• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Questions about rebuilding MC-30 and MC-240

jeffsplat88

jeffsplat88

Member
Hello Mc-Experts
I have a few questions that I would really appreciate some feedback on:

1) I've updated the bias circuits (diodes caps and entire bias resistor strings) on my MC-30 and MC-240. I'm curious why the MC-240 calls for -46 VDC whereas the MC-30 calls for -42 when both run at nearly the same plate voltages. I'm measuring around 20-25 watts of dissipation on the 6L6GC tubes on the MC-30. And more like 15 watts on the MC-240. Is there a reason why I couldn't back off the MC-30 to more like -46 and get more tube life?

2) The MC-30 has a hum balance pot between the heater AC voltages which is nice because I can null out the humm to near zero. The 240 has one side of the heater grounded so I can't add in a hum balance pot on the 240. Has anyone ever modified the 240 to lift the ground and add a hum balance pot? Is there a reason why this couldn't be done?

3) Tube Matching: I've got a collection of nice RCA black-plates to stick in these. I'm measuring them all on a Hickok 539b along with a plate current DMM. When selecting pairs to stick in these amps, is it more important to match GM or Plate current, or both (both being much harder to find pairs)?
 
Register to hide this ad
They are two generationally different amplifiers. They use different types of power supplies, MC 30 with tube rectification, the MC240 with a solid state doubler. They were also designed to use different output tubes, MC30 with 1614 and the MC240 with 6L6GC/7027A. You can use 6L6GC in the MC30 but do not use the 1614 (or anything less than the 6L6GC/7027A type tube in the MC240. As far as hum modifications needed on a MC240, I've never heard of one that benefited from it. Output tube matching with McIntosh unity coupled amplifiers, MC30 and MC240 being just two examples, is not critical.
 
ramjet said:
They are two generationally different amplifiers. They use different types of power supplies, MC 30 with tube rectification, the MC240 with a solid state doubler. They were also designed to use different output tubes, MC30 with 1614 and the MC240 with 6L6GC/7027A. You can use 6L6GC in the MC30 but do not use the 1614 (or anything less than the 6L6GC/7027A type tube in the MC240. As far as hum modifications needed on a MC240, I've never heard of one that benefited from it. Output tube matching with McIntosh unity coupled amplifiers, MC30 and MC240 being just two examples, is not critical.
Click to expand...

Thank you so much for your time in responding. It never dawned on me that the bias on the MC-30 was set up for the 1614 which may be a different tube than the 6L6. So It would not be out of the question to adjust the bias to -46 on the MC-30 when using 6L6?
 
The 1614 has more in common with the original 6L6 than does the 6L6GC as far as ratings. The MC30 and the MC40/240 are designed for two slightly similar but different output tubes. Run them as they were designed and they will be fine.
 
My restored MC40 mono blocks are printed 6L6GC or 7027A by the tube sockets. I'm using good vintage 6L6GC tubes in it at the moment and it sounds wonderful.
What is the difference between the two tube types? 6L6GC are common and I have quite a few but I don't know if I have run across any 7027A tubes. Actually, I need to check my tube stash, I may have some, they just never jumped out at me like the 6L6GC tubes.

BillWojo
 
I believe that the 7027A are higher plate dissipation than 7027, the MC240 calls for 6L6GC or 7027A. From my (limited) research they are equivalent.

I have found a vintage 7027A or two, but not a matched quad yet. Another option is the 7581A, Terry D had recommended them as good new-issue tubes and said that he has gotten good reviews from users.
 
SaturationPt said:
I believe that the 7027A are higher plate dissipation than 7027, the MC240 calls for 6L6GC or 7027A. From my (limited) research they are equivalent.

I have found a vintage 7027A or two, but not a matched quad yet. Another option is the 7581A, Terry D had recommended them as good new-issue tubes and said that he has gotten good reviews from users.
Click to expand...


I love Philips 7581as. I have them in my MC30s and both blackface Fender amps. To my ear, they are the pinnacle of 6L6 tubes. Originally, I got turned on to them by Fat Willie aka Lord Valve, in Denver in the late 90s. I think they were maybe $65 a quad. (he also hipped me to the nice NOS 5751s which he was selling for under $5 a pop). I've tried a number of tubes in my MC30s and all of them seem colored in a way the 7581as don't. I don't know about the new production ones, but I bet they are OK.

Back in the day, the Grateful Dead used to buy them by the pallet for their Fender amps. Part of that crystalline Fender tone, but in a Mac, they are also spectacular. With a MTB of 5,000 hours, I'm set for most of the rest of my life.
 
The MC40 is rated 40WPC and takes the 6L6GC or 7270 output tubes, the MC30 is rated at 30WPC and takes 6L6GC or lower-power 1614 tubes. The 7581A is a current-production tube that is similar to the higher-power 6L6GC and can be used on either.

The 5881 should not be used on the MC40/MC240 but I believe is commonly used on the MC30.
 
You must log in or register to reply here.
Back
Top Bottom