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Marshall amplifier cooling fans

Mode4 amps have a hybrid design with a valve pre-amp. But that amplification needs some cooling, and so, unlike most valve amps, the Mode4 has two small fans to boost airflow over the heatsinks in the solid-state amp. My Mode4 amp was basically working fine, but had one annoying problem. When the amp was first switched on, the cooling fans would buzz like angry bumble-bees. After a few minutes of noise, the fans would continue spinning at the same rate, but the buzz would disappear, leaving only a quiet whoosh of moving air. It was a very easy job, with satisfying results.


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WATCH RELATED VIDEO: How to: Mount the Cooling fans on to the AMP

Adding a Tube Cooling fan


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If you see it again it's because something has changed so please set your preferences accordingly. Question about power amp fan. I have a cheap Phonic Max power amp with a fan that is not working. I can't find much info or a schematic for this unit. It looks very very similar to the QSC RMX amps inside just sorta flipped backwards as far as power supply and power transformer location.

The fan doesn't spin at all. I measured the voltage to the fan at 54v DC. I'm not seeing any fans online at anywhere near that voltage, most are 12v some 24v. I'm wondering if someone who has some experience with power amps knows if most fans found in amps are 12v?

Maybe the high voltage explains why the fan failed? Would it be a terrible idea to just use a resistor inline to drop the voltage to 12v? Amp fans come in different voltages and types. It is also important to move air in the same direction as original design or the amp won't cool properly.

Perhaps trace the fan drive circuit back to see if you can identify a fault in that circuit. Lotus 7. Did you measure the voltage with the fan in the circuit? And, did you measure the DC resistance of the fan? If the power supply reads 54 volts into an open circuit fan disconnected , that tells you nothing about what the voltage may be with the fan load motor DC resistance connected. Also, do you remember if before failing the fan always ran at a constant speed or did it vary with the amplifier load?

Many newer audio power amps use active fan drive circuits that vary the fan current as the amp heat-sinks warm up. If your amp has one of those circuits, you probably should replace the fan with one that exactly meets original specs. In addition, if the amp does use an active drive current regulator for the fan, a voltage measurement with the fan out of the circuit is meaningless. It the fan actually is a 48 volt device, it probably draws much less current than a typical 12 volt fan. Using a series resistor is not advised.

Usually cooling fans have a manufacturer's number somewhere on the case. Finally, your voltage measurement may not be accurate. A switched DC fan may be driven by a simple half-wave rectifier with no filtering.

If you are using a simple not a "true-RMS digital volt meter, on a half-wave unfiltered circuit you will get a reading that is usually based on the peak voltage. On a half-wave circuit, that is far above the effective "average" DC voltage. Here you go So yes, might be a 48volt fan. Try to get the original from Phonic. Attached Thumbnails. Thanks guys. I ordered a cheap 48v off fleabay. I'll see how it works out. Its a sleeve bearing model but hopefully it will last a little while. Haven't had any problems with the amp running without fan never worked during my ownership but I don't run it that hard.

It's probably a 24 volt fan. A 48 volt fan will turn way too slow since it's never going to receive anything like 48 volts. As I suspected it's a temp controlled fan. The circuit shown is designed to put much less than the full power supply voltage on the fan. The NTC thermistor and the low ohm B-E resistor on the emitter follower form a current source into the fan resistance. Even at high temp when the thermistor drops to 1K the voltage on the emitter of the transistor will be less than 24 volts.

In fact, the output bypass C across the fan is only rated for 25 volts. That's a clear indication that the fan is not a 48 volt device. That circuit should never be operated with the fan unplugged.

The voltage will go up above 50 volts on a high-impedance meter because there is no path for the transistor's emitter current to return to ground with the fan disconnected. Unplugging the fan, or having the fan motor windings open up will result in an immediate over-voltage of C That's clearly already happened as per your voltage measurement.

If this were my amp I'd install a 24 volt fan and certainly replace C with a 47 uf 75 volt part. Using a 25 volt electrolytic cap in that circuit is just plain BAD circuit design since the cap will be subjected to over-voltage if the fan is unplugged.

Again, the lesson here is that with this type of circuit, voltages must be measured with a appropriate load. Are you sure the fan is actually defective?

It it's mechanically locked because of a seized bearing, then it's obviously a bad fan. However it it spins freely and is just not starting, the driver circuit may be to blame. That's why I asked if you measured the fan DC resistance. An unplugged thermistor, a bad connector on the thermistor, or a bad emitter follower transistor could result in not having enough current in the fan line to make it run.

