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Building a Valve Amplifier Part 5: Final Assembly

Dave Ives


I currently look after production at AB Open. I have a background in the arts, environmental conservation and IT support. In my spare time I do a bit of DJing and I like making things.


March 3, 2021 10:54

I am seriously thinking of building this valve amplifier. Are there any important facts i need to know before i order the spares?

0 Votes

February 3, 2020 13:37

Hi Dave,
Congratulations on an interesting article. I though it worth passing on some experience I have had with using large toroidal mains transformers in audio equipment. Although they are supposed to have low radiated EM field, my experience of using them in proximity to other (audio) transformers is that you can get some significant coupling, which manifests itself as hum over the speakers. If you do experience this, my recommendation would be to try rotating the mains transformer to see if you can achieve a "null" position. It may not be a problem with the relatively high signal levels in a valve amplifier, but I thought it worth mentioning.
I surmised the reason for this as being due to the transformer construction. An ideal core would be made up from concentric rings, but this would be expensive to manufacture. Instead, they are made up from a single long strip of material wound into a spiral. This seems to result in a mainly low overall field, but with the penalty of a very high peak in the radiated field in one particular direction. If you are unlucky, this can couple into other inductive components, or even wiring.

Kind Regards,

Graham Booth BEng CEng MIET

0 Votes

February 7, 2020 09:00

@gbooth1959 Thanks for the comment. There is only the slightest hum which is only really audible at full volume when there is no music playing. I had thought this was down to replacing the choke with a resistor in the power supply, but it could be the toroidal transformer as you suggest. I will investigate re-positioning transformers if it does become an issue. Thanks again.

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