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Old 31st January 2007, 02:58 AM   #1
gary h is offline gary h  United States
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Default please help diagnose preamp problem

Hello all,

I'm hoping to get help diagnosing a hum problem in my preamp going to speakers. It is a Dyna rebuild kit design. My understanding of tube amps is very limited but I did build this one and I have learned a lot troubleshooting, especially here and Tube Asylum.

I am pretty confident it is not a signal grounding issue. The hum doesn't have the sound of an ungrounded signal but rather that of AC harmonics making their way past the transformer. If I had to guess I would say it is at both 60 and 120 Hz, a little buzzy but low and smooth. (I have rewired all the inputs and outputs with new twisted pairs, new jacks and gone over their grounds carefully as a precaution.)

I suspect the transformer but don't know how to test it without replacing it with a new one. The transformer itself emits the exact same tone making it's way to the speakers and it vibrates noticeably causing the heater tube to rattle intermittently on the PCB.

This is what I have done:

1. Dismounted the transformer, rotated it in every direction, suspended it away from the chassis, placed a metal tray in between it and the tubes, put a brick on top of it, and run the preamp while alternately removing tubes.

2. Run it without any inputs attached through several amps and speaker combinations, off of several outlets, (without safety, with safety ground, and GFI direct back to the panel.)

I have enclosed a picture to account for my lack of descriptive ability. This design was originally intended to have a phono section (left half of PCB) but I was given permission to omit that section. The phono tubes are in place to rule out their absence as a possible culprit.

Many thanks in advance for any help on this,

gary

Picture at: http://home.comcast.net/~garyworld/preampguts.jpg
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Old 31st January 2007, 10:21 AM   #2
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Is it coming from both channels? Is it independant of your volume control? Is it 120Hz?

If it is 120Hz and not modulated by your VC, it's probably in the PSU.

I doubt that it's your transformer.
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Old 1st February 2007, 12:31 AM   #3
gary h is offline gary h  United States
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Hi,

thanks for the reply. The hum is independent of the volume, balance and selector switch. It is audible with the volume all the way up or down. I believe it is 120 Hz but could also be 60 Hz. (I don't have an ocilloscope but the tone is just off B flat 2 on my keyboard.)

thanks,

gary
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Old 1st February 2007, 12:31 AM   #4
gary h is offline gary h  United States
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Another question in regard to the PSU, what is the best way to determine this and what is the best solution?

Thanks again,

gary
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Old 1st February 2007, 11:50 AM   #5
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Given that the hum is equal in both channels, this points to your problem being common to both channels, i.e., the PSU becomes your main suspect. Since this is a rebuild/kit, I would further assume that it is probably a bad solder joint within the PSU, and not a 40 year old electrolytic cap giving up the ghost(I assume all caps are new). If this were a point-to-point build, I would recommend a visual inspection of all solder joints. However, this could be a pain in the **** with a pcb, so for starters, I would prod the different components on the board with a NONCONDUCTIVE probe, like a wooden chop stick, and see if I heard any cackling/pops/etc when I prodded each component. If you were really lucky, you would hear an increase or decrease in hum when you found the offending piece...probably a capacitor with a cold joint. You should start with your rectifiers.

If this doesn't work, you may need to pull the board and start measuring your different components for continuity just in case you have a bad cap, and reflowing all of your joints...make sure that all of your caps have bled off their charge before you start.

Hope this helps. These things can be very frustrating at times.
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Old 1st February 2007, 11:22 PM   #6
gary h is offline gary h  United States
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Thanks for the advice, I'll get to work. BTW, I'm a little confused by the term PSU. I assume it is an acronym for Power Supply Unit? Which component constitutes the PSU? Or does it refer to everything (except transformer) before the signal is introduced?

Thanks again for all the help.

gary
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Old 2nd February 2007, 01:13 PM   #7
Sherman is offline Sherman  United States
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Quote:
Originally posted by gary h
Thanks for the advice, I'll get to work. BTW, I'm a little confused by the term PSU. I assume it is an acronym for Power Supply Unit? Which component constitutes the PSU? Or does it refer to everything (except transformer) before the signal is introduced?

Thanks again for all the help.

gary
Gary,

The PSU is generally thought of as the power transformer, rectifier and filter. Basically all the stuff responsible for the B+ in an amp. Of course some PSUs can put out other voltages as well.

The filter of course can be simple such as a CRC (capacitor, resistor, capacitor) or more complex (with more RC stages, chokes etc.). Actually a single capacitor constitutes a filter but not on a hifi unit.
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Old 3rd February 2007, 02:06 AM   #8
gary h is offline gary h  United States
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Well that answers that. Thanks.

gary
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Old 3rd February 2007, 08:46 PM   #9
2wo is offline 2wo  United States
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Try reducing the load on the PS(U) You posted on another forum that you were not using the phono stage. Can you just remove the tubes? The question I have is I believe Dyna used a 24V heater supply and ran two tubes in serials. So if you pull them and the other two don’t light up, that’s why…John
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Old 3rd February 2007, 10:57 PM   #10
gary h is offline gary h  United States
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Hi John,

thanks for the reply. Actually the phono tubes were put in as an attempt to mitigate the hum, the thinking being that the load needed to be increased. I was told that this transformer can support 4 line stages. Note that the phono tubes are in but there is no wiring for a phono section. The hum at first diminished (except for 10 seconds at startup) but never completely went away. Now it is back as loud as it used to be. (I assume this is because the extra phono tubes took a few hours to break in. If there is such a thing?) The hum is present in either channel no matter how I switch tubes around.

It has been suggested that the problem lies within the Power Supply but not likely the transformer. How do I test the heater tube without having to get another to replace it?

Thanks again,

gary
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