Mcintosh mc-30 has no series bias resistor

M

Mickey

New Member
I'm carefully going over an mc-30 and comparing with a schematic. In both the schematic and my amp the selenium rectifier, which I plan to replace, has no series resistor. In other words, the negative side of the bias filter cap goes directly to the anode of SR1 and its cathode right to a tap on the M-152D transformer.

If I replace SR1 with a 1N4007, do you think I should add a series resistor or wait and see what the bias voltages look like? I have seen other schematics that have a 3.3K series R and have also seen the advice to go with a 9.1K. I will change the bias cap to 22 uF.

All opinions are appreciated.

Mike
 
Register to hide this ad
Never have seen one without the resistor, does your board look like this ?
 

Attachments

  • b1.jpg
    b1.jpg
    140.3 KB · Views: 157
  • b2.JPG
    b2.JPG
    24.7 KB · Views: 160
M152D power transformer has a bias tap while the M152B does not, having a bias tap does away with the extra voltage and resistor that the B transformer uses. Try the new diode without the resistor and measure the bias voltage, might be close and not need the added resistor.
 
Thanks Terry! Here is a photo of the circuit board in my mc-30. Sorry if it's a bit blurry. The greenish blue wire from the transformer tap goes to the cathode (top) of SR1. The anode is connected to the 8uF bias capacitor's negative lead by a black wire running behind the left 0.47uF bumble bee cap.

Thanks again for the advice.

Mike
 
Mickey said:
Thanks Terry! Here is a photo of the circuit board in my mc-30. Sorry if it's a bit blurry. The greenish blue wire from the transformer tap goes to the cathode (top) of SR1. The anode is connected to the 8uF bias capacitor's negative lead by a black wire running behind the left 0.47uF bumble bee cap.

Thanks again for the advice.

Mike
Click to expand...
Hello I have the same version of mc30 I’m doing the bias mod now What did you end up doing and how did you make out ??
 
detroithifi said:
Hello I have the same version of mc30 I’m doing the bias mod now What did you end up doing and how did you make out ??
Click to expand...

This thread now 14 years old and the OP hasn't been active since 2009. May want to start a new thread.
 
You must log in or register to reply here.
Back
Top Bottom