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Author Topic: 1977 Peavey CS-800 Amplifier not outputting fully?  (Read 11451 times)

Mitch Philips

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1977 Peavey CS-800 Amplifier not outputting fully?
« on: June 13, 2015, 10:32:04 pm »

I have an old Peavey CS-800 amplifier that is currently installed in an old nightclub in Los Angeles. It was installed when the club opened in 1978 and it's still used to this day. However, one complaint that was brought up to me was that it was not getting loud enough anymore and the "Overload" lights were going on much sooner than previously, even with the same set of speakers, which were installed in 1995.

I measured the output of each channel on the amplifier and it measured 200w which is exactly half of it's original output per channel.

What would cause an amplifier to start doing this (besides it's age of course)? The amplifier has been properly maintained, never overdriven for long periods of time, dusted regularly, kept in a well-ventilated rack, etc. Because it still works (sort of) fine, could whatever would be causing the issue be replaced or fixed?

Ultimately, if the amplifier could not be repaired, the club would just invest in a new one. But, I wouldn't mind trying to get it fixed.

Hopefully you can help!
Thanks!
-Mitch

 
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John Roberts {JR}

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #1 on: June 13, 2015, 10:41:36 pm »

I have an old Peavey CS-800 amplifier that is currently installed in an old nightclub in Los Angeles. It was installed when the club opened in 1978 and it's still used to this day. However, one complaint that was brought up to me was that it was not getting loud enough anymore and the "Overload" lights were going on much sooner than previously, even with the same set of speakers, which were installed in 1995.

I measured the output of each channel on the amplifier and it measured 200w which is exactly half of it's original output per channel.
How did you measure it? The CS800 was 400W at 4 ohm and something like 225W at 8 ohm.
Quote
What would cause an amplifier to start doing this (besides it's age of course)? The amplifier has been properly maintained, never overdriven for long periods of time, dusted regularly, kept in a well-ventilated rack, etc. Because it still works (sort of) fine, could whatever would be causing the issue be replaced or fixed?

Ultimately, if the amplifier could not be repaired, the club would just invest in a new one. But, I wouldn't mind trying to get it fixed.

Hopefully you can help!
Thanks!
-Mitch

If the CS800 is indeed going into DDT/limiting 3dB early that could be caused by external or internal faults. Do both channels act exactly the same? 

JR
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Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #2 on: June 13, 2015, 11:06:45 pm »

How did you measure it? The CS800 was 400W at 4 ohm and something like 225W at 8 ohm.

I used an old analog multimeter to measure the output of the channels. It read 35v and 5.8 amps and that works out to be about 6 ohms.

Both channels act exactly the same way.
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Brian Jojade

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #3 on: June 14, 2015, 02:09:08 am »

The CS800 was a rock solid amp. The fact that it's still in use nearly 40 years later is impressive. 

If both channels suddenly exhibited the lower power output at the same time, I'd suspect something going wrong with the supply voltage. Now, due to the age of the amp, I would seriously question the value of putting any effort into a repair.  A new replacement amp could be had for a couple hundred dollars at most.  By the time you take into account your time and effort to fix the original, it's not going to be worth your time.
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Brian Jojade

Ivan Beaver

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #4 on: June 14, 2015, 07:34:04 am »

I used an old analog multimeter to measure the output of the channels. It read 35v and 5.8 amps and that works out to be about 6 ohms.

Both channels act exactly the same way.
I will ask the same question JR asked.

HOW did you measure it?

Or to "pick apart your measurements.
1: WHAT is the 6 ohms?  The loudsepaker? The amp output?  If the loudspeaker, then that is the DC resistance-not the impedance.  An "old analog" meter cannot measure impedance.

2: How did you measure the current?  An "old analog" meter does not have the capability of measuring AC current.

3: 35V? As measured with what input signal-noise-sine wave-noise?

It makes a HUGE difference.,

Until we know the specifics of the measurement conditions, there is no way to guess what the issue might be.

I have used and worked on many of those amps and have never had a case that the output was low.
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Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #5 on: June 14, 2015, 08:17:36 pm »

I will ask the same question JR asked.

HOW did you measure it?

Or to "pick apart your measurements.
1: WHAT is the 6 ohms?  The loudsepaker? The amp output?  If the loudspeaker, then that is the DC resistance-not the impedance.  An "old analog" meter cannot measure impedance.

2: How did you measure the current?  An "old analog" meter does not have the capability of measuring AC current.

3: 35V? As measured with what input signal-noise-sine wave-noise?

It makes a HUGE difference.,

Until we know the specifics of the measurement conditions, there is no way to guess what the issue might be.

I have used and worked on many of those amps and have never had a case that the output was low.

Good questions! I'm sorry I didn't elaborate more in my post.

1. The amp output resistance is 6 ohms. The speaker the amplifier is usually connected to is also 6 ohms.

2. I did not use the multimeter to measure the amperage, I used an old analog ammeter I had laying around to measure the AC current output.

