Output transformer makes odd whining noise

[tr3]

I have been building a close approximation to a Mullard 5-20. The only way I could get it to work was to reverse the leads coming out of the output transformer. This all seemed fine but recently it has stopped working. The output transformer makes a horrible whining/groaning sound. The grid voltages coming from the output transformer also seem to have leapt up to about the same as the anode voltage.

Anybody any ideas???

Thanks.

 

[PassFan]

Before you reversed the leads coming out of the transformer was it whistling. I assume when you say coming out of you are talking about the speaker outputs. If it was whistling then you should have swapped the plate leads going into the transformer and not the leads coming out. If this is the case it sounds likeyou may have damadged your output transformer. When your plates get swapped around this causes the feedback from the output to be positive instead of negative. This causes the amp to oscillate which creates ringing through the speakers. Swapping the plate connections on the output tubes is the correct way to remedy this. You also could check any cathode resistors in the bias circuit. If these have gone bad then the bias on the tube could max the output to the transformer. Just a couple things to check I know, but if you get a little bit more specific help will be along shortly. Good Luck

 

[Brett]

The schematic I have shows feedback being taken off one of the output taps on the transformer. When you reversed the leads the feedback loop went from negative to positive and the amplifier became an oscillator, which probably accounts for the noise. You've probably cooked something and killed it. Transformers should not whine.

Disconnect the feedback loop R13/C9, attach a load resistor to each channel and start fault finding.

 

[tr3]

Thanks for the replies. Switching the plate connections seems to have solved my previous problems. But the output transformer does seem to have been fried.

cheers,

James

 

[PassFan]

Output Trans

If you look up the data sheet for the output tube (EL34) you should be able to find the plate resistance spec. It would seem that from the diagram this appears to be a class ab ultralinear output. Find that plate to plate resistance on the data sheet and match it to a suitable output tranny and viola, you have a replacement. I would go with one that has a higher output rating than 20 watts so I didn't push it too much. There is a guy on ebay that sells www.handwoundtransformers.com for less than you can buy hammond from Angela. He will also wind whatever you want for no extra charge. You may wish to stay close to Mullards original schematic as there are a few pitfalls to watch out for with this design and you don't want to cross over them. Good Luck

 

[Brett]

gentle word of warning

There's been a lot of talk recently about Handwound on AA and other places. Several people have bought and measured his trannies, and found them to be very poor in spec especially low primary L. I would question him hard before I purchased.

In the past I have mentioned him due to the good reports of others. I nearly bought a pair too, but by a great gesture of generousity I was given a pair of vintage trannies. If I was still looking, I'd go for the Hammonds, because of quality and consistency. Or maybe some Sowters or Lundahls if I were in Europe, and Lancrofts in Oz.