anatomy of an mc30 _ vintage vacuum audio

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These few pages are meant to help out those performing their own restoration of this classic amp. Pictured above is a pair of MC30s I completelyThese few pages are meant to help out those performing their own restoration of this classic amp. Pictured above is a pair of MC30s I completely

overhauled recently, both cosmetically & electronically. The output transformer is on the left, the power transformer is on the right. The tall metaloverhauled recently, both cosmetically & electronically. The output transformer is on the left, the power transformer is on the right. The tall metal

“tube” is the power supply multi-section electrolytic capacitor. The two tubes at rear are the power tubes. The big tube at front is the rectifier,“tube” is the power supply multi-section electrolytic capacitor. The two tubes at rear are the power tubes. The big tube at front is the rectifier,

whose only function is to convert AC into DC.whose only function is to convert AC into DC.

Let’s look inside an all-original, unrestored MC-30Let’s look inside an all-original, unrestored MC-30

Here is the amp resting on the top of its transformers with bottom cover removed. Front, from left to right – power supply filter cap, 2 powerHere is the amp resting on the top of its transformers with bottom cover removed. Front, from left to right – power supply filter cap, 2 power

resistors, 5U4 tube socket, 12AX7 (driver) tube socket, 12BH7 (voltage amp) tube socket, 12AU7 (phase splitter) tube socket, and 12AX7resistors, 5U4 tube socket, 12AX7 (driver) tube socket, 12BH7 (voltage amp) tube socket, 12AU7 (phase splitter) tube socket, and 12AX7(preamp) tube socket. Note that the 12AX7 furthest to the right is only used for one of its sections (pins 1-3). pins 6-7-8 are not connected to(preamp) tube socket. Note that the 12AX7 furthest to the right is only used for one of its sections (pins 1-3). pins 6-7-8 are not connected to

anything. Behind the tube sockets is a bakelite board containing most of the amp’s components. Behind that are two more tube sockets for the twoanything. Behind the tube sockets is a bakelite board containing most of the amp’s components. Behind that are two more tube sockets for the two

1614/6L6GC/5881 power tubes. The wires coming from the two holes on the left are out of the power transformer, on the right are out of the1614/6L6GC/5881 power tubes. The wires coming from the two holes on the left are out of the power transformer, on the right are out of the

output transformer.output transformer.





The bakelite board we mentioned earlier has mostly resistors on one side and capacitors on the other. As Mcintosh sold the MC30 for nearly aThe bakelite board we mentioned earlier has mostly resistors on one side and capacitors on the other. As Mcintosh sold the MC30 for nearly a

decade, at least four circuit variations exist. The pictures show what is by far the most common version.decade, at least four circuit variations exist. The pictures show what is by far the most common version.

“Resistors” (front side of amp)“Resistors” (front side of amp)

Letter R means resistor, letter C means capacitor, and all part names refer to their names in McIntosh’s schematic you will find in the MC30Letter R means resistor, letter C means capacitor, and all part names refer to their names in McIntosh’s schematic you will find in the MC30

owner’s manual in our archives. R2 is only in signal path when you are using the octal socket input and feeding audio through it. If you do not planowner’s manual in our archives. R2 is only in signal path when you are using the octal socket input and feeding audio through it. If you do not plan

to use the octal input, you can eliminate this part w/o any consequence on rest of circuit.R9, here shown as it is on schematic as 680K, is moreto use the octal input, you can eliminate this part w/o any consequence on rest of circuit.R9, here shown as it is on schematic as 680K, is more

often found to be 1M. Can be either while rest of circuitry is the same.often found to be 1M. Can be either while rest of circuitry is the same.

C7 – Yes! That resistor-looking thing is a capacitor, a small value (470pF), used in negative feedback circuitry in parallel with R13. It is sometimesC7 – Yes! That resistor-looking thing is a capacitor, a small value (470pF), used in negative feedback circuitry in parallel with R13. It is sometimesfound on the reverse side.found on the reverse side.

