SVT3Pro Bias Setting

Discussion in 'Amps and Cabs [BG]' started by arnocl, May 25, 2008.


  1. B-string

    B-string Supporting Member

    It is possible if they are all in "cut-off". Bias voltage keeps them "turned on" just enough to conduct a little to avoid low level signals from being lost before sufficient voltage is present to make them conduct.

    Personally I would start with at least known good tubes. Be sure no speaker connected, no signal and measure across resistors. Do NOT ground one meter lead.
     
  2. chria

    chria

    Jan 6, 2014
    Greece
    Thank you so much for your answer

    First of all sorry for my English.

    If I understood good you suggest me to change the tubes and then try to setup the bias, right?

    What you mean "do not ground one meter lead"?

    Thank you very much again
     
  3. B-string

    B-string Supporting Member

    "Known good tubes" means tubes that are known to work properly in use, new tubes are assumed good, not proven to be good yet.

    The voltage reading on those resistors is done across the resistor leads, do not clip one lead from your meter to the amp chassis. You are using a good digital meter? A cheap voltmeter or an old needle type meter should not be used. You will be measuring millivolts (0.001 volts = 001 Mv). Your english is pretty good so far.
     
  4. chria

    chria

    Jan 6, 2014
    Greece
    This is what i am trying to measure. Am I right?

    Mamy many thanks for your support Screenshot_2016-06-26-21-24-45.jpg
     
  5. B-string

    B-string Supporting Member

    Yes, correct!
     
  6. chria

    chria

    Jan 6, 2014
    Greece
    Ok dear B-String. I am really appreciate for your help!!!
     
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  7. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    I have a 1992 SVT-III (not pro, only three 12AX7 tubes in this) that I purchased new in February of '92. I'm getting ready to sell this amp (along with the SVR-215 cab that I bought new the same day), mostly due to weight issues (I replaced it with a GK MB212, and my spine thanks me), and in testing it out I'm getting an intermittent crackling/buzzing/sputtery sound, usually at low volume and usually on sustained notes on the D and G strings around the middle of the neck, though again, not always. Naturally I want to sort this out before selling the amp. A couple of questions:

    1) realizing this thread is about the -Pro version, does it still pertain to the non-Pro SVT-III?

    2) Does this sound like it might be a bias issue?

    3) Or might it be a tube issue? Way back when I bought the amp, I took out the crappy Chinese tubes that came stock and put in some Sylvanias. I'm going to go look through my tube stash to see what I might have (probably still have the original tubes somewhere).

    Thanks!
     
  8. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Intermittent crackling sounds more like a tube or cap issue to me than bias. I don't know if the 3 nonpro has a bias, but in the 3 Pro you get distortion on every note at any volume.
     
  9. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    There is a single trim pot that I can find (purple arrow).

    I'll first try swapping in other tubes and see what happens, since that costs nothing.

    image.jpeg
     
  10. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    I don't know enough to tell you that's a bias control or not. Could very well be. I still doubt it's the problem.

    One other thing that could be happening is dirty effect loop jacks. Stick an instrument cable in between the send and return (or preamp in/power amp in, depending on how they're labeled). If it fixes it, clean them with Deoxit. If not, back to the drawing board.
     
  11. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    Oh, I've had the effects loop jack problem. It was at the point where I just left a 1-foot patch cable plugged in all the time. I think I've finally effectively cleaned the jacks, though, and there's no audible difference right now between having them patched with a cable or not. Good thought though.
     
  12. B-string

    B-string Supporting Member

    Your symptoms are NOT bias level. Improper bias is a constant and any note played is affected. DO NOT TOUCH that pot.

    By the way @farace you have bipolar output transistors not the MOSFETs used in the 3-Pro being discussed. Different design.
     
    Last edited: Jul 9, 2016
  13. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    Noted!

    . . . and thank you. I saw they didn't look like chria's chassis in the post above mine, so I wasn't sure.

    I dug a half-dozen 12AX7s out of my tube stash, a couple being the original Chinese tubes from this amp, some JAN tubes, and a couple others, to see if there is any difference. Can you tell I'm trying to avoid having to take the power amp board out and replacing the caps? :/
     
  14. B-string

    B-string Supporting Member

    Power supply caps don't fit the symptoms either so best to save that effort and expense. Coupling caps "possibly" but they would not usually be affected by a frequency (note). Check all hardware especially ground connections and jack nuts for snugness (even pot nuts) since you said you eliminated the effects loop already. Work all the pots back and forth rapidly from stop to stop several times. Beyond these it will be likely a hunt for cold/cracked solder joints at the outside a failing resistor (especially plate load resistors), but those were much more common with old carbon comp resistors.
     
