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previous: Daver I'm about to retube a Mesa Mark III... -- 4/4/2000 11:06 PM

Re: Cold Boogie Bias

4/5/2000 12:29 AM

Gil Ayan

Re: Cold Boogie Bias quote: "I'm about to retube a Mesa Mark III for a friend. This will be the third Boogie I've retubed for someone and they all are biased drawing very little idle current. I'm using the 1 ohm cathode resistor method. The last Mark IV I did had the EL-34's idling at about 8 ma and the 6L6's at about 20ma. What have you guys seen? Is this just Mesa's attempt to get the amp through the warranty period or does it have something to do with the Boogie tone? I have always set the bias for more "normal" current draw and the owners always said the tone improved. Am I missing something? Please comment.   Daver   "     The problem with a Mark IV is that it is simulclass. The original idea was to have the two outer tubes running very hot, such that they would approach class A, and hence the name simulclass (i.e., Class A for one pair and Class AB for the other pair). Originally, Simulclass amps had two pairs of 6L6s; the outer pair running hot at 40+ mA or more and the inner pair running colder... say 25mA? What happened next is that the Sylvania supply dwindled, and Chinese tubes coult not take the heat... but the Simulclass thing was featured in the amps, and Boogie claimed it was part of the magic in the sound.     Then, Boogie started recommending people used EL34s in those socktes. Now, you know that for the same plate voltages, EL34s need a much lower voltage to bias at about the same point 6L6s do (call it around 10 volts LESS NEGATIVE or so). As you found out, if you put a bias voltage of -51VDC (or so) on an EL34, the tube will barley work at all... let alone will it be in class A. But, most 6L6s will be "ballpark" and so that explains the discrepancy you saw in that amp.     If you want the real sound the amp was designed for, for a rugged pair of 6L6s in the outer sockets and observe... if it doesn't melt them, then listen... and the amp will sound way better.     About the inner pair idling at 20mA, yes, that does sound on the low side, doesn't it? Only so much you can do with a fixed/fixed bias scheme...     You can try and make an adjutsble bias supply very easily in any boogie by replacing the *factory select* resistor with a pot and a tail resistor. However, watch out, because if you make the supply so that the inner tubes are hot, say at 35mA, then the outer tubes will be scorching hot if you use 6L6s.     I personally like a tight power amp, so I always set the bias such that the tubes will idle at say 25-30mA, for plate voltages of around 470VDC (typical in Boogies).     Do NOT try Chinese 6L6s tubes in the "Class A" sockets, they will not live too long. Maybe Svetlanas will do the trick, but again, be careful.     Good luck,     Gil

 

Replies:

Daver Gil,The Mark III I'm about ... -- 4/5/2000 1:19 AM