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Ampeg SVT 3 Pro Cooling Fan control

Discussion in 'Amps and Cabs [BG]' started by Heggy, Dec 13, 2012.

  1. Howdy,

    I just picked up a SVT 3 PRO and it had some issues. I have fixed 2 of them (broken EQ sliders and a bad tube) I calibrated the bias and plugged her in, sounds fantastic, except the fan runs full blast. I was looking at the board to see if there were any signs of damaged components and found a nut wedged in-between R61 and the MOSFET heat sink. It looks like there has been some overheating and I took the main board out to see if there is any other visible damage. I ended up breaking one of the leads ( as it was partially melted) on R1 and found another resister to replace it.

    However this was not the fan speed problem and I cannot visibly see any other component damage.
    I was looking for TS1 (LM-35 temp sensor) and not sure where it is located. And not quite sure how to test to see if it is working properly. Other than that there is IC1 and Q16 which look to be controlling voltage to the fan.

    Any insight or short cuts to fault isolation would be greatly appreciated.

  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    R61 is just a "bleeder" to drain teh capacitors to zero volts when power is off. The nut either shorted the high voltage, or just shorted ground to the heatsink.... one should have corrected itself with a big spark, the other probably would do nothing but maybe make extra hum.

    If the fan runs full speed, Q16 is not doing it's job of bypassing some current around the fan.

    When cold, Q16 should bypass most of the current, and as the amp warms up, it should let more go to the fan. IC1B controls that, but if anything were really bad with that, I'd expect the output relay to be open, since it opens if temp signal gets too high.

    I suspect bad connections, or similar issue, probably associated with Q16. Or maybe the fan was re-wired by someone.... that can happen when somebody decides the fan isn't doing what they think is right.

    TS1 is in a hole in the heatsink , between R36 and R38, near the back of the amp. But I don't think it is the problem if the relay closes.
  3. Jerrold,

    Your swift reply is greatly appreciated.

    I finally did find TS1 by tracing the PCB from R73. As you say it is hiding in the MOSFET heat sink!

    TS1 and the fan wiring look to be in factory original condition, however there seems to be signs of overheat (solder joints) on Q16 and another transistor (I suspect is Q17) which are physically attached to the heat sink. (BTW, i still don't understand the Q16 / Q17 circuit for the tube heaters. I was going to ask an Engineer at work today to see if he could figure it out :)

    It was the “hot” side of R61 that was shorted to the heat sink. It looks as though it was not a dead short (based on your prognosis) as the nut was oxidized aluminum or white metal.

    I will try and get the 2 transistors today and replace them tonight after work.

    Thank you again for your help.:)

    Best regards
  4. I finally got around to getting the parts (Christmas time...) and replaced Q16 and R61. Put the power transformer and board back in and plugged it in.

    The fan still is running at full and now the EQ IN/OUT SW has no effect (equalizer is always ON). In addition the amp seems to sputter every now and then (like when you twist the tube gain pot quickly from zero to full) and the clip light comes on momentarily when this happens. Normally it is after the note is played on the bass, so it is not due to normal input overload conditions.

    I tried to test the EQ switch, only to find out that i was testing a "dirty" ground which of course does not work until the circuit is powered.....

    The saga continues....:(

  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    As for the fan circuit..... which is also the tube filament power circuit.... and part of the +16V power supply.....

    Q17 is regulating the voltage across power amp V1 filament to 12V. By doing that, it also regulates the current in the power amp V2 filament to the proper constant amount. All 12AX7 are made, or should be, to have a given filament current with little variation about 150mA at 12V.

    TS1 provides a voltage that rises with temperature. At low temperatures, the output of IC1B is high, which gives Q16 more input voltage, and produces more current in it due to the emitter resistors R65, R66.

    The current coming to Q16 is regulated by the tubes and Q17, so there is a fixed amount. Any current in Q16 is not available for the fan, so it slows down.

    At higher temperatures, the output of IC1B (an inverting amplifier) comes down, reducing the drive to Q16, allowing more current to go to the fan, which speeds up.

    If the fan operates too fast, then either: the voltage at IC1B output is too high (is it good? is it's input OK?), or Q16 cannot bypass enough current (maybe there is too much flowing?).

    There should be only about 150mA available, if Q17 is working right (check for the 12V on V1 filament). IC1B can supply considerable current, so Q16 only needs a gain of 15 to 30 to operate correctly.

    Since the +16V is supplied from the same circuit, check if it is OK.... IC1B could draw so much current that it pulls down the 16V, especially if it isn't working right, or the circuitry around Q17 is not working right.....

    There's a long list of possibles, but some voltage checks will likely offer a good clue to the issue..


    A problem with switching gives a clue that the 16V supply may be not so good.... the JFET switches need a full swing on preamp IC3A to operate properly.
  6. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    So what ever happened here?

    Inquiring minds want to know..................
  7. Sorry for not responding sooner.

    After replacing the IC and the transistor, the amp went haywire, no sound lights on on front panel, nothing worked. And the fan was still running full tilt. So I put it on a bench b4 I smashed it with a sludge hammer.

    Months later I gave it to a co-worker to look at and he found the problems...

    It was the thermistor that was burnt causing the fan to run full speed. The main connector on the main board had a number of cold solder joints as well as 2 jacks on the back (power amp in and foot switch). After fixed, it worked, but drop out would occur in the audio, which is where I am now. The relay K1 is the problem, but have to disassemble to get a P/N as it is not on the schematic.

    Other then that the amp sounds awesome!