  15. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    I get the feeling this might be where I'm headed. I had already worked the pots and cleaned the jacks. But check this out: after I swapped out the tubes and found the amp still noisy, I started tapping components with the plastic handle of a screwdriver. Long story short, I got a lot of noise from two of the power transistors (the upper left two in the photo above). After I bounced the screwdriver handle off them a few more times, they got quiet, and there was no more noise in the amp. I played through it for about another half-hour without any sputtering, crackling, or undue noise. Now I'm letting it cool off again and I'll try again later to see if the noise returns. I started wondering about a cold or cracked solder joint; I guess it's not uncommon on these amps? (Or, another question: Are the screws holding the power transistors purely mechanical, or do they serve to ground the transistor case to the heat sink/chassis as well?)

    Thanks again. Apologies for getting so far afield from biasing an SVT-3 Pro, but I think we're near resolution here.
     
  16. B-string

    B-string Supporting Member

    Those screws on most designs are the connection for the collector of the transistor (+ or - power supply rail voltage). They are not grounded to the heatsink, there is a mica insulator between the transistor case and the heatsink with a thermal transfer compound on both sides. I don't remember ever seeing a heatsink at rail voltage (directly connected transistor "cases").

    EDIT to add: You could give those screws a gentle tweak with a screwdriver, don't get carried away with tightening them or you could cause fractured solder joints under the pcb.
     
    Last edited: Jul 9, 2016
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  17. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    OK. A few things....

    The 3PRO and SVT-III are totally different amps. The 3PRO uses mosfets, the III uses regular bipolar transistors. BOTH can have bias problems. But with the 3PRO it is MUCH worse, because there is no distortion-cancelling negative feedback in the amp, so any distortion comes right through. It really has to be bad in an amp like the III to show up, because the feedback will cut it way back.

    But, yes, that adjustment in the picture is in fact the bias pot in the SVT-III. Labeled "AP-1" on the schematic. And yes, the screws make an electrical connection. They can be tightened if loose, but do not get wild about it. Fixing by tapping on it is possible (dirt or corrosion), but it seems suspicious.

    Next. The symptom for low bias is NOT crackling. Low bias shows up as a buzzing noise (distortion) that creeps into the note as it dies out, ending up pretty harsh and "spitty" in bad cases. It sounds a lot like a bad speaker. If you have that, OK bias is likely. If it is anything else, no, it's likely to be some other cause, maybe dirty loop jacks, bad solder, etc.

    Then also... in the 3PRO, it should make zero difference to a bias problem if you change the tubes. It might make some other difference in the sound, but it will not affect the bias. The tubes are coupled into the power amp circuit in such a way that they only pass AC , but bias is a DC thing. if the tube affects bias, there is a different problem.

    Finally, About the bias pot in the SVT-3PRO. It's a piece of crap. Wasn't supposed to be, but it turned out to be too sensitive to vibration and roadie abuse. A tech who knows where to get parts and can find a good replacement that fits in the PWB would do his 3PRO customers a favor by replacing the factory pot if the PWB needs to be worked on. It's probably not worth fiddling with unless the board has to come out for some other repair, though. Not THAT bad a problem.

    The circuit also tends to make it more sensitive than it could be. It has a wide range. I understand why, because while the mosfets in any one amp are matched, the groups can vary quite a bit. Even if it had just enough range to handle the extreme high group, then it has way too much for the lowest group. Groups can vary almost 2:1 in setting. That goes with the territory, not a lot to do about it now. There could have been a different circuit, but it would have used two trimpots to do the bias, which looked like being big trouble in the field.

    As for the factory issue, I think there was one. The proper way to adjust the bias is with the unit at normal idle temperature, which is warm but not hot. In the factory, the Finished PWBs went from "wave soldering" to the test station. Wave soldering is basically dipping in molten solder at around 250C. Some probably did not sit long enough to cool off, and the bias was set hot. If correct when hot, it will be low at normal temperatures, and that is the cause of factory low bias. They were set to the right setting, but were too hot when set.
     
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  18. B-string

    B-string Supporting Member

    Welcome back Jerrold!
     
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  19. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    Thank you. Having powered the SVT-III up again after a long cool-down, the noise is back. I think I'm going to have to remove the power board and search for cold and/or cracked solder joints. :/

    Tapping various other things are causing quite a ruckus. For instance, if I tap against the red wire harness leading from the preamp board to the direct out board, right around the purple arrow, it's extremely noisy. Reseating the board connectors has no effect. Nor am I sure it's the harness itself, or just something connected to it (like the black wires coming from it and going under the direct out board). More tapping to come.
     
  20. farace

    farace Supporting Member

    Jul 9, 2016
    Connecticut USA
    Quick update: got lots of noise on the direct out board from tapping the connector joining the wire harness coming from the preamp board (even though I had disconnected and reconnected it more than once in this ordeal). Cleaned it really well (contact cleaner, then red De-Oxit, wiped off and followed by blue De-Oxit) then decided to clean all the other harness connectors as well. Powered it up and played it for an hour without the slightest bit of noise. I don't trust this to be solved yet, so I'll let it cool down and try it at least one more time, maybe more, before I button it up and declare it done.
     
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