Is my power transformer faulty?

[Chris Peacock]

I've recently had to fit a new power transformer to my home-build valve hifi amp, due to the original cheap one melting...

Since I fitted the new transformer and had to move wiring around, it's been suffering from bad mains hum. Whilst investigating this with a scope I found that the waveform on the 6.3V heater secondary looked like this:



The 5V heater secondary, along with the HT secondary, on the other hand looks like this:



The waveforms look like this even with the valves removed and therefore with no load on the transformer.

Do I have a faulty transformer here or could there be another cause, maybe that which is causing the hum such as induction or an earth loop?

I should maybe also mention that the 6.3V secondary is actually a plus/minus 3.15V one with no 0V connection, whereas the 5V one is 0V and +5V - I don't know whether that's relevant.

Thanks for your help,
Chris.

[DF96]

There is something weird happening here. Show us your PSU circuit, and what (if anything) you disconnected to get the scope pictures.

[Chris Peacock]

Hi there,

Thanks for replying. Here is the schematic for the PSU:



Note that this shows the old transformer; the new one is actually the TRA400 from here: Ask Jan First ® ; electron tubes and more The windings are:

2*270 V 0,2 A
2* 50 V 0,2 A
6,3 V 2 A
5 V 3,5 A
6,3 V 7 A

I've tried removing all of the valves from their sockets, so the 6.3V and 5V windings are not connected to anything; the HT winding again is going only as far as the pin on the PSU rectifier socket; I'm not using the 50V one.

Another strange thing (or maybe not) is that there is still some slight hum from the speakers with all of the valves removed; maybe the power transformer is inducing current in the output transformers, although the power TX is a drop-through one and the output TXs are framed mounted, so the windings should be at right-angles to one another. Actually, the output transformers are covered, so I'll double-check this...

Thanks,
Chris.

[Chris Peacock]

Yep, the new power transformer windings are in the same orientation as the old ones, and are out of phase with the output transformers, so that shouldn't be a problem.

[Chris Peacock]

...Oh and something else - I stated in my first post that the 6.3V secondaries didn't have a 0V connection - well according to the notation on the transformer they should do, but my scope tells me that each is giving out +/- 3.15V.

[Chris Peacock]

OK - problem solved, but not understood. It was my stupidity I'm afraid, I'd neglected to connect the 0V of the 6.3V secondaries to the chassis, so I was getting + / - 3.15V and a jagged waveform for some reason...

I still don't understand why this was causing the odd waveform though, it must have been down to inductance, I'm guessing.

Now I still have the problem of the mains hum to solve.

[Mooly]

You could always extend the wire from the tranny temporarily to move it away from the amp to see if it is induced hum.

I'm afraid my valve theory and practice is somewhat lacking but how about disconnecting just momentarily the heaters when it is on to see if the hum goes.

Does the phase of the heater windings matter ? I have recollections of seeing low value pots across heater windings... wiper to chassis ??? was that for hum cancellation.

[Chris Peacock]

Thanks Mooly - good idea to temporarily switch off the heaters. I'll try that first (I'm lazy and I don't fancy having to unbolt the transformer and so on).

I've played about with removing various valves and powering up, and it seems to me that the hum is originating in the input valves, which strangely are farther away from the transformer and have fewer possible culprit wires in close proximity than the output valves.

It's just weird that I had far less hum with the old transformer, and I've actually simplified the design too by removing a redundant NFB loop and switch that I never used.