2013.05.04.Anatomy of an Mc30 | Vintage Vacuum Audio search this site... Home Restoration Services Chrome Logos & Parts Amps for Sale Anatomy of an Mc30 Vintage Ads Slideshow Vintage Magazines Schematics & Manuals Contact Home >> Anatomy of an Mc30 vintagevacuumaudio.com/anatomy-of-an-mc30/ 1/9 2013.04.05. Anatomy of an Mc30 | Vintage Vacuum Audio vintagevacuumaudio.com/anatomy-of-an-mc30/ 2/9 . 04. Front.05. and 12AX7 (preamp) tube socket. The big tube at front is the rectifier. Let’s look inside an alloriginal. on the right are out of the output transformer.com/anatomy-of-an-mc30/ 3/9 . pins 678 are not connected to anything. Behind that are two more tube sockets for the two 1614/6L6GC/5881 power tubes. 12AX7 (driver) tube socket. Note that the 12AX7 furthest to the right is only used for one of its sections (pins 13). vintagevacuumaudio. The output transformer is on the left. unrestored MC30 Here is the amp resting on the top of its transformers with bottom cover removed. whose only function is to convert AC into DC. Behind the tube sockets is a bakelite board containing most of the amp’s components. both cosmetically & electronically. 12BH7 (voltage amp) tube socket. Pictured above is a pair of MC30s I completely overhauled recently. The tall metal “tube” is the power supply multisection electrolytic capacitor. Anatomy of an Mc30 | Vintage Vacuum Audio These few pages are meant to help out those performing their own restoration of this classic amp. The wires coming from the two holes on the left are out of the power transformer. from left to right – power supply filter cap. 5U4 tube socket. the power transformer is on the right.2013. 2 power resistors. The two tubes at rear are the power tubes. 12AU7 (phase splitter) tube socket. If you do not plan to use the octal input.76K and no more than 12. you can eliminate this part w/o any consequence on rest of circuit. used in negative feedback circuitry in parallel with R13. The pictures show what is by far the most common version. Anatomy of an Mc30 | Vintage Vacuum Audio The bakelite board we mentioned earlier has mostly resistors on one side and capacitors on the other.04. is more often found to be 1M.R9.24K. but have had very good results with (surprisingly) Vishay metal films or Tantalums or Kiwame and pretty dreadful ones with Dale and Mills (harsh. R2 is only in signal path when you are using the octal socket input and feeding audio through it. a small value (470pF). and all part names refer to their names in McIntosh’s schematic you will find in the MC30 owner’s manual in our archives. letter C means capacitor.com/anatomy-of-an-mc30/ 4/9 . R19 R20 – 12K 2 W plate resistors that MUST be matched within 2% of each other. C7 – Yes! That resistorlooking thing is a capacitor. “Resistors” (front side of amp) Letter R means resistor. here shown as it is on schematic as 680K. As Mcintosh sold the MC30 for nearly a decade. vintagevacuumaudio. Can be either while rest of circuitry is the same. It is sometimes found on the reverse side. If one is 12K exactly the other one must be no less than 11. I go through dozens of Allen Bradley carbon comp res to find two matched ones (not worth the effort). bright and metallic).2013. at least four circuit variations exist.05. vintagevacuumaudio. R23 – very often mounted on the other side of the board.com/anatomy-of-an-mc30/ 5/9 . 3. But the original 3. If your voltage is too high.2013. Note that R21 MUST be very close to its nominal 120K for all this to work according to plan. Those two components are part of the bias voltage circuitry. Anatomy of an Mc30 | Vintage Vacuum Audio Selenium rectifier and R30 — change the selenium rectifier IMMEDIATELY. Do not overlook it either way. if too low. (you’re ok between NEGATIVE 42 volts and NEGATIVE 45 volts from all those pins to ground). raise R30. either 8.1K.2K or 9. bring it down. 6. 7 of 12ax7 on the left side of amp (see 1st page).3k res must be increased nearly threefold as the 1N4007 will boost voltage some compared to Selenium rect. with a diode (1N4007 for instance) and replace R30 with a higher value resistor. R30’s value directly impacts the bias voltage you can measure at pins 2.04.05. the capacitors that will have the most audible impact on sound quality are C1 (seen on other side). They and only they are in the audio signal path. I have found that just a few brands of caps (most mylar/polyesters such as Mallory 150) and a couple of polypropylenes (most recently littleknown. – side connects to 820K.2013. Some choose to double its value (from 10uF/450v to 20 or 22uF/450 or 500v). Caution: + side goes to ground. etc. wholesaleonly STK. And by all means forget those old bumble bees. Hovlands. those are the ones to experiment with. New York State produced. I own a true.) will yield unsatisfactory results as they all tend to boost upper midrange in various but always unpleasant . managed to not harden/brighten the sound. C8 & C9. If you intend to engage in “capacitor rolling”. high voltage capacitance tester (not the hand held type which feeds 3 volts to the cap vintagevacuumaudio. Anatomy of an Mc30 | Vintage Vacuum Audio 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.04. essential to replace as it will make your output tube plates go cherry red if leaky or bad.05. selenium rectifier. You will find that most socalled “boutique” caps (read “expensive” — Mundorf. Otherwise you can snip off or eliminate altogether or replace it if you wish to remain true to original design of amp. 