Building a 300B PSE

[MGCRead]

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

[bob orbell]

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

[MGCRead]

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

[MGCRead]

Hello

I have had a busy summer but have completed the build of one monobloc. Applying a 1v sine wave ( from a good quality generator ) I get a good shape output wave from the amp BUT with a slightly offset ghost wave slightly to the right ( presumably ahead ) of the main wave. More alarmingly the amp sings/whistles at frequencies between about 5K and 50K Hz; I can't quite work out where the noise comes from.

I obviously don't wish to hook it up to my expensive speakers in its current state.

Any ideas of what I should try next would be very welcome.

Very many thanks, Malcolm

[MGCRead]

Sorry that should have said 500Hz to 5KHz ( needless to say I din't stress it with 50K !! )

[A Stuart]

Welcome to the forum.

I am unlikely to be able to solve the problem, but one thing which does occur to me is whether you had the oscilloscope across a dummy load (eg 8ohm resistor) on the speaker terminals. I understand valve amps must not be run without a load attached to avoid oscillations.

I am also slightly intrigued by this "ghost wave". You can attach jpgs (max size 175kB, max 1024x1024pixels) using the little paperclip symbol above when you are posting.

Also, is the earth of your oscilloscope probe attached to its chassis (therefore mains earth), or is it a true differential reading?

Alastair