MC225 Restoration

JohnWilliams

New Member
Here are some pictures of the restoration that I did on my MC225.

I put all power supply components on a single turret board that replaces the original bias supply board. It fits by turning the original can capacitor clips 90 degrees. This way, no permanent mechanical modifications have been made -- the old components could easily be put back in to restore the amp back to its original factory configuration. The two remaining can capacitors (voltage doubler) and associated bumble bee are no longer connected in the circuit. Those have been functionally replaced by new components on the turret board.
 

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Last edited:
Here are some pictures of the restoration that I did last year on my MC225.

I put all power supply components on a single turret board that replaces the original bias supply board. It fits by turning the original can capacitor clips 90 degrees. This way, no permanent mechanical modifications have been made -- the old components could easily be put back in to restore the amp back to its original factory configuration. The two remaining can capacitors (voltage doubler) and associated bumble bee are no longer connected in the circuit. Those have been functionally replaced by new components on the turret board.

Was the original bias board damaged? or did you replace it in order to mount the new caps?

A real restorer would have rebuilt the multi-cap can. :yes:


-Gregory
 
That's an interesting approach, for sure. So, all of the power supply parts are consolidated to one board. Well executed.

BTW, where did you manage to find a turret board of a height to fit into the chassis?

Cheers,

David
 
Gregory:
The original bias board was not damaged. The new board replaces all of the power supply components (except, of course, the power transformer and choke). It was done this way for several reasons. First, it isolates the modifications so that it is quick and easy to restore the amp back to the original, unmodified factory components, if desired. Second, it allows for better point/star grounding to reduce power supply noise due to ground loops. Third, it allows the use of higher quality components for improved performance. For example, the two rectangular blue components on the end of the board are 15uF high voltage polypropylene capacitors. One bypasses the main filter cap to reduce Equivalent Series Resistance by a factor of about 250 (ESR is 0.003 ohms) the other is the (ultra low ESR) filter cap dedicated to the 12AX7 input stage.

David:
The 2" wide turret board is Keystone part #15317 (Mouser Electronics part #534-15317).


-- John
 
Nice job and good thinking. What do think of the sonics compared to stock? What do you run for tubes? What speakers?
 
The current tubes are all original 1960's/70's vintage (post McIntosh clinic in the mid 70's):

12AX7: Valvo (Philips)
12AU7: Telefunken and HP branded (probably pulled from an HP instrument at some point).
12BH7: GE and McIntosh
7591A: GE full base x 4.

Don't really have any way to directly compare sonics with stock because the recap was really needed. Before the recap, lots of hum and sounded terrible. After recap, it is dead silent driving high efficiency headphones with inputs grounded. Sounds pretty good now with a really solid low end, but I need to roll the tubes to compare.

Any suggestions for good replacement tubes (without spending a fortune)?

Don't have appropriate speakers at the moment. Currently using some small planar magnetic speakers with subwoofer. They are not a very good match for this amp. They need a lower impedance source and sound better with a solid state amp.

-- John
 
Liked both the Gold Lion reissue KT-88s in my MC275/MC75s and also liked the Tung-Sol reissue 6550 in the same amps, so am thinking the GL KT-66s or Tung-Sol 6L6s in my MC30s. However, have also found some NOS JAN-Philips 6L6WHBs that am considering. The problem with the GL KT-66 is that it draws about 400mA more heater current per tube. Shouldn't be a problem with the MC30s, which are designed to run a preamp, but don't know if that is the case with the MC225. Might be safer to stick to the 6L6s. The Tung-Sol reissues are quite attractively priced. The GL, Tung-Sol and Mullard reissue small tubes (12AX7 & 12AU7) have all done well in my amps and preamps. The 12BH7 is a bit of a problem. Your choices are pretty much limited to EH, EI or NOS with them.
 
I use the EH 7591s and like them sufficiently to keep them in place. I actually sold off the original tubes back when they were fetching waaaaay more than I would spend for used-but-original tubes. I don't miss the old ones, either.

On the 12BH7s, the RCAs are quite good. The key tube, IMHO, is the 12AX7, where I typically use a 12AX7LPS for its low noise characteristics. The 12AU7 is the phase splitter circuit, however, if the resistors are properly matched and the operating voltages are in spec, the tube itself is of less consequence in this position as the designers really did a good job of keeping it within its operating parameters.

Cheers,

David
 
Recap mcintosh mc 225

Liked both the Gold Lion reissue KT-88s in my MC275/MC75s and also liked the Tung-Sol reissue 6550 in the same amps, so am thinking the GL KT-66s or Tung-Sol 6L6s in my MC30s. However, have also found some NOS JAN-Philips 6L6WHBs that am considering. The problem with the GL KT-66 is that it draws about 400mA more heater current per tube. Shouldn't be a problem with the MC30s, which are designed to run a preamp, but don't know if that is the case with the MC225. Might be safer to stick to the 6L6s. The Tung-Sol reissues are quite attractively priced. The GL, Tung-Sol and Mullard reissue small tubes (12AX7 & 12AU7) have all done well in my amps and preamps. The 12BH7 is a bit of a problem. Your choices are pretty much limited to EH, EI or NOS with them.

First off ditch the 12ax7 and use Sylvania 5751 gold brand gold pin triple mica.(Drop in replacement )Telefunken are not a good tube. Very dull & hazzy. Trust me! You are only fooled by the colored midrange that you mistake as being great. second change out the 12au7's to RCA 6cg7's.(this will need some resistors) Now you have a amp that was great that now is incredible. you will not believe the sound this amp will make. Of coarse this is my opinion.
 
First off ditch the 12ax7 and use Sylvania 5751 gold brand gold pin triple mica.(Drop in replacement )Telefunken are not a good tube. Very dull & hazzy. Trust me! You are only fooled by the colored midrange that you mistake as being great. second change out the 12au7's to RCA 6cg7's.(this will need some resistors) Now you have a amp that was great that now is incredible. you will not believe the sound this amp will make. Of coarse this is my opinion.

Good to know 2 year old threads still get attention too :D
 
I'd be concerned about the necessity for vintage "rarium" tube types, preferring the ability to operate properly with modern readily available stock.
 
The current tubes are all original 1960's/70's vintage (post McIntosh clinic in the mid 70's):

Don't really have any way to directly compare sonics with stock because the recap was really needed. Before the recap, lots of hum and sounded terrible. After recap, it is dead silent driving high efficiency headphones with inputs grounded. Sounds pretty good now with a really solid low end, but I need to roll the tubes to compare.

-- John

Hi John - You are saying you are driving headphones with your 225, how are you doing that ?
 
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