Bjorn Zetterlund
Well-known member
I just bought a Mesa Boogie 400+ Bass amp, and it needs all new valves. The output stage is 12 5881/6L6 valves in a push pull configuration. This amp (and all Mesa Boogie amps) has no bias adjustments, and it needs well matched Mesa Boogie valves to achieve best performance...no bias adjustment and properly selected valves is one of Mesa Boogie's main principles, if you like. There is a whole essay about it on www.mesaboogie.com The output stage is extremely simple; -63 V bias to grid via 2k7 (5W) resistor, with cathode directly to ground and the plate directly to the primary of the output transformer. The centre tap of the output transformer is at 540 V. The control grid input is via a 2k2 resistor for each valve.
I would like to learn more about how to select these output valves as one matched pair in the UK costs 50 pounds...with 6 pairs, that becomes a major expense!
My first query is; in this type of amplifier with many valves in parallel, how much of a benefit is there in having all valves matched as opposed to having six matched pairs, or simply having the idle current match overall? Specifically in my case, how well behaved (or not) would this output stage be if I simply got 12 valves and mixed and matched to get equal 'push' and 'pull' currents at idle?
Secondly, on a more practical level, what is the best way of measuring the idle current of any individual valve? I was thinking I could make a short 'valve socket extender' cable to enable me to measure the current at the cathode with a normal multimeter. Another possible solution would be to add precision current sensing resistors on the cathodes. You could even have a 12-way switch with a test point at the back to enable quick measurement of individual valves...would be nice to have to quickly diagnose a faulty valve.
Any and all thoughts on this are most welcome and much appreciated!
Thanks,
Bjorn
I would like to learn more about how to select these output valves as one matched pair in the UK costs 50 pounds...with 6 pairs, that becomes a major expense!
My first query is; in this type of amplifier with many valves in parallel, how much of a benefit is there in having all valves matched as opposed to having six matched pairs, or simply having the idle current match overall? Specifically in my case, how well behaved (or not) would this output stage be if I simply got 12 valves and mixed and matched to get equal 'push' and 'pull' currents at idle?
Secondly, on a more practical level, what is the best way of measuring the idle current of any individual valve? I was thinking I could make a short 'valve socket extender' cable to enable me to measure the current at the cathode with a normal multimeter. Another possible solution would be to add precision current sensing resistors on the cathodes. You could even have a 12-way switch with a test point at the back to enable quick measurement of individual valves...would be nice to have to quickly diagnose a faulty valve.
Any and all thoughts on this are most welcome and much appreciated!
Thanks,
Bjorn