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

[MGCRead]

Hello Alistair

Yes the probes were across a dummy load, but the scope was not earthed to the amp's chassis.

I will take a photo tomorrow. Thank you for your help.

Regards Malcolm

[A Stuart]

I meant the oscilloscope's own chassis. Mine is like that, which presents a problem reading off a circuit with earth lift.

[MGCRead]

Hello Alistair

I guess the probe earth must be mains earth via the scope, it is connected via a BNC plug.

Attached ( I hope ) are two photos. The multiple curves I get with an input of 1K Hz at 100mV; a signal level higher than this causes loud buzzing/whistling sounds from the amp which can't be good !

Any suggestions for where to look for the fault would be very welcome.

Malcolm

[A Stuart]

Hopefully someone here will have some concrete advice for you.

My best interpretation of the oscilloscope trace is that your main visible signal is superimposed on a smaller component of much lower frequency.

The lower frequency adds or subtracts from the trace as time passes.
So the trace crosses zero volts (thus triggering a the start of a new scan line) slightly earlier or later in the main visible signal causing the Left-Right shift
That low frequency component also adds or subtracts slightly from the level of the main signal's trace during that scan giving the Up-Down shift.

I cannot see the time base setting on your controls so cannot say whether the HF or LF component, whose existence I am suggesting, represents your signal-generator input signal.

However, I must suppose that being in possession of an oscilloscope you know at least as much as me, and definitely that there are are others here much more knowledgeable.

Also, regarding my previous comment re the oscilloscope ground; and not knowing whether your speaker 0 output is earthed to chassis or to circuit earth, you might try putting a resistor say 1k between the probe screen (croc clip near the end of your probe?) and the speaker 0 output to stop an earth loop through the oscilloscope.

Anyone, feel free to correct any error in my interpretation of the trace. I will stand in the corner with the Dunce's hat on.