I am building a stereo SE Amp with 6SN7's and KT88's. I built the Amp one channel at a time and got them running great! When the second channel was finished it tested fine as well. When I hook up my audio system pre-amp I get 120Hz hum. If I only drive one channel its fine(short the other channel to GND), both simultaneously I get hum. The hum is interesting (attached) It looks like a capacitor charging curve and is inverted on the opposite channel. I have attached my schematics and a picture of the scope connected to the KT88 coupling cap. I thought I was good at electronics until this one. Any input would be greatly appreciated I have been at this a while. I'm missing something!
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Ground loop. Place a ground loop breaking network between the internal audio ground and the chassis safety ground. (Anti-parallel diodes in parallel with a 4.7 - 10 ohm 3W resistor - size diodes for worst case fault current in event of a direct primary to secondary short in the power transformer, you can use a high current bridge rectifier if you prefer.)
![IMG_20171116_180425[1].jpg](/community/data/attachments/592/592147-16fe41daec6ebb8bdf9509cb079254e3.jpg)
I had a similar problem when I connected my st120 to my Preamp. The problem was I had a polarized two prong power cord on the amp and a three prong power cord on the Preamp. Replaced the two prong with a three prong and problem solved. This may or may not help, but thought I would pass it along.
I think you missed JonSnell's post. You must replace the power supply input capacitor
with a 50 or 60uF value immediately, or the 5AR4 will be destroyed.
That's why your ripple is so small now, because the input capacitor is much too big.
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In my amp I use a series dropping resistor 300R into a 2200uf cap, and have a time delay relay short the resistor after 10 seconds...
A similar dropping resistor could work on the OPs amp.
I use one of these: NE555 DC 12V Delay Relay shield Timer Switch Adjustable Module 0 To 10 Second | eBay
I also had a hum on the phono input of my amp. Went away with a "cheater" cord, but I own one of the few grounded turntables...
I have a good feeling this would fix it, and perhaps the OPs problem, too.
A similar dropping resistor could work on the OPs amp.
I use one of these: NE555 DC 12V Delay Relay shield Timer Switch Adjustable Module 0 To 10 Second | eBay
I also had a hum on the phono input of my amp. Went away with a "cheater" cord, but I own one of the few grounded turntables...
I have a good feeling this would fix it, and perhaps the OPs problem, too.
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Actually,
An externally hosted image should be here but it was not working when we last tested it.
Thanks everyone, I really needed the reminders to pay attention to grounds. I know that you probably answer the same question multiple times weekly but I appreciate your feedback. Ultimately I Pulled all my grounds back to a signal ground (this solved 85-90%) then I connected it to earth/chassis ground with 10 Ohm / 100nF Cap and diode network and that got it to showroom quality(undetectable hum/buzz by ear). The Diodes are a great idea to force the fuse to blow just in case signal ground changes significantly relative to earth ground. I will also drop the size of my tank capacitor to 60 uF as suggested but I don't have one handy just yet. Again Thanks!!
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