• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

to MC30 or not to MC30

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
 

Rodeodave

Member
Paid Member
2004-12-26 9:38 pm
in the alps

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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
 
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!