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SVT3Pro Bias Setting

A

arnocl

May 24, 2008
6
4
Belgium
#1
I recently bought a used Ampeg SVT3Pro. I had some problems with fuzz at low volumes, and after reading every post concerning this problem I came to the conclusion I had a problem with the bias setting. I noticed almost nobody adressed this common problem with the 3Pro in detail, hence my question.

I measured the voltage over the "flameproof" resistors in the poweramp-section, they all were around 1mV. I read the biascurrent can't be higher than 20-25mA per mosfet (equals +/-10mV measured at the resistors), so I've set the bias to a higher setting, carefully checking not to exceed 10mV.

The fuzz is gone now. The highest voltage is at 8.6mV (18.3mA), but all the others are way below this (the lowest around 1.7mV). Since none of the MOSFETs draws the same current, is there an optimal setting for the bias, or is the current current :hmm: setting, with only one or two mosfets approaching 20mA, good enough? I don't have an osciloscope, just a simple multimeter.
 
#2
if too low, you may have an issue in colder weather, at least until it warms up.

You should be OK if each bank AVERAGES the right current, and none are more than 1.5 x. There shouldn't be any that stay very low.

If they continue to vary widely, maybe yours does NOT have matched mosfets.... They should have a small color dot on them, and all the dots in a group of the same type mosfet should be the same color (there is a group of N-channel and a group of P-channel).
 
#3
Thanks for your reply! I appreciate you take the time to share your Ampeg wisdom.

I rechecked the voltages and highered the biascurrent, 2 are around 10mV now, 3 around 8mV, 2 are 4mV and the last is 2mV. All the mosfets have a blue dot on them, P-channel and N-channel. It's a 2001 made in SLME SVT3Pro, all electronics are original. When you say 1.5x, do you mean the current can be beyond 30mA, without doing harm. I'm a little worried I might blow the poweramp by tweaking it too much.

The amp works fine now. If you think the mosfets aren't functioning correctly, I'll take it to my technician, but I rather do simple reparations myself so I know what happens with it.
 
#6
Indeed it is, but mosfets need proper adjustments too. Too little current, harsh distortion, too much current, apparently a pyrotechnics show (glad I've never seen that last one).

Already saved 2 amps I tought were broken (with costly repairs) by simply adjusting the bias to a good setting. Don't mess with this if you don't know what you're doing, I'm no electronics wonder myself, but I read every spec about these mosfets, the schematics of my amps, ... So I guess I'm taking a rather safe risk here.

I'd really want to know more about electronics, but I guess you can't reach that without app. 3 years of proper schooling? Or are there any books worth reading?
 
#7
The current can be higher than what you have. The expectation was literally that the average would be "at" the bias and some would be a bit high, others low. This is for either group......... measure across each resistor in the group, and average them. Range should not have any really high ones, which should be Ok for matching color dotted parts.

There is a tendency for the bias to even out at slightly higher currents.

The actual spec for matching is that the "Vgs(on)" must be within a 0.3V range for all parts in one grade (color). That is the voltage from gate to source when the drain current is 1 mA.

Obviously, that works "backwards" also, so that if the gate voltages are all the same, the drain currents will tend to match due to the source resistors. The issue isn't at low currents, it is the matching at high currents. With about a half ohm of source resistor, the mosfets must share within about 0.6 A, ensuring nobody hogs all the current.

The bias is simply to get as much current in the parts as possible, within reason and without getting the devices too hot at idle, so as to avoid crossover "notch" at all likely temperatures (the usual bias circuits never quite track precisely as they should). The SVT-3PRO has zero feedback at audio frequencies (it has DC feedback), so crossover distortion is particularly critical.

When I wrote the original matching spec proposal, it was for matching to 0.25V. But it turned out that the manufacturer was already matching to 0.3V, and could do that very cheaply, so we took them up on it.
 
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#8
Im fixing to have to do this to my SVT as well as I just replaced my tubes but am still getting a slight distortion problem. I am an electronics tech by trade, though no experience with bass amplifiers, just radar equipment. Before I attempt this I want to make sure I understand what to do, If I understand correctly each of the eight mosfets has a corresponding power resistor, and the voltage across each one should be no higher than 10mV. (I am correct in measuring directly across each resistor, not from one side to ground, unless one side is connected to ground?). Adjustment is done with the pot located in the lower right corner, of the board that has the 12AX7 and 12AU7 on it, clockwise should lower the voltage counterclockwise should raise the voltage. Im also assuming that it doesnt matter if the load on the amp is 4 or 8 ohm. Am I correct in all this?

