Ampeg svt 3 pro Distortion

[Joe bags]

Hi guys

I have an Ampeg svt 3 pro bass head with a distorted output.
It was with a tech before me and he swapped out the 12au7 and replaced it with a 12ax7
and sent it back to the customer. The problem remained the same now it is in my hands.

I've read a lot of the posts on here about this amp. I know the 12au7 on the output section can cause distortion if it's bad.
So I have replaced this tube with a proper one. Still got a distorted output.

Tested pre amp and it is good.
Tested output transistors in circuit they seem to be fine.

I've gone through the test points on the schematic and measured voltages and none seem to stick out as being off the wall or not there.

I read another post that there is a bias pot that you can turn to get rid of distortion.
I've tried turning this with a sine wave on the input. Initially the distortion went away but then it came back.
Now I don't want to mess around with this pot too much as I started hearing a strange noise(like a high pitched sine wave!!)
with no input connected and the fan started working harder.

I got a triangular wave on pins 4 and 5 of the 12ax7 on output section with scope.
Got a strange waveform on pins 4 and 5 of the 12au7 like a distorted sine wave and much higher voltage than the 12ax7.
Not sure of the significance of this but just thought i'd mention it.

Any thought on narrowing down where the distortion is coming from?
Bad cap or resistor in output section?

Joe

[soundguruman]

seems that you should post the schematic

[Joe bags]

Abracedabra!

Not sure how to do pdf links. Hope this works ok!


SVT PRO 3 by Joebags1, on Flickr


SVT PRO 3 (2) by Joebags1, on Flickr


SVT PRO 3 by Joebags1, on Flickr

[Enzo]

[QUOTEI read another post that there is a bias pot that you can turn to get rid of distortion.
][/QUOTE]

This is dangerous. Distortion simply me4ans anything in the output that wasn;t present at the input. Adjusting the bias might kill any distortion CAUSED BY IMPROPER BIAS. It will not have any effect on distortiuon from any other source. And unlike a tube amp where mis-bias will result in tubes getting hot, thisw amp has those rows of matched MOSFETs, you won't be happy when you twiddle the little control and blow the lot of them away.

Make sure there are proper 12AU7s and 12AX7s and they are in the proper sockets.

Are we injecting the test signal at the power amp in jack?

You have a scope? Good. What does the distortion look like at the output - sine wave signal. Does it look the same without a speaker load?

[Joe bags]

Yea I figured it was a bit dangerous alright. Really don't want to blow the output transistors.
I'm pretty sure the 12AU7 and the 12AX7 are in the right place.
I installed a new 12AU7 from Banzai.
Yes test signal going in to power amp jack.
I will check outputs on scope with and without a load next.

Thanks

Joe

[MarkusBass]

Joe,

There is nothing wrong with the amp except that the bias is not set up correctly. Be aware that random turning of the bias pot can cause output transistors failure and a lot of costs - the pot is very sensitive (and you are quite close to this since the fan started turning with high speed). Search on this forum how to set up the bias in this amp and make sure you understand it before turning the amp on again. Of course you need 12AU7 and not 12AX7.

Mark

[Joe bags]

Quick test with a scope today. And amp is more distorted with load connected than without.

Found this on a search on biasing this amp.

"Adjust the trim pot until it is drawing between .8 and .85 amps from the wall. With a 4 ohm load connected inject a 10khz signal into the power amp in jack. Adjust the bias pot until the crossover notch is barely visible on the scope. Do not completely remove the notch. Remove the signal and confirm the amp is drawing between .8 and .85 amps from the wall. That's it.

At the risk of sounding stupid where do I put the probes to test for current draw?

[Jazz P Bass]

Found this on a search on biasing this amp.

"Adjust the trim pot until it is drawing between .8 and .85 amps from the wall.

You will be measuring the current that the amp is pulling from the mains outlet.
You 'break' one of the mains wires & insert a multimeter set to read current. The meter connects the two 'broken' wires.
Be careful as you will be messing with 'line' voltage.
Or, get yourself a 'Kill-A-Watt'

[Steve Conner]

You also need to figure out whether the guy who recommended 0.8 amps was running the same mains voltage as you. If he was 120 and you are 240 (as I suspect since most tech info available online is American) then you need to set for half the current: 0.4 amps.

[Joe bags]

@Jazz P. Cool I kinda thought as much. I tried breaking the live wire on the mains plug and putting the probes in series but didn't really get any readings.
Thought my meter might be suspect. Have never made many current readings in the past so might be finger trouble on my part. Will try again tmw.
What's a Kill-A-Watt? You mean a light bulb limiter?

@Steve. Thanks will look into the mains voltage used.

Regards

J

[MarkusBass]

Kill A Watt - Wikipedia, the free encyclopedia

[Joe bags]

Ok, I got a Kill A Watt pretty cheap in Maplin, nice one!

Amp is drawing 0.36 amps. I tried the procedure above but had no luck getting rid of distortion. I'm being real carefull turning that bias pot. Bit freaky it's kinda like waiting to be electrocuted!
I'm not convinced that it's a bias issue so I think I might have to back track a bit and do some more tests.

With the bias pot, is the dangerous thing with it when you randomly turn the pot?
So it's ok to turn the pot gently all the way to left or right in small increments?

J

[52 Bill]

The dangerous thing is to turn the bias pot without monitoring what effect it has on the amp. Sometimes these amps will start to draw too much idle current and heat up very quickly and will sometimes go into thermal meltdown mode.

So don't be afraid to adjust it, just use a little common sense and do it with your eyes and ears open.