R19 R20 – 12K 2 W plate resistors that MUST be matched within 2% of each other. If one is 12K exactly the other one must be no less thanR19 R20 – 12K 2 W plate resistors that MUST be matched within 2% of each other. If one is 12K exactly the other one must be no less than

11.76K and no more than 12.24K. I go through dozens of Allen Bradley carbon comp res to find two matched ones (not worth the effort), but have11.76K and no more than 12.24K. I go through dozens of Allen Bradley carbon comp res to find two matched ones (not worth the effort), but have

had very good results with (surprisingly) Vishay metal films or Tantalums or Kiwame and pretty dreadful ones with Dale and Mills (harsh, brighthad very good results with (surprisingly) Vishay metal films or Tantalums or Kiwame and pretty dreadful ones with Dale and Mills (harsh, bright

and metallic).and metallic).





Selenium rectifier and R30 — change the selenium rectifier IMMEDIATELY, with a diode (1N4007 for instance) and replace R30 with a higherSelenium rectifier and R30 — change the selenium rectifier IMMEDIATELY, with a diode (1N4007 for instance) and replace R30 with a higher

value resistor, either 8.2K or 9.1K. Those two components are part of the bias voltage circuitry. R30’s value directly impacts the bias voltage youvalue resistor, either 8.2K or 9.1K. Those two components are part of the bias voltage circuitry. R30’s value directly impacts the bias voltage you

can measure at pins 2, 3, 6, 7 of 12ax7 on the left side of amp (see 1st page). (you’re ok between NEGATIVE 42 volts and NEGATIVE 45 voltscan measure at pins 2, 3, 6, 7 of 12ax7 on the left side of amp (see 1st page). (you’re ok between NEGATIVE 42 volts and NEGATIVE 45 volts

from all those pins to ground). If your voltage is too high, raise R30, if too low, bring it down. But the original 3.3k res must be increased nearlyfrom all those pins to ground). If your voltage is too high, raise R30, if too low, bring it down. But the original 3.3k res must be increased nearly

three-fold as the 1N4007 will boost voltage some compared to Selenium rect. Note that R21 MUST be very close to its nominal 120K for all this tothree-fold as the 1N4007 will boost voltage some compared to Selenium rect. Note that R21 MUST be very close to its nominal 120K for all this to

work according to plan.work according to plan.

R23 – very often mounted on the other side of the board. Do not overlook it either way.R23 – very often mounted on the other side of the board. Do not overlook it either way.





Capacitors side (rear of amp and upside down to maintain orientation with previous page)Capacitors side (rear of amp and upside down to maintain orientation with previous page)

C2 is only in signal path if you use the octal socket input and not the RCA jack. Otherwise you can snip off or eliminate altogether or replace it ifC2 is only in signal path if you use the octal socket input and not the RCA jack. Otherwise you can snip off or eliminate altogether or replace it if

you wish to remain true to original design of amp. Good thing to do if you ever want to play with a C8 preamp.you wish to remain true to original design of amp. Good thing to do if you ever want to play with a C8 preamp.

C13 – Bias voltage capacitor, essential to replace as it will make your output tube plates go cherry red if leaky or bad. Some choose to double itsC13 – Bias voltage capacitor, essential to replace as it will make your output tube plates go cherry red if leaky or bad. Some choose to double its

value (from 10uF/450v to 20 or 22uF/450 or 500v). Caution: + side goes to ground, – side connects to 820K, selenium rectifier, and R25 R26value (from 10uF/450v to 20 or 22uF/450 or 500v). Caution: + side goes to ground, – side connects to 820K, selenium rectifier, and R25 R26

found on other side of board.found on other side of board.

