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SVT4-pro pics needed

Discussion in 'Amps and Cabs [BG]' started by Donzap, Mar 12, 2008.

  1. Donzap


    Mar 11, 2008
    Hi mates, I'm repairing a SVT4 and need pics of the bias adjust trimpots, I tried to measure bias across R265 and blew up a mosfet (I think my voltimeter has a different impedance than 10Mohm), so I thought on set pots at the same position than another svt4.

    Thx all.
  2. Donzap


    Mar 11, 2008
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    How will the picture help? The position of the pot for one may be different from the setting for another.

    The best advice is to set the trimpot for minimum bias, which is minimum bias voltage, and maximum resistance setting on the trim pot.

    Then turn it up slowly, ignoring any oscillation that may occur just as the bias starts to turn on the mosfets (can look scary, but is just in a certain range).

    Stop at the specified setting across the source resistors (average reading of all in a bank), and let it sit until warmed up, checking from time to time to see if it is stable.

    There is a second bias circuit, which does the temperature compensation. It is fixed, so you won't ever see zero bias voltage on the gates unless that circuit is bad.
  4. Donzap


    Mar 11, 2008
    There are two trimpots, Shall I make the same on both? With input conected?

    Thx a lot, I thought that a pic will be usefull at least to see the position on other amps.
  5. Donzap


    Mar 11, 2008
    Oh I forgot... Can I switch it on with no load?
    I'm pretty sure it's a silly question but the amps I build/repair are all guitar tube amps, so I feel uncomfortable with this solid state bass amp.

    Thank you.
  6. You can run a SS bass head with no load, but I'm not sure how this relates to bias setting. What are you biasing if there are no tubes? I'm genuinely curious.
  7. Jazzman


    Nov 26, 2002
    Raleigh, NC
    The mosfet's.
  8. Donzap


    Mar 11, 2008
    Output mosfets.

    Bias is mostly related to tubes by everyone, but really, bias is a reference voltage to work for many components, I have built a Clon centaur and it also have a bias reference voltage (Vcc/2)

    BTW, I have change 11 mosfets in this amp, and 2 resistors, then I switched it on for about 10 or 15 seconds and it works (the other time something blew up in a pair of seconds).
    Now I will try it with load and input, let's see what happen...

    I'll keep you posted.

  9. Neat, I figured that, but I wasn't sure if MOSFETs could be biased like tubes.
    If it keeps blow resistor and mosfets, maybe there's something wrong with the power supply? Just a shot in the dark.
  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The two different power amp channels each have a bias control. The channel with no problem you can ignore, don't do anything to it.

    Bias is ALWAYS set with no load and no signal on an SS amp, if you are setting with meter. If you set by looking at the crossover "notch distortion" on an oscilloscope, obviously you have a load and signal, but I don't recommend that approach for these amplifiers.

    Also, if you have replaced parts, did you make sure that all the mosfets in one parallel group are "matched"? They were matched at the factory with the same "threshold voltage". That helps ensure that they will share the load.

    You need to have them be matched. Trust me, I argued with the bean counters on that very issue, and they agreed to pay for matched parts, so you know it was important.

    The matched parts originally supplied will have a small dot of color on them, and all should be the same. Each color is a "range" of voltage of 0.3V. I wanted a narrower range, but the manufacturer already matched them to the 0.3V so we accepted that.

    If you do NOT match, when you bias the unit you will see that some will be at correct bias, some will be much higher, and some will have almost no current. That is "not good", and the amplifier may NOT be reliable at 2 ohms per channel, or 4 ohms bridge.


    For a tube, negative bias is applied to "hold back" the current to a correct level to avoid distortion. Tubes are a "depletion mode" device that will conduct a big current at zero bias.

    A mosfet, (or a bipolar transistor), is an "enhancement mode" device, and requires to have a voltage applied to MAKE it conduct. So to get the proper small current to avoid distortion, you must slightly turn it "on". But not too much. Also the bias must be reduced with hotter temperatures, because both Mosfet and bipolar have positive temperature coefficients for the parameters that matter.

    The Mosfets in that unit require a voltage somewhere between 2 and 4 volts each to make them start conducting, or 4 to 8 volts for the two sets.
  11. Donzap


    Mar 11, 2008
    Bad news for my friend, We have tested it on a ampega cabinet and after 1 minute or so we have seen smoke on the amp. I have tested 0,47Ohm resistors and a few are opened.

    Thx for all your support mate, Do you know where can I find these matched mosfets? I'm sure it will be impossible in my city, I bought the changed mosfets via internet in Barcelona, but can not choose matched ones.

    My friend has a Hartke Amp for practise and live, so I have time to test and try to repair (It would be so expensive in Ampeg service, and the amp is good enough to try it).

    Any idea of what can I test or change?

    Thx again.
  12. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Well, they SHOULD be available through Ampeg service, but I have NO idea how that works under LOUD, especially in the EC.

    You CAN match them yourself, but it might take buying some extra ones. Of course you kinda already did that....... so maybe it isn't that bad.

    If you have a source for the parts, even un-matched, that's great. But there are a couple manufacturers of them and only ONE is good. The OTHER manufacturer "improved" the parts and they are now more-or-less unusable.

    PM me and let's see if we can get it sorted out.

    First step is going to be figuring out what made it fail to begin with, because that affects where to look for problems. If you found and replaced the bad parts PERFECTLY, but didn't locate the original problem, then that original cause might have made it fail again, despite good work on your part doing the replacement.

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