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RE: How to know?

Posted by Yada (A) on April 12, 2010 at 19:45:44

In Reply to: RE: How to know? posted by drlowmu on April 12, 2010 at 19:18:11:

Hi, Thanks for the thoughtful reply but I'm not sure I could find the 10 ohm resistor or the 2k pot, much less add a resistor. I can solder and run a volt meter but the chances of me making an error here is moderately high; I might not shock myself but I might put a well operating amp into a less well condition.

I guess it's hard to know what the value is for an off the shelf Welborne DRD-45 amp? (these amps were made at the factory).

I'm still trying to figure out what happens if I plug in a NU45? Is it likely that the National Union tube is going to behave circuitry-wise in a manner that is differen that the RCA tube?

Thanks for trying to help without me being able to give much/any circuit specs.

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So, YOU can do this easily, VERY CAREFULLY !!!! measure across the 10 Ohm resistor, and adjust the 2K pot so that the VDC across it is 0.30 VDC to 0.28 VDC, which is 30 or 28 mills (Ohms Law). If for some reason, you can not get the adjustment low enough, contact me off line, and I will tell you where to add a resistor, to allow for more conservative tube operation, while retaining the convienience of the 2 K bias adjustment pot.

BE VERY CAREFUL, LETHAL voltages exist under the amp, so if you are inexperienced, use a technician as Somejoe warns you.

If this amp sounds worse at 30 mills than at 50 mills, the problem is NOT because of the lower the op point, but more likely, the general design and construction of the amp's power supply. This amp's two stage actual direct coupled audio circuit is simple and decent, so hopefully, 28 to 30 mA. will give you a nice relaxed, UNSTRESSESED-sounding music presentation.

Let us know how it turns out for you.

Jeff Medwin

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