mcintosh mc2105 running hot (or normal?)

I have an mc2105. I'm not the original owner, but without any comparison, I think it sounds great. Enjoyable sound, no noise or distortion.

It's in a wood case and when I put my hand over the top, it feels warm... Closer to hot, but I can touch and leave my hand on it... But not "running cool" as I've seen described on new well running amps. Maybe that is "cool" compared to untouchable. Out of the case, it feels warm... Hot. The power transformer has the most heat. I can keep my hand on it, but it's not cool at all. Would a power amp be cool or is cool relative to untouchable?

 

ALL regular class A or class AB power amps get hot. Only modern design class D amps stay cool.
Realize the wood case is a big oven mitt holding in all the heat so it has to go out the top.
Your amp is perfectly normal. Being able to 'leave your hand on it' is "yes that is great"
So your amp is fine..

 

I own an MC 2505 which is a similar design as the 2105 which runs quite cool unless output averages much over 15 wpc or so, continuously - over long periods - ie: with rock/pop music with a lot sustained bass content, for example.

However, it should not become that hot under relatively light use and/or music that does not have a sustained bass line.

 

I left it on overnight. It was idle. It felt warm/hot over the vents... Cool on the back and got on the power transformer.

How cool is cool? I'm just trying to figure out if what I'm feeling is hot, but normal cool operation for this amp.

 

Something does not seem right here: Of the several McIntosh amplifiers I own - including an MC 2505, (2) 2125 and 2155, none ever created the amount of heat you describe while idling - regardless of duration.

Power transformer may be slightly warm but that should be all.

Sounds to me like the amplifier may have a simple bias current issue which could account for the high idle temperature you report.

 

What do you suspect would remedy that type of issue? At most, complete recap?

 

A simple bias adjustment may be all that is needed in this case.

 

Anybody know anyone in MA who is a trusted resource with these?

 

Your 2105 is running hot. I have a 2100 and have inadvertantly left it on for days at a time. Surface temp was barely above room temp. I use the amp to power a pair of passive subs; even after vigorous use the amp is barely warm to the touch. Have yours checked out by someone who knows Mac equipment.

 

Only McIntosh repair centers with which I have direct experience are Audio Classics (NY) & DeWick Repairs (TN) and recommend them unconditionally.

Audio Lab in Cambridge MA may be worth looking into as they cite McIntosh.

I really think there is a very good chance the bias needs adjustment. Caps normally would not do this, Nevertheless a thorough going over would certainly be a good idea considering the age of the unit.

For whatever it's worth - all 4 of my McIntosh SS amplifiers still sport their original caps and none is newer than 30 years old.

 

I believe the service manual will describe the correct procedure and specifications. It is possible the problem is component drift because I can't remember if there is an adjustment pot for this, it may nhave been a select-on-test pair of resistors or something and they could have drifted.

 

A friend experienced exactly the same condition with his MC 2120 and corrected it by a bias adjustment - for which I believe there was a pot in that model.

I cannot speak for the 2105 but here's the service manual: http://akdatabase.org/AKview/albums/userpics/10004/MC2105 Service.pdf

 

If this is a tube amp, then some heat it quite normal. The bias current is applied to the grid in the output tubes. It works like a tap to adjust the flow of electrons (current) between the anode and cathode. Output tubes vary from one to the other in terms of the current flow they are happy to work with. This is why the bias current can be adjusted in class AB designs, to make sure your amp is working to peak performance without upsetting the output tubes.

Too little bias current and the electron flow will be too much, leading to hot operating conditions and reduced tube life. If you see the plates inside the output tubes glowing at all (not the heater filaments) then the tubes are being pushed too hot and hard.

Too much bias current and the electron flow is overly restricted, leading to cool operating conditions, long tube life, but a colder, more sterile sound as a result.

In the world of guitar amps, its generally accepted that they sound their best when the output tubes are run more on the hot side.

With hi-fi amps, they generally sound best when the bias is adjusted to the correct level for the tubes being used at the time. The sweet spot, not too hot, and not too cold . The bias is set using an oscilloscope to look at the sine waveform, adjusting the bias to achieve the correct waveform shape. Some audiophiles do prefer the sound of a colder bias though.

I would expect that you simply need to have the amp biased. You don't need to go to an expensive hi-fi service centre to get this done. Guitar amp technicians are setting bias all day every day - its a 20 minute job. Ring around to a few professional guitar shops and ask for recommendations on who to see for tube amp servicing. Often the best guys are ex-military electronics personnel, working from home and taking in work from these stores.

Cheers,

Mark

 

Oops! Scratch all that! I just researched the amp and found its a solid state design - no tubes!

 

Good info, all the same.

 

Just curious what where is the amp kept? What kind of ventilation do you have set aside for it?

 

I had a similar problem with a pioneer receiver that went up for sale. The problem was bad caps in the feedback loop causing heat at idle in the output devices, and a slightly warmer power transformer. The amp otherwise sounded normal, put out full power, just built up too much heat. After changing the caps (four total) and the filter caps, the amp ran luke warm at idle.

It seems to me your amp needs servicing. After replacement of defective components, the DC offset needs to be readjusted and the bias.
It is possible the power transformer has shorted turns and may not be long before it fails. If the heatsink of one channel gets noticeably hotter than the other, then the problem is not the power transformer.

 

Solid state amps have a biasing mechanism just like tube ones although it works "backwards" in that low bias current means low quiescent current.

Bipolar transistors are "normally off", current controlled current controlling devices.

Vacuum tubes are "normally on", voltage controlled current controlling devices.

With a fixed bias tube amp final you MUST understand that INCREASING the bias VOLTAGE DECREASES the quiescent CURRENT. This is a negative voltage relative to the ground reference whereas the other voltages are positive (in most cases). By knowing both the bias voltage and the quiescent current we can tell a lot of things especially in trend monitoring. In a well matched pair of tubes the needed bias voltages will be pretty close to get the currents equal. As the tube ages if drastic changes or different changes needed from one to the other we can make good predictions about tube life.