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Old 25th September 2005, 01:52 PM   #11
amperex is offline amperex  United States
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I can not say the PS is at fault. Others are giving good advice.

Try a choke in the B+ rail in-between C1 & the lead from R1. A gound loop issue can be the problem as well.

For good design practice the SS rectifiers should be a UF4007 type for lowest noise, but this type of rectifier would not improve a PS 120 Hz (60Hz mains) hum issue.
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Old 26th September 2005, 01:01 AM   #12
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As an aside, I'd use 600V caps in the PSU if you are using electrolytics...just for safety's sake.
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Old 26th September 2005, 03:51 PM   #13
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As previous mentioned ...brousing over the psu schematic, unless the chassis grounds are wildly adrift, it should be really tame with all that uF. Similiar circuits in the 1960's had only a fraction of the uF on the B+ rails. Have you double-checked the heater valve holder wiring that it isn't contacting ajacent lugs ? Circuits like this should be pretty reliable.
The circuit earthing of the front end does need to be away from the power end......with the low amount of nfb with a 3 stage amp you will defintely hear some noise from the LS around -60dB. Where have you placed the front end 6SN7 ?
If the output tubes were wildly out of match, using a common cathode resistor it is kind of self balancing. As cap coupling is used for o/p tubes, an easy test for hum in this type of output stage is to yank the phasesplitter tube out and then switch-on. You should only hear o/p stage noise.

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Old 26th September 2005, 07:37 PM   #14
Gilbert is offline Gilbert  United States
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Default Re: Troubleshooting First Tube Project

Quote:
Originally posted by tjotto1
but I have a 60Hz hum in the speakers. Looking with a DMM, I am seeing about 60mV RMS in one channel and 40mV RMS in the other. I was very carefull in wire routing and grounding to avoid ground loops. I tried repositioning some circuits to keep them as far away as possible from the 60Hz lines, including the heater lines, which are twisted. In looking for the source of the hum, I found that there was about 5V RMS of ripple on the 390V from the PSU. At first I thought it might be the 100uf cap, but now I'm not so certain. When I first power the amp on, the ripple on the 390V line is 40mV or less, but after things begin to warm up, the hum shows up, and then the 5V ripple is present on the 390V line. I have a feeling that this is what is causing the hum in the speakers but I am not sure what the exact source is to fix. Anyone have any input.
You'll always have some amount of ripple on the first stage, but in a push pull amp most of it is self cancelled in the output transformers. It's further down the chain in the preamp section you need to be careful about ripple amounts, not so much in the power stage, at least for push-pull.

Another important point to observe is how you've done your input jacks. A lot of people will make the mistake of mounting the RCA jack on a metal chassis, which grounds the jack, and then running a ground wire back to the input stage from the RCA jack, instant ground loop! If your RCA jack is isolated from chassis then you can run a ground back to the input stage, if it isn't then run just the signal wire back to the input.
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Old 26th September 2005, 08:30 PM   #15
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As suggested, pull the phase inverter tube and see if hum stays - if so, you've isolated it to output stage. DON'T pull the first stage tube, due to DC coupling.
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Old 26th March 2012, 03:08 AM   #16
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Default Hum on JN Still DIY Project

Stillís DIY audio projects: 3-prong, 3-wire 117V ac line system. A 60 Hz hum may occur with green/ground connected to amplifierís chassis.
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