I haven't got the ability to measure my plate voltage.I'm curious though. I've seen others post their plate voltage at somewhere between 425V and 455V. Running tung sol 6L6 GC STR which are 30 watt tubes and using an online calculator 60% dissipation leads you to a bias of between 40-42 mA. A cool 50% bias is 33-35mA. Why does Dr Z recommend 28mA? Seems Freezing when considering above and miles away from the maximum 70%. Would it be safe to go for 60%. Anyone?
Last Edit: May 25, 2017 8:24:03 GMT -7 by steviejay
First off New Sensor Tung Sol 6L6 GC STR will not output 30 watts each, and if they do it won't be for long. Also plate voltage is inversely proportional to currant draw, so increasing Plate currant decreases Plate voltage, but I'm sure you knew that. IR drop.
But I do set bias using three different Technics.
1) Using shunt bias method set to what my experienced tells me, and Beam Power tubes ( like 6L6's ) can be set between 10 and 40 mA safely each.
2) I look at my scope with a signal in the amp and look for cross over distortion at the FWHM ( Full Width at Half Max ), slight crossover sounds very rich on certain amp designs , gives a nice thick clean. Looking at the scope I will tweak the bias slightly.
3) I play test the amp and listen to it and if any final adjustment is needed I will adjust.
So 30 years of biasing experience goes into each Z Amp .
Thanks Dr Z. I'll take your word for it. I'm obviously missing something here. www.thetubestore.com/6L6-Tube-Reviews#30 These guys lump said tube in 30 watts group. Does this mean something different to what your referring?
First there is NO way to yield 60 watts from a pair of RI Tung Sol 6l6 GC STR's and have the tube operate for any length of time. Second do you know what Fender set there amps loaded with NOS 6L6's bias at, 28mA, good enough for Leo , good enough for Z.
The article you mentioned stated the TS 6L6 GC was initially set at 18 watts @ idle per tube which would peak at 26 watts, even less with plate voltage sag at max output. Also they state they set it lower for a nicer clean tone.
Don't take my word for it, set it how ever you like, you'll be contacting the Tube Store very often for replacement 6L6's.
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