It is essential that ALL caps and resistors be within specs for your amp to function and perform safely and as intended by its designers. Once youIt is essential that ALL caps and resistors be within specs for your amp to function and perform safely and as intended by its designers. Once you

have an amp that is indeed on spec, the capacitors that will have the most audible impact on sound quality are C1 (seen on other side), C5, C8 &have an amp that is indeed on spec, the capacitors that will have the most audible impact on sound quality are C1 (seen on other side), C5, C8 &

C9, and C11 and C12. They and only they are in the audio signal path. If you intend to engage in “capacitor rolling”, those are the ones toC9, and C11 and C12. They and only they are in the audio signal path. If you intend to engage in “capacitor rolling”, those are the ones to

experiment with. You will find that most so-called “boutique” caps (read “expensive” — Mundorf, Sonicaps, Hovlands, etc.) will yieldexperiment with. You will find that most so-called “boutique” caps (read “expensive” — Mundorf, Sonicaps, Hovlands, etc.) will yield

unsatisfactory results as they all tend to boost upper mid-range in various but always unpleasant , solid-statey ways. I have found that just a fewunsatisfactory results as they all tend to boost upper mid-range in various but always unpleasant , solid-statey ways. I have found that just a few

brands of caps (most mylar/polyesters such as Mallory 150) and a couple of polypropylenes (most recently little-known, New York State-brands of caps (most mylar/polyesters such as Mallory 150) and a couple of polypropylenes (most recently little-known, New York State-

produced, wholesale-only STK, used by McIntosh in one or several of their tube anniversary reissue models), managed to not harden/brighten theproduced, wholesale-only STK, used by McIntosh in one or several of their tube anniversary reissue models), managed to not harden/brighten the

sound.sound.

And by all means forget those old bumble bees. I own a true, high voltage capacitance tester (not the hand held type which feeds 3 volts to the capAnd by all means forget those old bumble bees. I own a true, high voltage capacitance tester (not the hand held type which feeds 3 volts to the cap

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and tells you nothing of its performance at nominal voltage). I have tested hundreds of those out of MC30, MC60, MC75 etc… Less than 1% areand tells you nothing of its performance at nominal voltage). I have tested hundreds of those out of MC30, MC60, MC75 etc… Less than 1% are

anywhere close to spec and not leaky. They are on their last legs if not already dead. Pull them! Or wait for them to fail, but at your own risk.anywhere close to spec and not leaky. They are on their last legs if not already dead. Pull them! Or wait for them to fail, but at your own risk.

Power supply circuitry:Power supply circuitry:

R27, R28, and the three caps contained in “can” (40, 80, 20 UF) regulate and filter voltage (known as B+) to various sections of the amp. A badR27, R28, and the three caps contained in “can” (40, 80, 20 UF) regulate and filter voltage (known as B+) to various sections of the amp. A bad

C12A/B/C is the primary cause of 60-cycle hum. Replace always, just like all other electrolytic caps. A shorted one will send 500 volts to your ampC12A/B/C is the primary cause of 60-cycle hum. Replace always, just like all other electrolytic caps. A shorted one will send 500 volts to your amp

chassis, to your ground network, back to your preamp, and anything else that is connected to your amp or stereo. No fun at all…chassis, to your ground network, back to your preamp, and anything else that is connected to your amp or stereo. No fun at all…

Lastly, the output tube sockets:Lastly, the output tube sockets:

CH1 and CH2 are neither caps nor resistors, they are inductors (chokes). In my experience, they are rarely bad, although they can be changed. ICH1 and CH2 are neither caps nor resistors, they are inductors (chokes). In my experience, they are rarely bad, although they can be changed. I

have not yet A/B’d new ones vs old ones to see if that makes any impact on sonics. If you decided to change them the service manual contained inhave not yet A/B’d new ones vs old ones to see if that makes any impact on sonics. If you decided to change them the service manual contained in

the schematics section of this site gives you the proper values.the schematics section of this site gives you the proper values.







AUDIBLE TROUBLE? WHERE AND WHAT TO LOOK FOR AUDIBLE TROUBLE? WHERE AND WHAT TO LOOK FOR

Let’s identify the basic symptoms and their likely causes (we assume all your tubes are properly and reliably tested, that your output tubes areLet’s identify the basic symptoms and their likely causes (we assume all your tubes are properly and reliably tested, that your output tubes are








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matched for current draw, that the rest of your equipment functions properly, that your cables are making proper contact etc.)matched for current draw, that the rest of your equipment functions properly, that your cables are making proper contact etc.)

