Now why would we be paranoid about adjusting a trimpot to where the adjustment is so small that you can't even see a difference between having it set just right and burning up your amp?
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SVT3Pro Bias Setting
- Thread starter arnocl
- Start date
Good that all went well and that the amp sounds like it has life in it once again.
Don’t bump your amp while transporting it, it could be enough to change the bias.
- Feb 12, 2006
- 61,035
- 5
- 220,950
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Perhaps because some of us have seen the results of this quirk, and it's been costly to those who have attempted to do this and missed it by "this much".
And hopefully everyone's cautions (including yours highlighted above) will be helpful to anyone else who sees this thread. There are some unhappy people who have come at the bias adjustment thinking they could fiddle with it like the idle mix screw on a carburetor without any lasting ill effect....
And yeah, with the bias adjusted correctly the 3Pro is a great amp!
Great, I am happy you read all the advice first. I did the same thing with the same results.
Being the paranoid type I adjusted mine some more, but the last little bit of adjustment made no significant difference to my ears, it just answered the "what if I ..." question.
Hi everyone,
Great thread here. I successfully removed the distortion by adjusting the bias. My resistances across the MOSFETS were 0 mV when I first measured it. Based on an Aragorn35061 post at the beginning of this forum, I started turning my bias slowly counter-clockwise to raise the voltage. This is not the right direction! No matter how far I turned it (which was quite far), the voltage remained at 0 mV. As with most knobs in life, turning CLOCKWISE results in an increase. Once it was turned only the slightest amount past the original position I started picking up mV. The first imperceptable turn put it up to about 2mV on average. The next one brought it up to around 45 mV average! I'm saying this because on the first page of this forum someone stated the opposite. In their defense, they were seeking confirmation and no one responded.
Also, as many have re-iterated: go imperceptibly slow with the adjustment. The placement of the bias now essentially looks like it's in the same place as when it started, (it probably moved a few degrees, definitely less than 10). And the average voltage moved from 0 mV to about 24.5 mV on each side. The original bump up to ~45mV didn't cause any smoke, luckily. It sounds great now.
Great thread here. I successfully removed the distortion by adjusting the bias. My resistances across the MOSFETS were 0 mV when I first measured it. Based on an Aragorn35061 post at the beginning of this forum, I started turning my bias slowly counter-clockwise to raise the voltage. This is not the right direction! No matter how far I turned it (which was quite far), the voltage remained at 0 mV. As with most knobs in life, turning CLOCKWISE results in an increase. Once it was turned only the slightest amount past the original position I started picking up mV. The first imperceptable turn put it up to about 2mV on average. The next one brought it up to around 45 mV average! I'm saying this because on the first page of this forum someone stated the opposite. In their defense, they were seeking confirmation and no one responded.
Also, as many have re-iterated: go imperceptibly slow with the adjustment. The placement of the bias now essentially looks like it's in the same place as when it started, (it probably moved a few degrees, definitely less than 10). And the average voltage moved from 0 mV to about 24.5 mV on each side. The original bump up to ~45mV didn't cause any smoke, luckily. It sounds great now.
Holy ancient mistype on my part! Sorry for years of confusion that I may have caused! I haven't owned that amp in many many years, and to be honest don't even remember that much about it anymore.
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This thread has proved very helpful to me such that a complete novice like myself who up until yesterday didn't even know how to use a multi-meter has just managed to re-bias my SVT-3 pro without setting it on fire. I'm quite happy with that.
One problem I could do with some advice on - I couldn't really gauge from the info here what the safe upper limit for a single resister is. My SVT is an old USA made model (blue logo) and the resistors do not appear to be matched so in the same bank I've got one at 36mV and another down at 6mV with the average hovering around 17mV. That's a big improvement from the 2 to 3mV average I started with but can I do better? I stopped where I was because I didn't know if it was wise to push the 36mV one any higher - did I do the right thing?
One problem I could do with some advice on - I couldn't really gauge from the info here what the safe upper limit for a single resister is. My SVT is an old USA made model (blue logo) and the resistors do not appear to be matched so in the same bank I've got one at 36mV and another down at 6mV with the average hovering around 17mV. That's a big improvement from the 2 to 3mV average I started with but can I do better? I stopped where I was because I didn't know if it was wise to push the 36mV one any higher - did I do the right thing?
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- Feb 12, 2006
- 61,035
- 5
- 220,950
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
The 36mV device is the limiting factor. What the bias instructions should include is an acceptable spread (which is a challenge when using unmatched, or poorly matched devices in this type of circuit) of values. You have a 5:1 spread ratio which is pretty wide, but as long as the lowest devices are conducting (which they are at 6mV) you should be ok.
Another test that "should" be done is an AC current sharing test, where the current through each source resistor is measured with a signal and a load. This should be within at least a 2:1 ratio (for bipolar amps I like to see 1.1:1 or better). I'm not saying measure this because it's easy to have a fatal accident, but this is how it's done on the design side over a sample of amps to validate acceptable current sharing. Power amp circuits that do not naturally share current are notoriously challenging to make reliable IME.
As many had a problem with their Svt3 pro and i just got mine used yesterday i wanted to check if the bias is in range.
I should start up the amp with no cab hooked up and the measure with master = 0 the voltage drop across the resistors right ?
The main question was towards no load and master to zero.
I was just not used to start up an amp without load (as this is always said to cause damage to the power amp)
I should start up the amp with no cab hooked up and the measure with master = 0 the voltage drop across the resistors right ?
The main question was towards no load and master to zero.
I was just not used to start up an amp without load (as this is always said to cause damage to the power amp)
Only with tube amps. SS amps are almost always fine with no load. Honestly, if the amp sounds fine, then your bias is in range and you don't need to worry about it. It's very obvious when the bias is out of whack.
These documents may help. All were gathered from the interweb. Whatever you do, be careful.
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- Feb 12, 2006
- 61,035
- 5
- 220,950
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Seems like everything is fine. Every resistor has some voltage across it so i left the amp as it is and screwed everything back into the rack.
Now i can happily play with it knowing that this is how it is supposed to be and i can enjoy the sound also in a few years.
Also did i learn that apparantly solid state amps must be biased without load compared to tubes which i knew.
Thank you all for the quick and good advices
Now i can happily play with it knowing that this is how it is supposed to be and i can enjoy the sound also in a few years.
Also did i learn that apparantly solid state amps must be biased without load compared to tubes which i knew.
Thank you all for the quick and good advices
