Techincal Amp Question

[Marc Piane]

I have a buddy that claims that amps have a 'life-span' and after a certain age start to lose power (spl, watts). Is there any truth to his argument?

[drurb]

Originally Posted by fingers

I have a buddy that claims that amps have a 'life-span' and after a certain age start to lose power (spl, watts). Is there any truth to his argument?

Here is my understanding. I hope fdeck will chime in. Traditional amplifying circuits (not the "switching" types that are so common now), could, I suppose lose a marginal amount of their current and power capabilities as a result of sagging power-supply capacitors. In addition, I recall an age-related effect within conventional power transistors such that the semiconductor material develops "pin-holes" that weaken it's performance. Beyond such effects are age-related changes in bias and quiescent currents.

Let's keep this in perspective. Suppose that, as a result of aging, an amplifier lost a whopping 25% of its power. That would translate into only a 1.25 dB decrease in output. You'd be hard-pressed to notice it. In other words, don't worry, be happy, and play on!

[drurb]

Originally Posted by jsbarber

I have a pair of 1956 McIntosh MC-30 tube amps. About three years ago I had them gone over and they needed a couple of replacement capacitors. They sound great.

Of course, with tubes, one replaces the output devices (the tubes) over the years. Your Macs should sound great for many more decades. As one who has restored hi-fi tube amps, basically all they usually need are new coupling caps and electrolytics in the power supply. Those electrolytics, unlike the case for transistor amps run at voltages in the hundreds.

[larry]

How about a related question-
When an older amp fails, what is the most likely cause (assuming no physical damage)?

[Marc Piane]

I ask because I overheard this agrument between two guys in a group I was playing with but didn't chime in.

[Ric Vice]

Originally Posted by larry

How about a related question-
When an older amp fails, what is the most likely cause (assuming no physical damage)?

Larry,
I'm sure that DURB or Fdeck are better at this because of their technical expertise. From my personal experience with a B-15 it was the power transformer that failed, but I'm told that the filter caps or the rectifier tube could have been faulty and ultimately caused the problem.

Ric

[drurb]

Originally Posted by Ric Vice

Larry,
I'm sure that DURB or Fdeck are better at this because of their technical expertise. From my personal experience with a B-15 it was the power transformer that failed, but I'm told that the filter caps or the rectifier tube could have been faulty and ultimately caused the problem.

Ric

This is a tough one to answer. Filter caps often go somewhat gradually, introducing hum and noise as they do. As to total and sudden failures... it depends.

[robgrow]

Originally Posted by Ric Vice

Larry,
I'm sure that DURB or Fdeck are better at this because of their technical expertise. From my personal experience with a B-15 it was the power transformer that failed, but I'm told that the filter caps or the rectifier tube could have been faulty and ultimately caused the problem.

Ric

Ric,

That is usually the case. Transformers seldom fail on their own. It is the related components in the power supply that fail and take the transformer with them. This is far more common however in vacuum tube amps which have relatively high operating voltages. I first started repairing musical instrument amps in the late 60's, and, in that time, I have seen a lot more power transformers fail in vacuum tube amps than in solid-state amps. Still it does happen in transistor amps. I have an early-80's Polytone amp here now that needs a new power transformer.

[robgrow]

Originally Posted by drurb

This is a tough one to answer. Filter caps often go somewhat gradually, introducing hum and noise as they do. As to total and sudden failures... it depends.

While filter capacitors usually go gradually, with an unusual hum often being the first clue that there's a problem, filter capacitors can also fail catastrophically. Fortunately this isn't very often, but when it does happen it's not a good thing.

[drurb]

Originally Posted by robgrow

While filter capacitors usually go gradually, with an unusual hum often being the first clue that there's a problem, filter capacitors can also fail catastrophically. Fortunately this isn't very often, but when it does happen it's not a good thing.

How true. The worst thing one can do is take an amp (especially a tube amp) that has not been used for a long time and just plug it in and turn it on. That'll blow them up.

[PB+J]

Originally Posted by drurb

How true. The worst thing one can do is take an amp (especially a tube amp) that has not been used for a long time and just plug it in and turn it on. That'll blow them up.

