Yamaha B-4 bias

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Member
Joined 2019
Paid Member
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.
 
Member
Joined 2015
Paid Member
A few pictures, if you have none.
 

Attachments

  • M4_adj1.jpg
    M4_adj1.jpg
    78.7 KB · Views: 167
  • M4_adj2.jpg
    M4_adj2.jpg
    28.7 KB · Views: 158
Member
Joined 2019
Paid Member
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.
 
Member
Joined 2019
Paid Member
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.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.