Amp blows fuse immediately upon powering:Amp blows fuse immediately upon powering: Bad transformer (bad news) or bad filter cap (C12A/B/C). Also possible — one of the PVCBad transformer (bad news) or bad filter cap (C12A/B/C). Also possible — one of the PVC

covered wires inside has gotten stripped or burnt and is causing short.covered wires inside has gotten stripped or burnt and is causing short.

Amp powers on, stays on, but no sound at all:Amp powers on, stays on, but no sound at all: Check that speaker are properly connected, and that gain pot is turned all the way to the rightCheck that speaker are properly connected, and that gain pot is turned all the way to the right

(max setting). Also make sure that you can se all tubes nicely lit, esp. the 5U4GB. If that tube socket is making no or bad contact, amp will produce(max setting). Also make sure that you can se all tubes nicely lit, esp. the 5U4GB. If that tube socket is making no or bad contact, amp will produce

no sound because no voltage is going to amps’ various sections.no sound because no voltage is going to amps’ various sections.

Sound is heard but accompanied by steady low (60-Hz) hum:Sound is heard but accompanied by steady low (60-Hz) hum: Most likely culprit: C12A/B/C (assuming you’ve already adjusted the hum pot onMost likely culprit: C12A/B/C (assuming you’ve already adjusted the hum pot on

side of chassis to its quietest spot)side of chassis to its quietest spot)

Sounds good at first but distorts increasingly as amp stays powered:Sounds good at first but distorts increasingly as amp stays powered: Most likely bad coupling caps (C5, C8, C9, C10 or C11)Most likely bad coupling caps (C5, C8, C9, C10 or C11)

After a few minutes the grey/black plates of one or both of your 1614/6L6GC etc. output tubes turn cherry red (usually accompanied byAfter a few minutes the grey/black plates of one or both of your 1614/6L6GC etc. output tubes turn cherry red (usually accompanied by

hum):hum): replace C13, check R30, R2.replace C13, check R30, R2.

Constant or intermittent sizzle, like the sound of frying bacon/eggs in a panConstant or intermittent sizzle, like the sound of frying bacon/eggs in a pan Either one (or several) bad resistor(s), a bad contact in one ofEither one (or several) bad resistor(s), a bad contact in one of

the tube sockets (esp. the small tubes at front), or a bad volume pot.the tube sockets (esp. the small tubes at front), or a bad volume pot.

As a rule of thumb, never plug in one of these for the first time without at the very least carefully visually inspecting the proverbial guts for anyAs a rule of thumb, never plug in one of these for the first time without at the very least carefully visually inspecting the proverbial guts for anyburnt, badly discolored, or shrivelled resistors, cracked or gook-oozing capacitors, stripped or burnt wires, and other evidence of past or currentburnt, badly discolored, or shrivelled resistors, cracked or gook-oozing capacitors, stripped or burnt wires, and other evidence of past or current

trouble.trouble.

And of course, never plug in this or ANY tube amp without speakers connected, without input connected, a fuse of the right value (3 amp slo blo),And of course, never plug in this or ANY tube amp without speakers connected, without input connected, a fuse of the right value (3 amp slo blo),

and leave them on unattended, especially if they have not been properly restored.and leave them on unattended, especially if they have not been properly restored.

The variac technique is NOT recommended for tube amps with tube rectification like this one, but is OK for amps with solid state rectification. YouThe variac technique is NOT recommended for tube amps with tube rectification like this one, but is OK for amps with solid state rectification. You

risk to do more damage than good. And since we’re myth-busting — tube amps should not be left on at all times. Your tubes will not last forever.risk to do more damage than good. And since we’re myth-busting — tube amps should not be left on at all times. Your tubes will not last forever.

Every hour the amp is on is an hour less in your tubes’ life.Every hour the amp is on is an hour less in your tubes’ life.

Good luck with your amps and remember what got you here in the first place — the music, not the hardware.Good luck with your amps and remember what got you here in the first place — the music, not the hardware.

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anatomy of an mc30 _ vintage vacuum audio

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