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the original schematic calls for a 50 OHM RESISTOR...BUT I've seen some posts where folks are using a 82 OHM RESISTOR...IS THERE ANY benefit regarding the differences between the 2?
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The larger the resistor, the cooler the bias. It's been revealed here that the AC30 only operates class A at lower clean volumes. After that the cathode voltage rises and this cools the bias to class AB1. So, IMHO, biasing the amp closer to class AB1 at idle isn't really a big issue WRT being "vintage correct". It may have something to do with the plate voltage. If modern replacement and reissue PT's aren't made to compensate for modern wall voltages the plate voltage will be higher than what it was in, say, 1963. So to keep the tubes from overdissapating the bias must be cooler. So...
If your plate voltage is corrected for vintage specs, go ahead and try a 47R (closest modern standard value) cathode resistor. If your plates are 20V higher, probably best to go with the higher cathode resistor value."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz -
very cool...I have the replacement tranny "bundle" from TRIODE...They are robust size (one would only speculate that the "new" iron has been compensated for raised wall ac V's...the 50 OHM is doing just fine...I've also installed (from power switch) a 120 V cooling FAN...after playing the amp hard for 2-3 hrs. you can touch the PT all you want...it feels cold!Comment
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sure wish I could try (afford) a quad of TRUE MULLARD 6BQ5's in this rascal...I have some pretty cool "mint" pedals if anyon'es interested in a trade (wink)...frank.Comment
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The issue isn't heat in the PT, it's heat in the tubes. The fan isn't a bad thing, but dissapation is the issue. Can you measure the voltage from plate to cathode on the power tubes, and the voltage from the cathode pin to chassis. With these numbers you can figure your bias and KNOW if it's fine or not."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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I'll check it out as soon as I get a new battery in my MM...Comment
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