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. and R25 R26 found on other side of board. C13 – Bias voltage capacitor. and C11 and C12.com/anatomy-of-an-mc30/ 6/9 . C5. Good thing to do if you ever want to play with a C8 preamp. used by McIntosh in one or several of their tube anniversary reissue models). solidstatey ways. Sonicaps. Once you have an amp that is indeed on spec. Replace always. just like all other electrolytic caps. the output tube sockets: CH1 and CH2 are neither caps nor resistors. and the three caps contained in “can” (40. MC60. I have not yet A/B’d new ones vs old ones to see if that makes any impact on sonics. back to your preamp. No fun at all… Lastly.05. vintagevacuumaudio. Pull them! Or wait for them to fail. In my experience. 80. and anything else that is connected to your amp or stereo.2013. R28. 20 UF) regulate and filter voltage (known as B+) to various sections of the amp. they are rarely bad.04. I have tested hundreds of those out of MC30. Anatomy of an Mc30 | Vintage Vacuum Audio and tells you nothing of its performance at nominal voltage). MC75 etc… Less than 1% are anywhere close to spec and not leaky. Power supply circuitry: R27. but at your own risk. They are on their last legs if not already dead. A bad C12A/B/C is the primary cause of 60cycle hum.com/anatomy-of-an-mc30/ 7/9 . although they can be changed. If you decided to change them the service manual contained in the schematics section of this site gives you the proper values. to your ground network. A shorted one will send 500 volts to your amp chassis. they are inductors (chokes). 05. that your output tubes are vintagevacuumaudio.2013.com/anatomy-of-an-mc30/ 8/9 .04. Anatomy of an Mc30 | Vintage Vacuum Audio 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. check R30.com/anatomy-of-an-mc30/ 9/9 . C10 or C11) After a few minutes the grey/black plates of one or both of your 1614/6L6GC etc.04. Your tubes will not last forever. The variac technique is NOT recommended for tube amps with tube rectification like this one. And since we’re mythbusting — tube amps should not be left on at all times. Sound is heard but accompanied by steady low (60Hz) hum: Most likely culprit: C12A/B/C (assuming you’ve already adjusted the hum pot on side of chassis to its quietest spot) Sounds good at first but distorts increasingly as amp stays powered: Most likely bad coupling caps (C5. without input connected. Amp powers on. that the rest of your equipment functions properly.2013. especially if they have not been properly restored. Anatomy of an Mc30 | Vintage Vacuum Audio matched for current draw. output tubes turn cherry red (usually accompanied by hum): replace C13. but is OK for amps with solid state rectification. badly discolored. stripped or burnt wires. cracked or gookoozing capacitors. And of course. Also make sure that you can se all tubes nicely lit. C8. Constant or intermittent sizzle. As a rule of thumb. a fuse of the right value (3 amp slo blo). the 5U4GB. Also possible — one of the PVC covered wires inside has gotten stripped or burnt and is causing short. Good luck with your amps and remember what got you here in the first place — the music. and other evidence of past or current trouble. never plug in one of these for the first time without at the very least carefully visually inspecting the proverbial guts for any burnt. a bad contact in one of the tube sockets (esp. and that gain pot is turned all the way to the right (max setting). that your cables are making proper contact etc. R2. stays on. or a bad volume pot. not the hardware. If that tube socket is making no or bad contact.) Amp blows fuse immediately upon powering: Bad transformer (bad news) or bad filter cap (C12A/B/C). like the sound of frying bacon/eggs in a pan Either one (or several) bad resistor(s). or shrivelled resistors. C9.05. never plug in this or ANY tube amp without speakers connected. You risk to do more damage than good. the small tubes at front). Copyright 2012 Vintage Vacuum Audio | Designed by Apogee Design Systems vintagevacuumaudio. amp will produce no sound because no voltage is going to amps’ various sections. Every hour the amp is on is an hour less in your tubes’ life. but no sound at all: Check that speaker are properly connected. esp. and leave them on unattended.
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