Thanks

John
 
#9
Aragorn35016 said:
I am an electronics tech by trade, though no experience with bass amplifiers, just radar equipment.
Click to expand...

Just think of it as an extremely low frequency radar. ;)

Im also assuming that it doesnt matter if the load on the amp is 4 or 8 ohm. Am I correct in all this?
Click to expand...

The bias voltage will be the same (maybe very slightly higher at 4 ohms due to B+ sag) but the current will change with the load.
 
#10
Psycho Bass Guy said:
Just think of it as an extremely low frequency radar. ;)



The bias voltage will be the same (maybe very slightly higher at 4 ohms due to B+ sag) but the current will change with the load.
Click to expand...

Hehe my tech abilities have waned dramatically, I work mostly in logistics now, that and most of my Troubleshooting work involved using a test plan and mostly "chasing the signal" when things didn't work. Trying to use this amp experience as sort of a refresher course to some of the stuff I studied in my college days.
 
#11
The adjustment MUST be done with NO LOAD. Any load will draw a slight current due to small DC offsets, and messes up the reading. Just about any SS biasing should be done no-load.

Then also, it is 25mv, not 10 mv, measured across individual source resistors. That is the average per "set" of N-channel or P-channel mosfets.

This is for a unit with graded and matched mosfets. There should be a color dot on each device, and all N type or all P type devices should have same color, although the N and P may be different.
 
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#12
Jerrold Tiers said:
Then also, it is 25mv, not 10 mv, measured across individual source resistors. That is the average per "set" of N-channel or P-channel mosfets.
Click to expand...

In one of your posts in an older thread you said: "app 20-25mA per device"

Since the source resistors are rated at 0.47 Ohm, and knowing U = R x I, I tought the average Voltage measured should be around 10 mV?? Or am I completely wrong here?
 
#13
arnocl said:
In one of your posts in an older thread you said: "app 20-25mA per device"

Since the source resistors are rated at 0.47 Ohm, and knowing U = R x I, I tought the average Voltage measured should be around 10 mV?? Or am I completely wrong here?
Click to expand...

Probably I fat-fingered it when typing.......... and saw what I "knew was there" when I looked at it.............

I assure you that if we wanted 25mA bias the factory procedure would have said 12.5mV, since volts are easy to measure, but current is hard. However the number is 25............................mV. The 25 mV gives approx 0.05A, which is around 3W bias dissipation per device..... no problem.

Sorry for any confusion.......
 
#14
ok here is what mine reads across the resistors, it doesnt look good. Taking the top off and looking from the front of the amp they read as follows (in mV):

(Rear of Amp)

2.0-------2.2
1.7-------1.1
1.4--------.3
1.4-------3.2

(Front of Amp)

I let the amp run about 10 minutes and as instructed did not hook a load to it. I expected to be a little off from 25mv, but not that far. The one that reads .3 kind of bothers me. I dont know if I should go for turning the pot or not, I have a gig tonight, so I may wait until tommorrow to make sure the thing doesnt blow up when I crank the pot.

John
 
#15
Against my better judgement (and lack of patience) I went ahead and turned the pot ever so slightly, man that thing is sensative! The resistor that read 3 mV shot up to 44, so I backed it down to about 24, no smoke or extra heat so I think Im good. I cant check it with a speaker yet as Ive brought it to work with me to do this but if this took care of my distortion problem Im not going to worry about "optimizing it" I just cant get over how sensitive that pot is, I cant even tell I turned it thats how little I actually gave it a nudge! The resistor that read .3mV now still only reads about 2.5 mV so Im still not sure what the deal is there. Maybe it wont smoke on me tonight, wish me luck!!

John
 
#16
Well hooked it up to my cabs and the bias adjustment was a complete success!! Amp is as clean as a whistle! As sensitive as that pot is, Im hoping that the torque seal I put on it is enough to keep it from drifting due to vibration/general transport from one place to another. We'll see. Thanks again Jerrold for your insight, this thread should be a sticky for SVT3 users!

John
 
#17
Biased my SVT-3 Pro last night - was set at about an average of 18 mv per side, finally got it settled down at about 25 mv per side [average across the 4 resistors on both sides] and WOW what a difference! Once you get a good 12au7 in it and the mosfets balanced - this is a much better amp than I thought.

Watch out for that bias pot though... its unbelievably sensitive = very hard to not turn it more than you need to.

Special thanks to Jerrold Tiers for the details. :bassist:
 

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