Problem: Severe power transformer and choke vibration

audio_moksha · 25th February 2008, 06:34 AM

I recently complete yet another EL34 SE. Problem is that the Hammond power transformer and choke have a severe mechanical vibration. This vibration can be felt throughout the chassis and well as on the opts (via mechanical coupling). The result is an audible 50Hz/60Hz hum on the speakers that can be heard from a meter away. What could this mean? Is my power transformer and choke saturated?

The transformer is 375CT @ 200mA. My current draw is all within limits. My PSU is a CLC filter rectified by a 5U4G. The first cap is a 5uF Solen followed by a 10H hammond choke and then a 100uF panasonic (at 1000V). The supply provides 420V for the EL34's and 290V for the 6SL7. The power transformer in question had been in another amp that had an electrolytic leak with fumes (yuck).

Any suggestions would be helpful and I would also go buy a new transformer if I had to. What do you think it is?

SHiFTY · 25th February 2008, 07:43 AM

Try a larger first cap- instead of 5uF try 20uF.
With 5uF at the front it's effectively a choke input filter and many transformers (& chokes) don't like this.

You can also try switching on with no tubes installed- if you still get vibration then it's the power transformer. Also double check you don't have any shorts to ground- measure the current draw across the dropper resistor...

audio_moksha · 25th February 2008, 06:49 PM

Thanks for the suggestions shifty. I checked all the grounds and they seem to be ok. Also if I change the 3uF cap won't it change my pus output voltage? The reason I had 3uF was because it gave me 420V; any higher capacitance gives me 450V; I simulated this in PSU designer. Also I've gone through a lot of fuses between yesterday and today. The fuse goes off with a pop and a bit of light suggesting that the power transformer is pulling too much current (spike?)

Today I will try swapping the power transformer with another one (the same model. I really hope my choke isn't saturated too....

SHiFTY · 25th February 2008, 06:58 PM

It will give you a higher voltage yes- but you should use the correct transformer for the job or design the circuit to handle it.

Try connecting it with a dummy load (5Kish) after the last cap instead of your circuit.

audio_moksha · 26th February 2008, 02:09 AM

Turns out it was a bad transformer; I replaced it and the hum disappeared. Thanks for the input Shifty.

25th February 2008, 06:34 AM	#1
audio_moksha diyAudio Member Join Date: Oct 2006 Location: Los Angeles	Problem: Severe power transformer and choke vibration I recently complete yet another EL34 SE. Problem is that the Hammond power transformer and choke have a severe mechanical vibration. This vibration can be felt throughout the chassis and well as on the opts (via mechanical coupling). The result is an audible 50Hz/60Hz hum on the speakers that can be heard from a meter away. What could this mean? Is my power transformer and choke saturated? The transformer is 375CT @ 200mA. My current draw is all within limits. My PSU is a CLC filter rectified by a 5U4G. The first cap is a 5uF Solen followed by a 10H hammond choke and then a 100uF panasonic (at 1000V). The supply provides 420V for the EL34's and 290V for the 6SL7. The power transformer in question had been in another amp that had an electrolytic leak with fumes (yuck). Any suggestions would be helpful and I would also go buy a new transformer if I had to. What do you think it is?

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25th February 2008, 07:43 AM	#2
SHiFTY diyAudio Member Join Date: Jul 2002 Location: New Zealand	Try a larger first cap- instead of 5uF try 20uF. With 5uF at the front it's effectively a choke input filter and many transformers (& chokes) don't like this. You can also try switching on with no tubes installed- if you still get vibration then it's the power transformer. Also double check you don't have any shorts to ground- measure the current draw across the dropper resistor...

25th February 2008, 06:49 PM	#3
audio_moksha diyAudio Member Join Date: Oct 2006 Location: Los Angeles	Thanks for the suggestions shifty. I checked all the grounds and they seem to be ok. Also if I change the 3uF cap won't it change my pus output voltage? The reason I had 3uF was because it gave me 420V; any higher capacitance gives me 450V; I simulated this in PSU designer. Also I've gone through a lot of fuses between yesterday and today. The fuse goes off with a pop and a bit of light suggesting that the power transformer is pulling too much current (spike?) Today I will try swapping the power transformer with another one (the same model. I really hope my choke isn't saturated too....

25th February 2008, 06:58 PM	#4
SHiFTY diyAudio Member Join Date: Jul 2002 Location: New Zealand	It will give you a higher voltage yes- but you should use the correct transformer for the job or design the circuit to handle it. Try connecting it with a dummy load (5Kish) after the last cap instead of your circuit.

26th February 2008, 02:09 AM	#5
audio_moksha diyAudio Member Join Date: Oct 2006 Location: Los Angeles	Turns out it was a bad transformer; I replaced it and the hum disappeared. Thanks for the input Shifty.

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