Building a 300B PSE

[John Caswell]

I am a bit puzzled by three things
1) I would have expected that when you swapped the leads to the speaker terminals it made a difference. I would have expected it to break into violent oscillation - so that makes me think that the feedback is correct.
2) This "singing" - as you have a dummy load on the amplifier where are you hearing it from? Is it coming from the output transformer? Does it occur with no signal? If it is in sympathy with the input signal ie it sounds like the 1khz test tone, then the transformer is ringing with a phenomenon known as "magnetostriction" whereby the windings and transformers are changing size, albeit very, very small at the same frequency as the test tone.
3) I suspect that you have the humbucking pot wired incorrectly.

John

[MGCRead]

Hello John

I can't really pin point where the singing is coming from. Its volume varies with the input amplitude, it is inaudible below about 0.1V. The frequency of the singing ( it is quite a clear single tone not at all like vibration ) also follows the input frequency. Probably the same frequency but I don't have perfect pitch.

I will take the humbucker out and give it a check, it looks to be correctly wired.

Thanks for your help.

Regards Malcolm

[MGCRead]

Hello John, et al

I checked the humbucker and resoldered the connections, all seems OK.

Then I read up on magnetostriction and I suspect I know what may be causing that. The copper sheet on which the transformer and valves are mounted ( see photo posted earlier ) is backed by a sheet of 1/2" ply and I bolted the transformers through the ply. The only way I can deal with this is to cut a a hole in the ply large enough to allow me to bolt the transformers directly to the copper sheet. But the sheet is only 0.6mm thick ( which is why I backed it with ply ). I can try and remove the ply around the mounting bolts ( not easy although there are no cables in the immediate area, a spade bit and slow and careful drilling might work ).

Still leaves with too much mains hum so a redesign might be necessary anyway. I will ponder but I am very grateful for everyones help.

Thank you, Malcolm

[MGCRead]

By using a length of wood as a stethoscope I can comfirm that it is the o/p transfomer that's singing ( acting as a loudspeaker in essence, and quite a good one ). I have tightened the bolts all I can. I sprayed the laminations with a good coat of primer and black car paint - would that be significant ?

I dug out an old Richard Allan speaker driver from the attic that was used in a louspeaker I built in the late 1960s and hooked that up. Only fed a signal from the generator ( sine wave ) and there is a horrendous mains hum much more than I would have expected from stray pickup.The humbucker reduces it ( when turned to either limit ) and a larger value pot might reduce it further.

When I connected the scope in parallel with the speaker I got a strange signal (first photo ). A square wave ( no loudspeaker but dummy load connected ) is second photo. It would seem the amplifier circuits are working.

I don't know if this gives you any further ideas ?

Cheers Malcolm

[MGCRead]

Not sure I managed to attach photos.

[John Caswell]

Hi Malcolm,
Both of those traces have significant dollops of hum on them so......
Changing the hum bucking pot for a larger one will not reduce the hum. The hum should be at its minimum with the pot roughly central.
As the amplifier is dc coupled you will need to short out the both the grids of the 300Bs to chassis with a capacitor on each, say a 0.47µF /400V to remove any injected hum from the previous stage, that leaves you with the output stage. You should then be able to adjust the hum bucker for minimum. If not you have problems in the output stage.

John

[MGCRead]

I have maximum hum at middle position of the humbucker, it decreases by much the same amount regardless of whether I turn clockwise or anticlockwise.

One thing I have remembered is that I used 120uF resevoir caps in the power supply as printed in the parts listing but someone ( not me and therefore at WAD ) had inked in 82uF in the parts list and circuit diagram. I have no idea whether that is relevant.

I will try bypassing the grids of the 300B. Just to be doubly clear ( I would hate to damage such expensive valves!), I connect a 0.47uF cap ( electrolytic I assume ) from the g terminal of each 300B to 0V.

Talking of earth reminds me that I remain unclear whether the power supply circuit should earth to the chassis or to the signal 0V. I have done the latter. But the power supply circuit diagram shows the 0V lines directly connected to earth (ie the usual three line triangle symbol ) which by my reckoning would not be the same 0V as the signal circuit. The tag board layout is also ambiguous but seems to imply all circuits connect to R24 and hence to chassis earth.

Sorry to bother you John with all these problems but I am really having trouble with this amp and your advice is greatly appreciated.

Regards, Malcolm

[Greg.]

You live not that far from John. I’d PM him.

[MGCRead]

Originally Posted by John Caswell

Hi Malcolm,
Both of those traces have significant dollops of hum on them so......
Changing the hum bucking pot for a larger one will not reduce the hum. The hum should be at its minimum with the pot roughly central.
As the amplifier is dc coupled you will need to short out the both the grids of the 300Bs to chassis with a capacitor on each, say a 0.47µF /400V to remove any injected hum from the previous stage, that leaves you with the output stage. You should then be able to adjust the hum bucker for minimum. If not you have problems in the output stage.

John

I did that and the hum ( which is much less than before but still too loud to be acceptable ) is at a minimum in the central position. One other note ( if you'll pardon the pun ) of interest is that the o/p transf whisle is now a constant pitch and does not vary by signal amplitude or freq. It is very loud.

I will re-wire the amp circuit first to tidy up the wiring and make sure I have a proper star earth.

Cheers Malcolm

[A Stuart]

Originally Posted by MGCRead

... I remain unclear whether the power supply circuit should earth to the chassis or to the signal 0V. I have done the latter.

Not wanting to have too many different voices in your ear, but Bob was pretty clear about this (post #2). The earth on your mains input socket goes to the chassis.
Thought I should point this out before you re-did your star earth.