to MC30 or not to MC30

[no gas]

I've got an old Mcintosh MC30 that was hacked to death by the previous owner. All the iron is good but the circuit board has been desecrated beyond description. What would you do with it? Rebuild the MC-30 as it was or make something better out of it? Ideas and schematics are welcomed.

[danFrank]

If you plan on using the same transformers, I don't think you can improve on the original circuit. If the circuit board is all that is gone, make a new one with some G10 fiberglass circuit board material. IIRC the original board is 3" X 8" or something close to that.
Get a schematic and wire up a new circuit board and put it in the amp. It will sound nice.
The resale value will not be as much as an original unit but it will work good as new.
Daniel

[JLH]

I agree with Dan, rebuild it using a new board. The schematic is not hard to find on-line. It would be pretty easy to rebuild it using perf board cut to the correct size. Your other option is to send the transformers to me for proper disposal.

Rgs, JLH

[Rodeodave]

I'd definitely rebuild it. Do some research, confirm that the transformers are ok, and bring it back to life.

Over at audiokarma there's a dedicated McIntosh forum, you'll find plenty of info.
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MC30 Restore X2

[Bandersnatch]

hey-Hey!!!,
The basic McIntosh circuit is excellent use of those OPT's; can't think of any other topology that will improve them. That is not to say that the originals can't be tweaked.
Input stage: the 12AX7 can be replaced with a 12AY7. Changes to the cathode R to deliver an appropriate plate voltage to the direct coupled phase splitter would be needed.

Phase splitter: re-wire heater connections to run 6CG7, equalize plate loads at 33k, and install a CCS in place of the original 18k tail resistor.

Driver stage: The 12BH7 is getting positive FB from the plate winding; there is enough gain to do without this, but drive voltage requirements require the additional headroom provided by the PFB. Replace the plate loads with CCS. This will maintain the voltage headroom, but eliminate the positive FB.

Cathode follower: Replace 12AX7 with a 12AT7 and decrease the value of the loads to increase current a little bit( or install a CCS here; useful to deal with high-ripple negative rail ) and set appropriate idle current.

Bias supply: Improve filtering, consider a pi-filter built around appropriate voltage caps and the 30Hy/40 mA Hammond choke. Also add adjustments to the cathode follower grids to set power tube idle current.

Main PS: Replace the 50R resistor with a choke; consider a Dynaco St.70 part here( 1.5 Hy/200 mA/50R DCR ). Use at least a replacement can cap for the main filters.

Same topology, improved implementation.
cheers,
Douglas

[no gas]

Originally Posted by Bandersnatch

hey-Hey!!!,
The basic McIntosh circuit is excellent use of those OPT's; can't think of any other topology that will improve them. That is not to say that the originals can't be tweaked.
Input stage: the 12AX7 can be replaced with a 12AY7. Changes to the cathode R to deliver an appropriate plate voltage to the direct coupled phase splitter would be needed.

Phase splitter: re-wire heater connections to run 6CG7, equalize plate loads at 33k, and install a CCS in place of the original 18k tail resistor.

Driver stage: The 12BH7 is getting positive FB from the plate winding; there is enough gain to do without this, but drive voltage requirements require the additional headroom provided by the PFB. Replace the plate loads with CCS. This will maintain the voltage headroom, but eliminate the positive FB.

Cathode follower: Replace 12AX7 with a 12AT7 and decrease the value of the loads to increase current a little bit( or install a CCS here; useful to deal with high-ripple negative rail ) and set appropriate idle current.

Bias supply: Improve filtering, consider a pi-filter built around appropriate voltage caps and the 30Hy/40 mA Hammond choke. Also add adjustments to the cathode follower grids to set power tube idle current.

Main PS: Replace the 50R resistor with a choke; consider a Dynaco St.70 part here( 1.5 Hy/200 mA/50R DCR ). Use at least a replacement can cap for the main filters.

Same topology, improved implementation.
cheers,
Douglas

Great ideas! I'll consider doing them all. Thank you!