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Balance cathode biased tubes?

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2/2/2001 8:44 AM

Balance cathode biased tubes?

I spent the day tracking down the hum in my amp. I removed the ground loops from the input jacks, that helped a bit. I also regrounded the power transformer, poor mechanical connection now soldered, that also helped a bit. Now it only has a mellow hum and the volume control has little effect on it. I checked the bias current, one tube is at 36.5mA and the other is at 40.5mA. It is cathode biased, how do I even them out? They are 7591A tubes and I am broke so replacing them is not an option right now. Any help is much appreciated.

Thanks

2/2/2001 12:53 PM

Usually a 50ohm hum balance pot installed in

cathode resistor circuit let let you null out

all but the worst tube imbalances. Just install

between the cathodes and the existing cathode

resistor/bypass cap. the added resistance may

nessesitate a reduction in the value of the

cathode resistor but I havn't had to do this yet.

I done this on several amps and it works well.

These are available from http://www.triodeel.com

Greg Z

to thine own sound be true

2/2/2001 9:37 PM
Richie{~}==:::	You can use a resistor for each tube.instead of both sharing the one. You may have to do some figuring to get what you are looking for voltage wise,but less inexpensive way to do this. Kinda like the post about using the pot. Richie{~}==:::

2/2/2001 10:22 PM
Shea	If your speaker output jacks are grounded to the chassis, I wonder if it would help to connect the cathode resistor & bypass cap right to the ground terminal of one output jack. Just an idea. Shea

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