Yamaha B-4 bias

codyt

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Paid Member
2019-08-26 7:05 pm
IN, USA
So, I have an old Yamaha B-4 power amp, the Japanese 100v variation of the M-4. That said, there are a few differences, namely dual mono PSU, output impedance selector, and slightly different Toshiba transistors.

All that preamble to ask the question: does anybody know what the appropriate bias measurement is for this amp? The only documentation I can find is an owner’s manual in Japanese.
 

baeijsman

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2015-03-20 3:47 pm
Copenhagen
A few pictures, if you have none.
 

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codyt

Member
Paid Member
2019-08-26 7:05 pm
IN, USA
One other thing I forgot that differentiates the two: the B-4 has button for forcing the amp into either class A or class B (off). I’m testing now and the class B is biased at 15mv, while enabling class A shoots the bias up to around 400mv. The amp runs super hot in class A, and part of me is thinking it might be better to bias the normal operation up a bit and just leave the class A option off all together.
 

codyt

Member
Paid Member
2019-08-26 7:05 pm
IN, USA
Looks like the 400mV class A bias pot corresponds well to the 30w class A spec, so leaving those at 400. Knowing that the amp holds up well at that level of bias, I’m experimenting with the ‘class b’ bias pot now, which was at the m-4 recommended 15mV. I’m running it up to 67mV, which should give me the first 5 watts in class A. So far it sounds great and the heat is reasonable. Could be best of both worlds - class A at normal listening levels and less heat. When the class A button is on the amp becomes a real heat monster.