Lifting the coupling caps made no difference so I don't think this is the problem.
Sorry my mistake, I didn't see where you had confirmed this.
^From post 20
If I pull out the 12BH7 would it isolate the bias and go back normal if C7,C8 (.047uf) are bad?
I replaced C9,C10 (.25uf)and it didn't resolve the issue.
The bias circuit is new (diode, cap, resistors) so its something else causing the bias issue.
I paralled a 2.2M and got about 14 volts out of the bias. Tubes where at about -44v with about 435volts so bringing up the bias brought the power supply voltage back up.
Not sure what this means since all the resistors in the bias circuit test fine.
I just noticed a 16uf cap at the junction of R19 and R22 on both amps, was this a service bulletin? Crap, I think I just discovered the problem.....
I replaced C9,C10 (.25uf)and it didn't resolve the issue.
The bias circuit is new (diode, cap, resistors) so its something else causing the bias issue.
I paralled a 2.2M and got about 14 volts out of the bias. Tubes where at about -44v with about 435volts so bringing up the bias brought the power supply voltage back up.
Not sure what this means since all the resistors in the bias circuit test fine.
I just noticed a 16uf cap at the junction of R19 and R22 on both amps, was this a service bulletin? Crap, I think I just discovered the problem.....
Last edited:
First off lets take a voltage check of R17 and R18..These are 12k plate resistors that feed the 12Bh7 voltage amp..On one side it should be almost 400vdc and on the plate it should read between 325 and 360..If it reads much less,you need to change the .047s C5 and C6 I believe feeding G1 of each side of the 12Bh7. Lets start with that.
It's the cap at the junction of R19 and R22, its not in the schematic and I didn't pay much attention to it until I just started reverse engineering the board....
The cap was also fairly new looking and must be going bad, I lifted cap and voltages came back up. What the heck is this cap doing in the circuit? The last update was done about 15 yrs ago by a McIntosh service center. I'm within a volt of bias now.
The cap was also fairly new looking and must be going bad, I lifted cap and voltages came back up. What the heck is this cap doing in the circuit? The last update was done about 15 yrs ago by a McIntosh service center. I'm within a volt of bias now.
That was loading down your neg bias voltage and it probably was for parisitic oscillation issues.
Dear everybody,
I already got a pair of MC60s from my friend.
He told me that he has use normally but one day 1 channel had buzzing sound like much distortion.
I just check it follow manual and compare with another channel that has normally working.
And found that both grids of 12BH7 have -20V to ground, not 0V as manual said.
I try to replace C7, C8, C9 and C10 coupling capacitors but nothing has change.
Other voltage points seem normal like manual and another channel.
If somebody has experience with this issue, please let me know.
Thanks a lot,
I already got a pair of MC60s from my friend.
He told me that he has use normally but one day 1 channel had buzzing sound like much distortion.
I just check it follow manual and compare with another channel that has normally working.
And found that both grids of 12BH7 have -20V to ground, not 0V as manual said.
I try to replace C7, C8, C9 and C10 coupling capacitors but nothing has change.
Other voltage points seem normal like manual and another channel.
If somebody has experience with this issue, please let me know.
Thanks a lot,
- Status
- Not open for further replies.