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Old 2nd March 2008, 05:13 AM   #1
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Default bad buzz in an EL34 amp

I finished wiring an EL34 SET amp yesterday but It's putting out quite a bit of buzz and I can't seem to get rid of it. The circuit is a straightforward EL34 SE tapped at 47% UL. The quiescent current is about 55mA with plate dissipation about 25W. The power supply is a Hammond 375-0-375V rectified by a 5U4G. The CLC filter is 47uF-10H-47uF followed by 50uF reservoir caps for the two EL34's and the 6SL7 driver. The Hammond power transformer has a slight mechanical buzz to it and when I put my ear to it its the same buzz I hear on the speakers. Today I used rubber grommits to mount the power transformer hoping that this would help but it didn't. I can also feel this buzz on the psu caps. I've tried two different hammond power transformers and they both had the same buzz. I also tried different tubes to no avail.

I used star grounding with the chassis connected to the star point by two ying-yang diodes. I have the 6V heaters' CT grounded at the star ground but when I tried grounding the 5V rectifier heater CT, it caused the 5U4G to spark. Also, the psu caps are 2 JJ chassis mounted 50uf+50uF. However, I am not using one of the 50uF sections on one of them.

The buzz starts as soon as I flick on my standby switch (which basically makes or breaks the psu CT. The buzz is the loudest when my volume pot is slightly more than midway and softest when the pot maxed out. The pot is a 100k alps and is it forming some kind of filter with the transformer induced chassis vibration?

All wiring looks good with similar wires twisted and ac wires running at the chassis edge. The chassis is a Hammond 17"x10"x3" steel chassis. The transformers run left to right; 2 OPT's the hammond choke and finally the power transformer. The Choke and power transformer is 90 degrees to the OPT'sThe rca inputs are isolated and run from the back of the chassis to the front where the pot is located. I used Belden STP for this and the heater wiring.

There seems to be something fundimentally wrong here especially with the rectifier sparking and all. Shouldn't you be able to ground the CT of the 5V heaters? I don't have an oscilloscope but the buzz is mostly 60Hz. Any ideas??
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Old 6th March 2008, 11:11 AM   #2
nkg is offline nkg  Australia
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Hi Audio Moksha
First of all I'm not that experienced and I'm a bit perplexed that none of the brains in the group has not bothered to answer to address your problem yet, but that is another issue. If the bolts that used are not completely isolated from the chassis the vibration will free to travel anywhere ie metal to metal anywhere.
If it a mechanical buzz he laminations of the transformer may be loose have tried tightening up the bolts that hold the the laminations of the transformer together. It does seem odd that two transformers are doing the same . Its a pity your not here in perth WA I use Universal Transformers to make my power tranies much better quality than hammond and not that much more expensive .
Nigel
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Old 6th March 2008, 02:42 PM   #3
korey is offline korey  United States
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I can't guarantee anything, but I can probably help ya out a little as I am just a tube hobbyist for a little over twenty years now. I have rebuilt many magnavox and zenith tube amps. Have you got any pictures especially of the underside? In my experience, you usually cannot ground any center taps and it sounds like your 5U4 tube is shorted directly to ground if it is sparking inside.
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Old 6th March 2008, 04:59 PM   #4
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The 5U4G is directly heated, meaning that the 5 volt winding has B+ on it! Do not ground!

A few ideas... others may elaborate on some of these...

Reduce the first filter capacitor to about 20uF to relieve some of the load from the rectifier and power transformer. The following capacitors, after the choke, can be about as large as you want them to be.

Connect the centre tap of the 6.3 volt winding to a point that is several volts above ground. A voltage divider at the end of your filter string that results in about 25 or 30 volts should be fine. With cathode bias you may be able to connect it to the cathode of one of the EL34s.

Is the ground end of your volume control and the ground of your input connected to the same spot that your first stage is grounded?
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Old 6th March 2008, 05:00 PM   #5
kevinkr is offline kevinkr  United States
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Can you post some construction details of your amplifier and schematics?

Yes, Hammond transformers buzz as do many others - this has nothing to do with your problem.

The volume control is a clue. You either have electro-static or electro-magnetic pick up in the input circuitry. It is worst at the pot center because this is the point that results in maximum source impedance at the grid of your input tube.

How is your filament lead dress? Tightly twisted and not looped around the socket?

Elevating the filaments above ground usually helps with hum pickup in the driver and input stages. In this case if you used cathode bias in the EL34 you can use the cathode voltage to elevate the filament supply by connecting the filament center tap to the cathode of just 1 of the two EL34. If you used fixed bias divide down from the B+ and bypass the divided down voltage with a small electrolytic. Usually something like 40 - 50V will be enough - use a 10uF/100V electrolytic. A couple of mA through the divider is all you need.

Do you have a stopper resistor on the grid of your input tube - this could be a manifestation of vhf oscillations. (Hard to see without a fast scope.)

Center tap of 5V winding is at B+ potential, grounding = dead short across the rectifier and transformer secondary. No wonder the rectifier arced. Might be good to replace it soon.

How have you oriented the power transformer and choke relative to the output transformer and the input circuitry?

What is the chassis made of?

pix pix pix pix pix.....
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Old 6th March 2008, 05:57 PM   #6
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Hey fellas
thanks for taking all this time to help me. The good news is that I fixed it a couple of days ago. Turned out that the buzz was mechanically induced through the chassis! I isolated the power transformer and choke using some sturdy rubber grommits as well as a sheet of rubber between the laminations and the chassis. It sounds great now!
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Old 6th March 2008, 06:40 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by audio_moksha
Hey fellas
thanks for taking all this time to help me. The good news is that I fixed it a couple of days ago. Turned out that the buzz was mechanically induced through the chassis! I isolated the power transformer and choke using some sturdy rubber grommits as well as a sheet of rubber between the laminations and the chassis. It sounds great now!
Steel chassis by any chance? I'm thinking eddy currents in the chassis sheet metal resolved by the insulation you've added. Otherwise that could be the most microphonic pot of the century...
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Old 7th March 2008, 08:10 AM   #8
nkg is offline nkg  Australia
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Tis good its fixed.
Nigel
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Old 7th March 2008, 05:41 PM   #9
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yep; steel chassis it was. I've never had a power transformer with such a severe mechanical problem before. Anyway I'm glad its fixed.
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