Mcintosh C11/C22 Power Supply Question

[AA6U]

I'm in the process of restoring a Mcintosh C11 and a previous owner did a very poor job rewiring/changing the power supply section. I am trying to bring it back to the original circuit (with new rectifiers and caps). However, because of their rewiring, I only have the schematics to go by. I compared the C11 and C22 power supply schematics and found they are nearly identical except that the C11 shows the negative terminals of first two 20uf caps (C101C and C101D) after the voltage doubler grounded, while the corresponding 20uf capacitors on the C22 (C30C C30D) are not grounded. Should I or should I not ground the negative terminals of those capacitors? See the schematics. Also, would the power supply benefit from additional capacitance? If so, how much and where?

 

[dcgillespie]

AA- The negative side of C30B, C & D in the 22 must be grounded, or there will be no B+ voltage. The lack of a ground connection on the schematic is clearly an error of omission.

As for adding capacitance, if you do, the appropriate points would be at the B+ 1, 2 & 3 decoupling points. If you do increase the values, going more than doubling the original value represents a point of very diminished potential returns. All of the circuits in the preamp run in Class A, so therefore, current draw is steady from the power supply. As a result, raising the value of the original caps will not produce any improvement due to increasing the reservoir capacity, but simply due to lowing the decoupling impedance at the supply point. At that however, the existing values are plenty low for that job, so increasing the values will largely be one of emotional satisfaction. Lab testing will show no difference in performance if the values are doubled -- if that helps with the decision process.

Dave

 

[dewickt]

For convenience I use a quad 50 uF at 350 V, the added capacitance does no harm and the can is the right size to look correct.

 

[AA6U]

Thanks for the insight and advice. I rebuilt the power supply and all voltages are correct. However, I have a slight hum when I turn up the volume, so I added some capacitance to both the B+ and filament supplies, but it didn't help. I've already replaced the old .01uf coupling capacitors.

However, interestingly, the preamp came with tubes labeled 12AX7 RCA made in Poland. However, given the tubes' plate structure, they are certainly 12AT7 tubes with new markings which was confirmed by some of my tech buddies. When I use the 12AT7 tubes, the preamp works okay, but I get no hum. If I put in real 12AX7 tubes, I get the hum. When I change the input selector switch through the different inputs while using the real 12AX7s, the hum gets louder when it is set to the low level inputs (and it the hum gets louder when you turn up the volume). Do you think it is the bypass capacitors?