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Post by roly on Aug 10, 2022 2:56:11 GMT -5
Ampeg Gemini 1 very noisy.
Replaced the 6.3 secondary center tap with 100 ohm resistors.
Now...nothing but expected amp idle noise.
Thoughts Peter?
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pdf64
Wholenote
Posts: 557
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Post by pdf64 on Aug 10, 2022 11:37:08 GMT -5
Seems a bit weird.
The only Gemini 1 schematic I could find showed the heater winding CT connected to chassis ground via a 0.1uF cap. Which isn’t a great way to do it.
Maybe one of the valves in there has poor insulation between its heater and cathode.
Instead of connecting to ground, you could try DC elevating the heater circuit, by connecting it (or the 100ohm resistors) to the output valve cathodes.
DC elevation can be useful in getting rid of some types of heater hum. It’s like magic sometimes!
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Post by willie on Sept 17, 2022 9:32:56 GMT -5
Hey Roly...you might describe "amp idle noise" a bit more. Is this noise you hear with the amp idling with nothing plugged into the input jacks? Or is it noise with an instrument plugged in? Of course, heater hum is quite difference than white noise or extraneous atmospherics most often picked up by the preamp stages. Shielding in older amps wasn't the greatest and many times the input jacks either arent the type that shut down the preamp when nothing is plugged into the amp or they have lost their ability to do so due to excessive wear or "crud" on the contacts....just a few thoughts...most likely irrelevant.  willie |
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Post by ironlion on Oct 3, 2022 19:41:49 GMT -5
It's been a while since I went into an amp, but wasn't the reasoning here that you were creating a different ground reference? By introducing that small amount of difference between the chassis ground and the center tap ground, you get rid of the hum?
Also, two resistors were cheaper than a center tapped transformer.
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Post by roly on Oct 13, 2022 20:33:22 GMT -5
Hi Willie
My definition of "idle noise" is the amp on with nothing plugged in and all pots at zero.
All the input jacks have been replaced.
Ironlion
"By introducing that small amount of difference between the chassis ground and the center tap ground, you get rid of the hum?"
Yes, that worked.
I guess failed center tap is the issue.
Thanks to all for your advice.
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