I don't think this is true--or maybe, I'd like to know what counts as a long time, Six months? Or six years? My understanding is tht the old paper in oil caps could simply degragde with time, but that more modern stuff doesn't.

And what's the alternative? You could attach the amp to a variac and slowly bring the voltage but, but if it's going to fail it's going to fail when it hits the voltage that produces failure

[Johnny Atomic]

Originally Posted by robgrow

Ric,

That is usually the case. Transformers seldom fail on their own. It is the related components in the power supply that fail and take the transformer with them. This is far more common however in vacuum tube amps which have relatively high operating voltages. I first started repairing musical instrument amps in the late 60's, and, in that time, I have seen a lot more power transformers fail in vacuum tube amps than in solid-state amps. Still it does happen in transistor amps. I have an early-80's Polytone amp here now that needs a new power transformer.

Cooked transformers are usually the result of some other component failure and over working the transfomer. Examples, shorted filter cap, shorted tube, shorted transistor, failed bias cap (tubes draw too much current). Rare examples I've seen transformer so rusted they shorted themselves.

It's usually one of these things, plus complicating the matter is some numb nutz will have previously installed the wrong size fuse thing. Fuses are there for a reason.

NEVER OVER SIZE A FUSE!

[drurb]

Originally Posted by PB+J

I don't think this is true--or maybe, I'd like to know what counts as a long time, Six months? Or six years? My understanding is tht the old paper in oil caps could simply degragde with time, but that more modern stuff doesn't.

And what's the alternative? You could attach the amp to a variac and slowly bring the voltage but, but if it's going to fail it's going to fail when it hits the voltage that produces failure

Well, I assure you it is true. I was thinking more along the lines of six years as opposed to the six months. Yes, you can attach a variac and that is usually the safe way to approach the situation. It is not true, however, that when it hits operating voltage it will fail if it would have. When you take an old electrolytic and slowly bring up the voltage, it can actually reform the dielectric. This is known as "reforming the caps." Sure, you should probably just replace them anyway but it is not true that a cap that would have failed if the full operating voltage were suddenly applied will fail if it is slowly brought up to that voltage.

In addition, if such a cap does fail, bringing it up slowly can limit the failure to a less catastrophic event.

Here are some helpful links:

One, two, three

[fdeck]

Chime, chime.

I can't really add anything to the above discussion. Caps degrade over time, but it seems likely that caps made today have a longer lifespan ahead of them than ones made 40 years ago. Likewise for semiconductors. My little GK combo still reaches its rated power after almost 10 years.

Really old tube amps should probably be re-capped, and also checked for some of the electrical safety issues related to those historical designs. I would hesitate to re-cap a modern amp just for prophylactic reasons because a circuit board is always damaged just a bit when parts are replaced. So nothing is risk free.

I have read about blown output transistors. If I had to guess, such a case occurs when a particular transistor in an amp is marginal from the git-go, and may be carrying more than its fair share of the output current.

Virtually all of the repairs that I have done relate to mechanical parts such as jack, switch, and pot contacts. Sometimes there are connectors associated with internal wiring harnesses that just have to be plugged and unplugged a few times.

Just a few ideas. Right now the greatest danger to my amp seems to be the EA Micro300 head.

[RWP]

I have an old Crown DC300 I bought in the 70s' with original semiconductors and PS components. That unit has been run hard over the years. I recently ran some basic power output measurements on it using a straight 4 ohm resistive load and found it still met specs. I did have to replace some small caps in the driver board that sat under a small transistor heat sink and were dried out from heat. Otherwise nothing has failed. That amp is a tank however, and may not be a valid comparison.

[ToneRanger]

Originally Posted by RWP

I have an old Crown DC300 I bought in the 70s' with original semiconductors and PS components.

I've had several DC300's - at one time I had 3 in my PA rig (back when they were amongst the most powerrful amps you could buy). I too had excellent long-term service, but as they got old they lost low frequencies. A tech did some checks and told me the power supply caps needed replacing, which brought them back like new.
(I remember reading a survey in the 80's that said more US recording studios used DC300s to power their monitors than all other brands combined. I wish I still had one for my bass rig or home hifi)