Again, the open circuit voltage measurement is meaningless. Even at high temp whne hte thermistor drops to 1K the voltage on the emitter of the transistor will be less than 24 volts. In fact the output bypass C is only rated for 25 volts.

The voltage will go up above 50 volts because there is no path for the transistor's emitter current to return to ground with the fan disconnected. Ah yeah my bad, I gave him bad advice. You're right they do use a temp controlled fan on this amp. Ruh roh! Not that the 48v won't work, it just won't work as well Duhh, missed the 25volt rating of the cap You could ofcourse replace the cap for a higher voltage one. And maybe raise the value of the ohm resistor if needed. I don't see any big problems.

A 48v fan will likely draw less current, so the voltage on the fan 25 degrees C is already higher. And a degrees C 10k NTC will be about 3kohm. Enough to give the fan close to 48 volt. The 2 previous posts are both correct afaik. Last edited by Rylan; 12th August at AM.. Reason: added wink? The original fan still spins freely just doesn't turn on.

I can try swapping out the cap for a v one and try a new transistor and see what happens. The fan itself may, or may not be OK. Even though you were able to read 54 volts at pin-1 of the fan connector, that may have been simple leakage current in the transistor assuming your 54 volt reading was with the fan disconnected.

What is the voltage reading with the fan connected? You need to confirm that the thermistor and it's connections are OK: To do that, unplug the NTC 10K thermistor and measure its resistance. At room temperature, it should be around 10, ohms when isolated from the circuit. Warming up the thermistor should lower the resistance.

See the attached chart of resistance values for a NTC 10K thermistor. To check the transistor in the circuit, you must add an emitter load that simulates the fan resistance. With the proper resistance in the circuit it is possible to check the current source with a voltmeter. The circuit is not a simple voltage divider. It's a current source with the ohm resistor shunting the base-emitter junction in the transistor. That means the current gain value of the transistor is important.

If you want to maintain original operation, you can't just replace the transistor with anything - it should have about the same forward current gain hfe as the original part. The MJE transistor is actually a "darlington-pair" and has a hfe of or more. Of course, a defective fan won't run at all.


Marshall Mode4 Amplifier Repair

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marshall mg hdfx fan I AM TRYING TO FIND A REPLACEMENT COOLING FAN FOR A MARSHALL MGHDFX AMP HEAD. CAN'T EVEN - Marshall MGHDFX Watt Amplifier.

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marshall amplifier cooling fans

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Fan Cooling System for a Guitar Tube Amplifier


The Idea behind this instructable is that tube amplifiers get very HOT! Although heat can kill an amplifier. So this system can keep your amp cool and save you money!!! I have ran many tests with this system on many different amplifiers, on average a power transformer gets to deg. Tools tape measure, pencil drill, small drill bit, 2" hole saw table saw or circular saw? Measure and drill for the back panel mounting holes, if you have the original back panel use it as a template.

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Discussion in ' Marshall Amps ' started by treetrunk , Aug 1, Log in or Sign up. Marshall Amp Forum. Joined: Jan 9, Messages: Likes Received: Sorry if this is a dumb question - hope it is not though Can tube amps get overheated and get damaged due to the tube temperatures escalating over a period of time - say a couple of hours of playing?

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Finding the best combo amp for you can be tough. The right electric guitar feels like an extension of your limbs, whereas finding a great combo amp can take a whole lot longer. A combo is an amp that contains both the amplifier and speaker in the same box - making it ideal for small spaces or where portability is a consideration. A lot of amps on the market are configured as a head and cabinet, meaning that the bit that actually amplifies your signal is separate from the speakers that your signal comes out of.

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An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical instruments such as an electric guitar , an electric bass , electric organ , synthesizers and drum machine to convert the signal from the pickup with guitars and other string instruments and some keyboards or other sound source e. Combination "combo" amplifiers include a preamplifier , a power amplifier, tone controls, and one or more speakers in a cabinet , a housing or box usually made of hardwood, plywood or particleboard or, less commonly, moulded plastic. Instrument amplifiers for some instruments are also available without an internal speaker; these amplifiers, called heads , must plug into one or more external speaker cabinets. Some amplifiers are designed for specific styles of music, such as the "traditional"-style "tweed" guitar amplifiers, such as the Fender Bassman used by blues and country music musicians, and the Marshall amplifiers used by hard rock and heavy metal bands.

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