3. The 35v was first observed whilst playing some pink noise. I wanted to find what frequency was giving me the most power from the amplifier and after some testing, found out it was around 57-60hz.

Hopefully this helps you.
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Mac Kerr

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #6 on: June 14, 2015, 09:06:19 pm »

Good questions! I'm sorry I didn't elaborate more in my post.

1. The amp output resistance is 6 ohms. The speaker the amplifier is usually connected to is also 6 ohms.

2. I did not use the multimeter to measure the amperage, I used an old analog ammeter I had laying around to measure the AC current output.

3. The 35v was first observed whilst playing some pink noise. I wanted to find what frequency was giving me the most power from the amplifier and after some testing, found out it was around 57-60hz.

Hopefully this helps you.

Here is a question that was implicit in Ivan's question. Was there a speaker connected to the amp while you measured it, and if so, what was it? It seems unlikely that the output impedance of the amp is 6Ω and without a speaker you would only be measuring the amp's ability to create voltage across a very high impedance, not the real situation.

Since the amp was apparently originally rated at 225W into 8Ω it isn't clear yet that the output power is down.

Mac
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Stu McDoniel

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #7 on: June 14, 2015, 09:24:39 pm »

I have an old Peavey CS-800 amplifier that is currently installed in an old nightclub in Los Angeles. It was installed when the club opened in 1978 and it's still used to this day. However, one complaint that was brought up to me was that it was not getting loud enough anymore and the "Overload" lights were going on much sooner than previously, even with the same set of speakers, which were installed in 1995.

I measured the output of each channel on the amplifier and it measured 200w which is exactly half of it's original output per channel.

What would cause an amplifier to start doing this (besides it's age of course)? The amplifier has been properly maintained, never overdriven for long periods of time, dusted regularly, kept in a well-ventilated rack, etc. Because it still works (sort of) fine, could whatever would be causing the issue be replaced or fixed?

Ultimately, if the amplifier could not be repaired, the club would just invest in a new one. But, I wouldn't mind trying to get it fixed.

Hopefully you can help!
Thanks!
-Mitch
Power supply caps? They do have a shelf life.
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Ivan Beaver

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #8 on: June 14, 2015, 09:32:34 pm »

Good questions! I'm sorry I didn't elaborate more in my post.

1. The amp output resistance is 6 ohms. The speaker the amplifier is usually connected to is also 6 ohms.

2. I did not use the multimeter to measure the amperage, I used an old analog ammeter I had laying around to measure the AC current output.

3. The 35v was first observed whilst playing some pink noise. I wanted to find what frequency was giving me the most power from the amplifier and after some testing, found out it was around 57-60hz.

Hopefully this helps you.
If your measurements are indeed correct-the question should be "Why is my CS800 producing more power than it is rated for?

If you are getting 35V-with pink noise-lets assume the lowest crest factor pink noise (6dB).

Then that  would mean peaks of 70V.

Into a 6 ohm load that is over 800 watts-far GREATER than the rated power.

Something is either wrong with the measurement gear-the test procedure/setup or just the idea that the power is low.

I suspect that people are assuming that it is low.

Even it it is HALF power-that is only 3dB and I HIGHLY doubt anybody listening would be able to pick out that an amp is now 3dB lower than it used to be.

It would have to be a LOT LOWER for people to easily notice.

I honestly feel you are chasing something that does not exist.

I also doubt that an "old school" clamp on amp meter can easily measure the current created by the wide freq response of noise.

Also-to be honest-I have never thought of hooking up an ohm meter to the output of an amplier-but I doubt the RESISTANCE is 6 ohms.

Typically the output IMPEDANCE (you can't measure impedance with a typical "old school" ohm meter) is very low parts of an ohm-nowhere near 6, but I could be wrong.

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Mitch Philips

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #9 on: June 14, 2015, 09:58:29 pm »

If your measurements are indeed correct-the question should be "Why is my CS800 producing more power than it is rated for?

I assumed the power output was lower because the speakers are not nearly as loud as they once were even a few months ago. I can understand speakers not performing as well as they once did when they were newer, but for BOTH of the speakers to all of a sudden stop being loud and for the amplifier to be driven into overload WAY before the volume even gets half way up leads me to believe the amplifier is the culprit.

The speakers in question are a pair EAW KF300s. I know the speakers are being slightly underpowered but they operating well within their boundaries. The have never been clipped for long durations of time (few blips here and there).

Save for a few special occasions where we really cranked the volume, such as Super Bowl parties, New Years, etc, the amplifier and speakers were not pushed at all. Starting a few months ago, even playing normal, relatively-quiet, background music would drive the amplifier into overload.

As for the ammeter, it was not a clamp-on ammeter. It was a needle gauge type.

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Re: 1977 Peavey CS-800 Amplifier not outputting fully?
« Reply #9 on: June 14, 2015, 09:58:29 pm »


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