Bias for AMPEG SVT 3 pro

[Davis]

Hey everyone. I have an AMPEG SVT 3 pro and I am noticing the amp is running a little hotter than it probably should, and pops and cuts out intermittently after running it for about 25 minutes.

I've had the amp for about 4.5 years. Ive just replaced the tubes. Ive heard that doing a bias check after tube replacement is common practice. Unfortunetly where I am from trying to get someone to service (let alone buy) an amp that wasn't assembled in mexico/china is quite difficult. They are reluctant to repair a brand they don't carry.

I am experienced with electronics and I'm not intimidated to poke around inside an amp, only I don't quite know how to go about biasing this amp. If anyone out there knows how I can do the bias myself without an oscilloscope, and would be so kind as to leave me some kind of instruction, I would be extremely grateful. Thanks guys.

[Twist]

Are your talking about the pre amp tubes? There is no bias adjustment for those. There is an idle adjustment for the output FET's. I wouldn't suggest doing it unless your very familiar with the procedure. The popping and cutting out could be poor solder joints on the bridge rectifier.

[Davis]

The pre-amp tubes seem to be fine. I was more concerned with the power tubes. The 12AU7 and 12AX7 on the power board.

I completely stripped the amp, removed the boards, took out the Tx, and checked every solder joint. Everything looks fine and dandy. The bridge rectifier is mounted directly on the chassis and is run to the board via quick connects, and they're in there pretty good.

I thought bias because it was getting hot sooner than it used to and was causing an inbalance of current due to an untuned bias leading to cutting and clipping. In college I had built an audio amp, and when it came to biasing, I mostly did it by ear. I don't really want to do that with this amp, especially since I didnt build the sucker, and its considerably more expensive than my college built amp (which still works very well.)

I pretty much just wanted to know if anyone knew the ideal voltage and currents I should be tuning the bias to, and where the test points were to stick my meter. Thanks a lot.

[Amp Kat]

You may be talking about the Pro 4 Twist. I think the Pro 3 has 6 -6550's like the Classic. Part of the repair cost is getting that big ass thing up on the bench. Doesn't that amp have the LEDs to set the bias like the classic ?
The board with the eq on it rarely fails. If the bias is off the TLO-72'S may be bad. You really need to do a current test and see just where the draw really stands. The LED's when working are nice but if the comparator is off so will the bias be and there is some leway there. Lots of voltage on those plates like 700 volts of it so be exteremely careful when inside and around the caps and plates of the 6550's.

[Stoffie1974]

Nope amp Kat ... that's the SVT2 PRO.
I also have a SVT3 PRO and i "poke around" inside amps too
Normally you shouldn't bias them, they're fixed, and with preamp tubes there's a larger margin then with paralleled power tubes (bias is more important here).
I did replace mine too with EH (standard was sovtek), and when i tested a pure sine-wave on my scope, there was assymetric signal on the output (the neg. alternance clipped much sooner than the pos.), re-replaced the 12AU7 and it was solved.
I guess you could have a faulty tube, just take them all out and restart your amp and see how it goes, allways measure if there's DC on the output of the mosfet, if it still runs hot ... it's NOT the tubes. If it doesn't, put it off, place one tube and restart the same procedure. But you should know this ... allways check the voltages of the supply first !!
If you want, i have the schematics.
Good luck ...

[catstrat]

svt 3 pro

I have an svt 3 pro on my bench now. According to the service data i have there is a bias adjustment for the output transistors. The following is right out of the Ampeg tech notes of the service packet. Turn unit on. The relay will engage and it might take up to 75 seconds. 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. I have to assume this is the correct way since i paid Fifteen bucks for the info from Loud technologies.

[Joe bags]

Hey Catstrat, I'm trying to do this bias adjustment also. What mains voltage you were using?

Regards

J