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MODIFYING THE AMPEG SVT3 PRO This is a manual to make some improvements to the SVT3 PRO. I’ll explain how I installed a better cooling, a real

mute, a soft start and changed the LED's. As you can see in its original state the cooling is not adequate to

handle 4ohm loads (that is: it runs unnecessarily very hot which shortens the MOSFETs life).

Don’t mind the white cable between the preamp & amp board, I’ll get to that later. As you can see I added a soft

start between the fan and the transformer. This will reduce the inrush current when you put the amp on, the 4

capacitors will be grateful to you !! It is NEVER a

good thing to power up a heavy transformer

without a soft start, because the capacitors have to

be charged from zero which results in a very high

load current. A soft start adds resistors in series

with the primary of the transformer which are

shorted by a relay after 1 or 2 seconds.

This way the capacitors can be charged with a reasonable

current without early aging. Any soft start circuit will do, I

found this one in a Electronics magazine. It can handle transformers till 2000W.

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Before you start you need to know that you have to disassemble EVERYTHING !! The next picture is to give you

an idea… Don’t forget to write down which tube

goes where, they’re different types !! Remember

where you removed the long screws.

VERY IMPORTANT: Write down which color/cable

goes where in the power supply circuit, this is a

international transformer for 100V / 110V / 230V /

240V.

Now let’s move on to the hardest part:

THE COOLING

A cooling element must transport the heat from the

component to the ambient with a as high as

possible efficiency. So the greater the surface of

the cooling element, the better it dissipates the heat.

This can be done by adding fins … as you can see

there are none !! What you need to do is put the 5W

resistors on the other side of the board, drill some

holes in the original “heatsink”, and install a real

heatsink … the bigger the better. But, there’s a but

… there’s a small disadvantage to this. If you

should have to remove the MOSFET’s for servicing

the power amp, you’ll need to desolder ALL the

MOSFET’s, than first remove the heatsink before

you can release the screws of the MOSFET’s. This will add 20 minutes of work, but hey, I’ve seen worse

scenario’s than this. On the other hand, with the added heatsink they wouldn’t have to work “on the edge”

anymore, overheating is a common failure in power amps. The heatsink dimensions should be no more than

60mm x 60mm x 120mm with LONGITUDINAL fins. This is to insure a good airflow through the fins and over the

MOSFET’s.

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Follow these steps:

Remove the power transformer (1 screw & all cables to amp board)

Remove the small board hanging above the amp board

Disconnect the 2 large connectors on the amp board

Remove the screws of the amp board and the 2 red marked screws in the heatsink, then take the amp

board out of the housing

Remove all the 5W resistors, you’ll need a good iron to do this, there’s a lot of copper that takes away the

heat !!

Remove the screws in red and with a small screwdriver gently release the MOSFET’s from the isolation

WITHOUT damaging it, I used a flat screwdriver as a wrench between 1 pin and the heatsink. They stick

pretty hard !!

Solder the 5W resistors on the other side of the board. Here you have to make sure the pins don’t stick

out more than 1mm on the other side, otherwise they will touch the heatsink and short circuit the output.

This might be a good time to check the other solderings, since you have the board taken out. I had

several bad ones, so I resoldered almost everything, just to be sure. Just wiggle a component on the

other side of the board and check if the pins move along with it … Bad solders are also a common failure

in power amps (especially those “made in China”- ones), and it happens always … on stage !!

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Next steps are for the mechanical part of the heatsink, this is what you’ll need to do this:

o 2 clamps

o Threading tool M4

o Lubricant

o drill M4

o drill M3.2

o countersink M4

o 2 pieces of wood

Because I don’t have industrial computer & laser

guided machinery, everything will be done by hand.

Follow these steps:

Put the heatsink in the middle of the frame, place 1 piece of wood on top of the fins and tighten it with a

clamp. You can see the 4 holes, they are drilled

just between the fins. In the next picture you’ll see

the backside. Don’t forget to mark the hole (M4)

for the thermistor as it sticks out when you

reassemble the amp board !! The other holes are

drilled with M3.2.

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Take the other clamp, tighten it to the other side and remove the first one. This to be sure that all the

holes will align perfectly. Drill the other 4 holes with M3.2.

After drilling the 8 holes, remove the clamps & heatsink and use the M4 drill with countersink on the

original heatsink, the screws must be countersunk when you reinstall the heatsink on the amp board.

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Now you must drill the threads in the heatsink. Each hole has to be done with 3 drills. Be sure to use

lubricant on the drill, otherwise the screws wouldn’t fit tight. (yes you have to lubricate 24 times !)

This is how it should look afterwards:

Reinstall the original heatsink WITHOUT the new heatsink, tighten the 8 MOSFET’s (not the 2 drivers).

Desolder the 8 MOSFET’s, again a good iron is

needed due to large copper surfaces, and pull out

the heatsink with the MOSFET’s bolted on it.

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Apply enough thermal paste on the original heatsink, reinstall the new heatsink and reinstall gently the

whole unit on the amp board. Resolder the MOSFET’s (don’t forget to tighten the 2 drivers).

Reinstall the amp board and you’re done for this part. Don’t forget to put some loctite on the screws,

because they tend to loosen due to vibrations after a while … mine were ALL loose after the first service !

NEXT STEP … The mute modification:

This amplifier uses a FET mute, it pulls the input signal to ground … but not completely. When you open the

master at ¾, you can still hear the sound, and when you’re playing through a PA, the audience will hear it too !!

I own 2 SVT3 PRO’s and they BOTH had this, must be a design flaw. So I decided to modify the mute circuit:

1) Mute the input with the FET

2) Release the speaker relay

Here’s how I did:

Take out all the buttons, bolts & screws of the preamp board. (the transformer must be removed)

Take out the board and remove the wire jumper JW27 (next to the double switch) and install a 10K

resistor in series with a 1N4148 diode (cathode towards the board). Be sure to install them on the

opposite side of the boards end (where the jumper was).

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Flip the board over and remove the traces

Add 2 wires as shown in the picture. This will NOT affect your sound, it’s just DC voltage supply to

“give” the relay the release signal when you push the mute button. Again, check for bad solders,

onboard jacks very often get loose connections !!

Replace the 2 green LEDs with blue ones (looks much cooler)

Reinstall the preamp board, with all the buttons & bolts, use loctite on the board screws.

Solder a wire from the anode of the added diode to R45 of the amp board. DO NOT solder the wire on the

other side of the resistor or it will not work !!

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Reinstall the transformer, hopefully you wrote down the correct combinations of wires/colors.

YOU’RE DONE !!

Here’s a hint:

When testing a serviced amplifier, ALLWAYS test it with a light bulb (100W) in series at the mains. Errors are

human, and it would be very painful to see your amp go up in smoke after all this work, when you push the ON

button. If the light bulb shines bright … there’s a short circuit and your amp survived !! Otherwise it flashes

shortly, until the capacitors in the power supply are charged, then you’re OK. Here’s the schematic of a box I

build:

BY THE WAY …

The amp is rated 450Wrms @ 4ohm.

I tested mine (both) on the bench, they rated 554Wrms @ 100Hz & 1KHz @ 4ohm for hours and they did not

overheat !!