so the frame is bolted solidly to the chassis, this can be confirmed with a meter or better still a mega. Bob
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Old 12th April 2021, 08:51 AM
MGCRead MGCRead is offline
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Join Date: Apr 2021
Location: Malmesbury, Wilts, UK
Posts: 2
Default Re: Building a 300B PSE

Thank you Bob for a speedy and very helpful reply. That clarifies the ambiguity nicely and is what I expected. I will continue planning with more confidence now particularly having discovered some very helpful youtube videos by Nick of hificollective. I doubt I will spend hundreds of pounds on expensive components though as I very much doubt I will be able to hear any significant differences not having 'golden ears' but I do look forward to listening to a valve amplifier having known only trannies since my first hi-fi over 50 years ago.

Once again many thanks for your help, Malcolm
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