WEM Custom 15

[Murphyv310]

Hi.
That looks to me like a slightly clipped sine wave.
Incidentally the EZ81 as wired is a full wave rectifier not half wave.

[Bushbaby]

Is there anything else that is worth investigating with the scope?

[Murphyv310]

You need to have all valves fitted and scope the output of the EZ81 cathode which is connected to the reservoir capacitor. Measure the DC volts there with a meter and also see what peak to peak voltage ripple there is on the scope. Then do the same on the smoothing capacitor C4, there should be only a very small ripple just a few milli volts.
I'll ask again is the new transformer wired the same way as the original?
Unless you have wired the heaters without the centre tap or earthed one end of the heaters it's not there you should be looking at. You should check the HT side first then look for hum loops, badly routed wiring, heater/cathode leaks in the ECC82's etc

[Murphyv310]

The behaviour of the volume control is very much like a hum loop in your amp.

[Bushbaby]

Hi Trevor
Yes its wired exactly the same as the original. The original had about 1" of lams and this one has about 2" albeit it has exactly the same ratings for primary and secondary wiring. The originals were built at a cost and are underpowered hence the melting of the wax. This was the reason for replacement.

Best regards

[Murphyv310]

Thanks Rob.
So in fact the old transformer was working other than getting too warm. Quite common for transformers to be under rated. There is always the possibility that the problem could be induced hum, some times rotation of the transformer 90 degrees can be a cure or even the addition of steel screen can make a big difference. If the old transformer is OK it might be an idea to refit it for a test. I've repaired these in the long distant past and I'd say due to the gain they have there is a little residual hum but nowhere near excessive. If you have a YouTube channel why not do a demo or if there is a way to upload a video it might be helpful.
Certainly here to help if you need it.

[Murphyv310]

Try removing V1 & V2 and see if there is residual hum.

[Bushbaby]

I have removed V1 & V2 and there is still residual hum!

Best Regards

[Murphyv310]

Thanks Rob.
OK so now lets gets this fixed tonight.
Question time.
1.... is the hum heard at three feet or more or is it ear to the speaker?
If its ear to the speaker then I'd say that is normal. On a Push -Pull output stage like your amp the output stage effectively cancels out hum if it is working OK which if it is very slight I'd say it is working as it should.

2...... Not so much a question but the next step to take if the hum can be heard at a distance from the speaker. Remove all the ECC83's leaving just the two EL84 & EZ81 in place. Does doing that remove the hum you have?
If not then you will need to measure the the Anode voltages on both the EL84's (pin7) screens (pin9) the control grids (pin2) and the cathodes on pin3. There will be a slight difference between the voltages on the anodes due to the way the transformer is wound, there is normally a difference between the windings.

3...... Again if there is significant hum I want you to scope the ripple on C14 (reservoir) cap and the smoother C4. Take a picture if you can and tell me the p-p value of the ripple on both the caps.


4........ If the hum is quiet does it get louder with all the valves in place? if so between V1 & V2 which one is worst (V3 needs to be in place for this test)


5......... Back to significant hum with just the EL84's in place........ One further thing does pulling the EZ81 stop it altogether? Not such a stupid question as if the transformer is not well screened it could be inducing hum to the output transformer.

[Bushbaby]

Hi Trevor
1) yes the hum can be heard at least 10 feet away.

2) with just the EZ81 and EL84 no hum at all.

3) excuse my naivety, whats easy way to set up measuring the ripple current using a multimeter / scope?

Best regards