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RE: 300B OPERATION POINTS .. SURVEY!!!

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Hi,

Your 300B should last for 10,000 hours in your amp, or your amp is just plain JUNK.

This is NOT a 300B design problem at all, it is an AMP DESIGN PROBLEM.

I have made many posts on DIY Forum about running tubes at their Golden Ratio proportions, at 62% of their rated NORMAL use plate dissipations, ( not design maximum ratings BTW.) Threre is only one manufacturer who is savvy to this and does it routinely with excellent result, all else is JUNK in design.

What is the plate-to-cathode VDC measurements you get now?

What is the ohmic value of your 300B's cathode self-bias resistor....from that, one computes actual current draw. ( Multiply VDC difference of the P-K by current in AMPERES, and you get POWER - a reading of plate's power dissipation, in WATTS.)

Then, look up the tube Manufacturer's "suggested" operating point. How does your plate's dissipation rating in watts compare with the Manufacturers? Is yours at 100% of suggested, 95%, 85%, 75%?? If so, THAT IS YOUR PROBLEM. It will 1) sound STRAINED in a SEer, and 2) show short tube life ...which is NOT a 300B problem, but a problem of an unconcious design engineer.

I suggest YOU go ahead, become proactive, and set up the tube so it has a 62% plate dissipation in your amps. Thats the Golden Ratio number that works well in all things. Thats right, if it is self bias, increase the bias resistor some. The trade-off is that you UNLOAD the current draw on the supply, so B+ also goes up some, depending upon the stiffness of the supply. So,

1) measure, change, remeasure, listen,

2) change, remeasure, listen, etc.,

3) till you solve your own problem, mis-use of the tube.

Your amp will sound FAR less strained, far better, and your 300B tubes will last - for years. Wise up. The manufacturers rating is designed so that they can $ell more tube$.

Jeff Medwin

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