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Old 25th May 2006, 07:03 PM   #11
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One of the worst offenders is input connectors, such as RCA jacks, that ground to the chassis. I've gotten some wicked nasty hums from that. Now, it may not seem such a big deal having a couple of ohms across a chassis, but it is. Take those few mV of PS hum, add 200X or so open loop gain, and BZZZZZZZ!!!!. Make certain that your input jacks are grounded at one point only: I always return them to the same place on the circuit board where the input stage cathode (emitter, source) ground return is located.
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Old 26th May 2006, 02:28 AM   #12
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Pics are up. I have larger files if anyone wants them via email.

top view
input section
right output
left output
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Old 26th May 2006, 03:26 AM   #13
rdf is offline rdf  Canada
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Was the area around them scraped clean of paint prior to mounting the input jacks? There also appears to be a very large path between the ground reference for the input tubes and the input jacks. You might consider putting the ground tabs back on the input jacks, making that point your chassis ground reference and lifting the one you currently have.
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Old 26th May 2006, 08:12 AM   #14
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The chassis around the input jacks should still have a coat of paint and primer, but it's possible that bare metal might be contacting one of them within the mounting hole itself. I've checked once, but I'll double check again tomorrow. I'll switch the chassis grounding point as well.
Thanks.
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Old 26th May 2006, 10:13 AM   #15
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Can't stress that too highly- there should only be two places where ANYTHING contacts the chassis, the safety ground (the green wire from the power cord) and the circuit ground, which absolutely should be at the input. The power supply ground should be at the far end of the ground bus, and the ground bus should be directly connected to the PS cap's negative terminal. Since the transformer-rectifier-cap ripple currents are high, even moving the ground bus a short distance from the cap terminal along that loop will cause surprising amounts of hum.

If the hum goes away with nothing plugged into the inputs but reappears when something is plugged in, then the signal ground should be lifted from the chassis. It can be referenced back to chassis ground through a small (10-30 ohm) resistor, which breaks up the ground loop.
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Old 26th May 2006, 04:00 PM   #16
rdf is offline rdf  Canada
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Sorry tokyofist, I was fooled by the use (very scary IMO) of RCA jacks as bias sample points. On closer look it appears the designer chose a point on the input sub-plate, beneath the paired Solens in the diyhifisupply pic, as the signal ground-to-chassis point. I don't see it in your amp though. Where does your signal ground connect to the chassis?
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Old 26th May 2006, 04:16 PM   #17
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The ground bus connects to the chassis at one of the mounting screws for the power supply PCB. Look in the top view picture.
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Old 26th May 2006, 04:27 PM   #18
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Quote:
Originally posted by tokyofist
The ground bus connects to the chassis at one of the mounting screws for the power supply PCB. Look in the top view picture.
Well, that'll be your problem, then. When you said you had hum with buzz coming out of the tweeters I thought it had to be power supply ripple getting where it shouldn't. Lift the chassis connection from the power supply PCB and make a new chassis connection from the end of your 0V bus bar that's furthest from the power supply. At the same time, make sure there aren't any other connections to chassis.

Another thing to attend to is the heater wiring. I can't see any connection from the 6.3V to chassis, unless there's a transformer centre tap that I can't see. Connect a 47R resistor from each side of the 6.3V directly to chassis at the first valve where the 6.3V leaves the transformer. If you're feeling posh, add 100nF in parallel with each 47R.

Make sure you use a serrated washer between your earth tag and chassis and that you do the screw up tight. I use M6 (that's 1/4" to you) and do it up with a spanner. You're looking for a low-resistance bond that stays that way for years. Scrape the chassis clean of paint or anodizing before attempting to make a chassis bond. Use this method for the mains safety earth and any other connections to the chassis.
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Old 27th May 2006, 05:18 PM   #19
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You are all gods among men. I first swapped the chassis grounding point to a location near the inputs. This had no effect, however. I then added a resistor and cap in parallel from each 6.3V winding to ground. Bingo ! No more noise, even with my ear at the speaker.

I owe you all a beer. We'll of course figure something else out for the teetotalers. I should have come here two months ago and saved myself much agony and frustration.

Thanks much again, although I'm sure I'll be back for future projects.
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Old 27th May 2006, 06:55 PM   #20
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Congrats! Another good trick if the filament secondary has a centre tap is to create a resistive divider from B+ to ground - the series string doesn't need to draw much current - bypass the lower resistor to ground with a cap and tie the filament secondary centre tap to the positive end of the cap. Choose a centre point voltage that comfortably exceeds the maximum positive swing any cathode will see. Also make sure not to exceed the heater-filamant rating for any connected tube. This biases the filament positive relative to the cathode and effectively elminates the possibility of the first inducing noise on the second.
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