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I have a nice set of MC-60's that I purchased from the original owner. They sound fantastic at low and medium volumes but when pushed higher or with faster music it begins to distort until I turn down the volume a little. I have changed out all of the tubes one at at time and I still have the problem. It happens with both amps. Any Ideas?
Follow Ups:
The first and foremost thing you want to do on the mc 60s is change the .047 caps from the phase inverter to the voltage amp..Underneath you have a pair of 12k 2 watt resistors and if they are getting exceptionally hot,you have to change the .047uf caps at 600v..Next the filter caps have to go...You cant check filter caps at idle in a class B amp becuase they are not drawing any energy from the supply at idle.Next the bias diode and cap should be changed to a fast diode and a new filter cap bypassed with a polypro or pio cap..Just those mods will wake that amp up. THe mac sound can be really nice and I learned this from elecktron and sector7 but they need attention and have to be brought back to tolerance specs.I really love the mac sound now and the reason a lot of mac amps sounded poorly is they had the original filter caps in them and the bias diode has to be a fast switching like a schotkey or a hex fred.This keeps the audio signal from getting absorbed by the bias.
dean, hi. The McIntosh circuit operates the output tubes in Class B. I have found if the bias on the output tube grids is off it can have a bad effect on sound. In my MC-225 I have the bias set to within 1/10 Volt, and it needs to be. If your amps are original and stock you need to make sure all the resistors specified on the schematic to be matched at 1% are just that. The McIntosh circuit tends to balance itself, but if you have resistors out of tolerance you can change tubes 'til doomsday and not get good sound. I would suggest replacing the capacitors in the power supply as a matter of course. These parts degrade over time and they do have a life. If you are unsure about doing the circuit work yourself find help. There is enough room under the chassis to fit polypropylene caps or BlackGate electrolytics. If you are willing to take the time to redo what time has undone you will be rewarded with superb sound. I started out redoing the power and bias supplies in my 225 and wound up changing all the coupling caps: Rel Cap TFT on input and Multicap polystyrene and tin for the rest. I changed all the resistors and matching the 1% values to 0.01%. The results were spectacular. You have some classic amps with the 60's. Replace parts and upgrade as you would a classic anything (car, airplane, etc). You will discover just How Good the McIntosh circuit Really is! Have fun.
as Electron with both a set of MC-225s and then a set of MC-30s. Remember that those MC-60s are well-on 40+ years old. I ended up swapping all the resistors in the MC-30s due to their values having drifted by as much as 75% - original Mac spec'd them at 20%! This shouldn't be surprising due to the age.I am here to report that these amps will *sing* with proper component replacement - not modification, just replacement of aging components only.
Electron is also correct on the bias circuitry set ups. I missed not getting it set correctly first time around and there is a definite difference, sonically.
Enjoy them as they were designed!
Cheers,
David
It sounds like you are driving them into clipping. How loud is loud? What speakers are you driving? If you have low efficiency speakers and you try to drive them at higher levels the amps will run out of power. Also you didn't mention what impedance your speakers are or what tap you are running your speakers off. It could also be that the power supply caps could be in need of replacing. In particular the caps for the front end of the amp. It's hard to say without additional information.
"Nothing quite like high voltage DC to sharpen the mind and body!"
I'm using the 8 ohm tap of the amps. Loud is not crazy loud: perhaps 100 db. The speakers are Legacy Focus 96db efficient. I could try the 4 ohm tap because the Focus is a 4 ohm speaker but the terminals are so close together. The amps have all original parts in them.
Dean, You should be able to drive those speakers comfortably if your amps are up to the task. You are correct that you should be using the 4 ohm tap instead of the 8 ohm tap. I would have to agree with the other inmates about replacing caps and resistors. It is very unlikely that your stock MC60 has all it's resistors within spec. Carbon compostition resistors are one of the biggest sources sound degradation. Carbon comp resistors will change their value over time. Humidity can affect them, temperature affects them. Replace them with good metal film resistors. I use Dale/Vishay RNC70J, RNC65J, RN60J, resistors. For the wirewound resistors I have used Dale/Vishay NH-10 and NH-25 non-inductive types. Caps should be replaced as well. For the tubes safety, change the coupling caps at the very least. As another inmate had mentioned, I also use MIT RTX Tin-Foil Polystyrene caps in my amps. Power supply caps should also be replaced. I use United Chemicon 36DA series caps 550V rated which can easily be placed under the chassis. At the very least, replace them with new production twist-lock types. Also the selenium rectifier used in the bias network should be replaced with a modern soft-recovery diode and resistor valued changed to to correct the rise in bias volatge (due to lower voltage-drop) which will reduce the idle current to the output stage. Having the output stage under-biased can cause the early on-set of distortion. Have a qualified tech do the work for you if you are not comfortable with doing it yourself. You will not be dissapointed with the restored amps. MC60's are great amps which will reward you with great sound.
"Nothing quite like high voltage DC to sharpen the mind and body!"
All original parts? No wonder you have problems. You're listening to defective amps.The paper coupling caps are surely resistors by now. Most resistors are most likely way out of tolerance too. Some resistor pairs in this amp, although nominally 5% were matched to each other to within 1%. You should consider an extensive refurbishment.
MIchael Samra and others have discussed the benefits of a total blueprinting of the Mac Unity Coupled amps including the 60s. Search on his name here, in "Tubes" and possibly "Tube DIY" and you should get a good perspective on what you need to do and the expected results.
Be aware that some don't care for the Mac "sound" so even after repairs you may remain dissatisfied. If this turns out to be the case, there are many who appreciate the Mac sound, esp 60s and will gladly take them off your hands for more $$ than seems sensible, esp if totally stock.
Two things come to mind:Are you overdriving the amp? Using a CD player direct with controls all the way up for example? Notch the input level controls back a bit if so.
Has this unit been refurbished? Power supply capacitors are suspect if not. Is it both amps or just one?.. have you tried another source altogether to eliminate the source of distortion further up?.. Other capacitors may be passing dc causing this too if they have not be checked for leakage they should be..
A good tech will send a signal through the amp and look at the B+ too to sort it out quickly.
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