Peavey CS 800 heating

I recently acquired one of these amps with the intention of using it for sub duty in a HT system. I live in a small condo and my sub is a 10" Peerless that never gets to play anywhere near it`s 200w rating so needless to say, the Peavey is waay overkill.
I found the fan, even at the lowest speed to be distracting. I disconnected it to see how the amp would react just idling, essentially what it would be doing even when watching a movie.

The amp is all original and has never been repaired and puts out a healthy 853.33 watts into a mono 8 ohm load at clipping, and it doesn't object to 3 ohm loading either as it does 912.6w mono into a 6 ohms.
With the fan unplugged and top cover removed, the left channel runs a little warmer than the right. Even after an hour, either heat sink can safely be touched but that doesn't sit quite right with my OCD :(.
I`m assuming there is a difference in biasing between the two modules. Should that be reason for concern?
 
You could increase the resistance on the fan drive board to make the fan run a little slower even when cold. If you leave the full speed hot detect circuit alone that should protect the output transistors against fan stalls.
Schematics are often on eserviceinfo.com, if you don't want to wait on the phone to call them for one.
If yours is too early to have a 2 speed fan, look at some of the pv**** schematics for various fan drive circuits. Some fans were 120 VAC some were 24 VDC. Instead of the hard to buy in onesies snap action thermostats, I use a 15 k ntc thermistor pull up, a 10 k pull down resistor, on the gate of a 2n7000 nfet, to provide useful signals to power devices to turn them on or off. You need 50 ma for a triac in AC so you may need a TIP41C boost transistor for that purpose. You can adjust the turn on point of the 2n7000 with a pot parallel pulldown or pullup. The thermistor head touches the OT heat sink in a little thermal grease. You can sometimes salvage NTC thermistors from PCAT power supplies, the dead ones.
 
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I should have mentioned that the amp in question is a "C" variant. I`ve just replaced the top cover to further restrict heat dissipation and will see how it reacts. Placing a higher value than the 250R series fan resistor may be necessary but I`d like to avoid that if at all possible.
 

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No actually, replacing the top cover HELPS the amp cool. With the cover off the heat sink fins can only radiate. The fan blows underneath them. With the top on, an air duct is formed, with the airflow path through the fan, up the center and out through the heat sink fins to the end vents on top. With the lid off, nothing draws air past the heat sinks.

There likely is a small bias differential between the two sides, and unless they overheat, it is absolutely nothing to worry about. It is perfectly normal for the heat sinks to warm up some while ti runs, yes even at idle. I'd be concerned it it stayed cold. Your car engine heats up even at idle. Allow your CS800 the same.

Also, it may well be that one side is a little warmer than the other due to the stuff under it. One side has a lot of space under the module, while the other side has two big cap cans and a transformer. Is the transformer side the one running warmer? Transformers generate heat.

There is no bias adjustment on these amps, the bias is set by a pair of dual diodes on the driver card. Well, one is on the card perched on a transistor, the other is on the heat sink, but has a two-wire cable back to the driver card. They might drift a bit in forward drop, but I have never seen it get to be an issue unless the part just outright fails.

Noisy fan? Instead of modifying the circuit, try lubricating the fan. Most muffin fans have a hub label on the motor end, remove than and the end of the axle is exposed. A drop of real light oil usually shuts up a noisy fan. And if not, well, a new fan. Really. These stock fans are not little airplane engines in my experience. Noisy ones tend to be just that, noisy fans, not insufficient design by the engineers. Oil them or replace them.
 
Putting the top back on (with fan still disconnected) generated more heat as was expected. While it didn't get terribly hot or trip the sensors, I deemed the level to be too high for comfort, especially when you take the amps vintage into consideration, I dont think it would be wise to thermally stress the amp for the sake of quieter operation . The amp cooled down completely within a few minutes of re-engaging the fan on low speed with no discernible temperature differences between the modules.

That said, I would be inclined to believe that some bias circuit drift has taken place as Enzo had suggested but it is well within the fans capability to deal with. I still need to address the fan noise which by the way is a consequence of turbulence as opposed to being mechanically induced, ie bearing noise, it`s surprising how much turbulence a fans finger guard creates. Removing it is not really a good idea so I think I`m going to play around with adding some more series resistance in an effort to slow down the RPMs.

I`m also toying with the idea of reversing the direction of the airflow as I have done with BGW 750 series amplifiers who share a similar chassis and component lay out. I`ll start with the fan speed and report back with my findings.
 
Hi Michael

I have not seen the inside of the CS800 in 30+ years. It was a copy of the BGW750 family. The top cover must stay on. It is part of the thermal management and HUMAN SAFETY. The ±80 volts and the AC LINE are open to touch when the cover is OFF. Many of our 750 were modified with small low speed fans (Special service amplifiers). The thermal switch on top of the TO3 has one side of the AC line that feeds the fan. The fan runs very slow as we had a series power resistor that was bypassed when the amp was hot.
Duke
 
Hi Michael

I have not seen the inside of the CS800 in 30+ years. It was a copy of the BGW750 family. The top cover must stay on. It is part of the thermal management and HUMAN SAFETY. The ±80 volts and the AC LINE are open to touch when the cover is OFF. Many of our 750 were modified with small low speed fans (Special service amplifiers). The thermal switch on top of the TO3 has one side of the AC line that feeds the fan. The fan runs very slow as we had a series power resistor that was bypassed when the amp was hot.
Duke

I wouldn't dream of running the amp "topless", it was removed strictly for testing purposes. While not quite as deadly as the BGW with its exposed rail voltages, fan switch and output switch, the Peavey will provide a nasty zap if you're not careful.

I used to run a PA rig that consisted of 16x 750 B and C series amplifiers that had the fans reversed. They were configured to draw cool air from the sides and exhaust through the rear. There was no perceivable increase in heat production and reliability wasn't affected either but it made cleaning the amplifiers so much easier. All that was needed was an occasional brushing of the top side heat sinks. The chassis, power supply, protection circuit, potentiometers and mother boards remained so much cleaner, and for longer periods.
I`m sure BGW and subsequently Peavey had valid reasons for configuring the amps the other way around though.
 
UPDATE

After some experimentation, an additional 500R/20W in series with the 250 ohm resistor already in there turned out to be just what was needed.
It dropped the RPMs and subsequently the noise level to barely a whisper and still managed to dissipate the heat while idling quite nicely.
I relocated both resistors to the metal plate behind the Channel A DDT circuit board. Not only are they mounted more securely but the plate serves a heat sink too.

I performed some final power tests into various loads (1khz into 6 ohms mono yielded 1003.62 watts :) ) and the amp barely broke a sweat.
 

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wg_ski

Member
2007-10-10 5:21 pm
PV calls it the “DDT circuit”. It just detects when the amp clips, and feeds back a proportional DC signal to a gain reduction circuit at the input of the amp. Over a useful range, it will keep the amp out of gross clipping when over driven. Most modern PA amps have this feature, and you can turn it on or off.