DC Offset on Input

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DC Offset on Input

Postby skidave » Mon Dec 19, 2011 8:30 pm

I need some help with one of my St-70s. I have one with what I think is a rather large DC offset on the input. I'm measuring -129mV. The other ST-70 has -49mV

These are rebuilt units with the stock circuit but new components and a cap board. I use one for the left channel and one for the right channel bi-amping the speakers.

I was quickly making some measurements using my voltmeter and a calibration CD. There is so much DC offset in one amp, above about 10K, the meter won't measure AC signal. Since I am using the amp with one input cable, I have the switch in mono mode and I have my meter connected to the unused input RCA connector.

I checked my preamp and there is minimal DC on the output and I even switched the left and right input signal cables-no change. I did not switch the 7199s yet, but that is next.

I'm not sure if this is an issue or not. I'm used to SS gear where you can null the output offset (if it is non-servo) and usually there is no offset on the input side.

Thoughts?
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Postby 20to20 » Mon Dec 19, 2011 9:15 pm

Check your channels seperately with no signal input so you can see a differential between channels. In mono you're seeing the combo voltage. You may have just one tube with an above normal grid leakage.

20
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Postby EWBrown » Mon Dec 19, 2011 9:19 pm

THis isn't actually a "real" problem, just the result of an ages-old electrical phenomenon:

If your ST70s have 470K grid resistors for the 7199s, try replacing - or shunting - them with 100K.

There will be a small "offset" voltage caused by the phenomenon called "grid leak" caused by the tube's internal cathode to plate current flow. Even though Class A amplification stages don't draw grid current, there will be a tiny amount of grid current (microamperes) generated inside the tube, and a with relatively high grid resistance like 470K will make the grid float slightly above "zero" volts relative to the power / signal ground.

Just about any preamp made will easily drive a 100K resistive load, most will drive even lower loads than that, as low as 10-20K.

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Postby skidave » Tue Dec 20, 2011 12:57 pm

Hey Guys, thanks for the responses. It definitely is with the tubes because I moved the 7199s between the two amps and the offset follows. I thought about previously changing the 470Ks to 100Ks, but I left it alone. If it's not a performance concern, then I am going to let it be.

I appreciate you sharing your knowledge!

Dave
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