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Old 26th June 2011, 09:54 PM   #1
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Location: Smoky Mountains, NC
Default Rail voltage imbalance.

I have put together an F5 type amp using a tubeshunter kit.
I checked the power supply with no amps connected and the rectifier and output voltages remained stable at +/_ 26VDC for 30 minutes. I am using an Avel 250VA 18+18 transformer.
When I connected the left channel, the rail voltages immediately began to drift upwards on the negative side with the bias pots at 0 ohms(or close). The voltage peaks at close to -46VDC and +6VDC.
Right channel ditto.
I have checked the mica insulators, made sure of amp ground to PSU, looked for broken tracks and dry solder joints.
Before I realized this was happening, I did manage to adjust the bias current to !.3amps, but the amp shut down. No blown fuses. either 1 amp or 2.5amp fast blow.
Has anyone had a similar experience or any bright ideas?


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Old 26th June 2011, 10:23 PM   #2
semi932 is offline semi932  United States
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Definitely something in the boards as you say the voltage is ok coming out of rectifier and caps.

Check and make sure fet's are installed correctly for starters.

Also the ztx transistors are oriented correctly.
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Old 27th June 2011, 12:17 PM   #3
Join Date: Oct 2005
Location: Central Florida
I hope you didn't expect a lot of help, seeing that you are using pirated Chinese boards.
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Old 27th June 2011, 12:39 PM   #4
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Try loading the PSU with resistors just to check that everything is OK.
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Old 27th June 2011, 05:43 PM   #5
h_a is offline h_a  Europe
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Ground of +-rail and --rail connected to eachother and connected to safety ground? (Sounds like a floating ground to me).

High Octane phono preamp Wiki
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Old 27th June 2011, 09:03 PM   #6
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Thank you for the responses.
After further inspection of the solder joints and component orientation and functioning, I'm pretty sure that it is a grounding issue. Have to check when I return from vacation.

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