SVT III Pro bass amp distorting, low output

[nevetslab]

this is an interesting thread tome because today I started working on a friends amp (SVT3 PRO) that has similar symptoms. Only on my amp, the bottom half of the sine wave, as viewed from the output, is distorted and low magnitude compared to the positive half. In the schematic I have of the power amp section, the sine wave signal looks healthy and symmetrical at TP2 (R4), but starts to look bad at TP3 (R5). Looks like R4, R5, R6, and R7 are OK. +-100 v supplies good. +-65V supplies good. 310v present for the V11 v12 B+. I dont see that the above thread ever resolved the issue so that I might apply the knowledge to this amp. Anyone have suggestions for this one?

Welcome to the Forum!

TP 3 is still ahead of the drive point to the MosFET Output Stage. TP 3 IS subject to the front panel's TUBE GAIN control, which the schematic shows as the Plate Voltage. That circuit is shown on both the Preamp schematic, output which goes out Pin 11 to the Power Amp schematic, controlling Q1 as the Series Regulator stage to set the plate voltage for V1 & V2.

I'm assuming the signal appearance at TP 4 and the Output from the amp share the same waveshape. If so, that tends to point to the 12AU7 circuit, and/or the level of the Plate Voltage controlled by front panel TUBE GAIN knob. Does changing that control affect the linearity of your test waveform?

You're right....we never did see a conclusion on the original post. I did see a comment regarding the touchiness of the bias control, and would add a warning to that, having killed the output stage on the SVT3-Pro's bigger brother SVT6-Pro, where I was monitoring the wrong output section when I tweaked the main PCB's bias pot. You CAN fully turn on the output stage to self-destruction. I posted on that calamity earlier this month.

[tomtech]

thanks for the quick reply Nevetslab,
From your reply: "I'm assuming the signal appearance at TP 4 and the Output from the amp share the same waveshape."
This is correct. TP4 is where things go south with respect to the waveform and the rest of the amp follows it exactly to the output. And yes, I am on board with the 12au stage as the suspect, for that reason.
The plate voltage control does affect the wave as you suggested. But I can not get the amp to deliver rated clean output with any setting of the plate voltage front panel control. Is still a puzzlement to me. ;-)
BTW: I have not touched any bias controls or other internal trims pots that might be in there.

[tomtech]

OK looks like "case cracked". Did I say R5 was good earlier? If so I lied. :-) Its open. I clipped it out and clip leaded in a value close (180K 1/4 W) to see if that helps and its good now. Clean output full rated power. Now to get the right value( 150K 1/2 W). I wish ampeg was more service tech friendly cause it looks liek a lot of work to get the board out and do it right.

[nevetslab]

OK looks like "case cracked". Did I say R5 was good earlier? If so I lied. :-) Its open. I clipped it out and clip leaded in a value close (180K 1/4 W) to see if that helps and its good now. Clean output full rated power. Now to get the right value( 150K 1/2 W). I wish ampeg was more service tech friendly cause it looks liek a lot of work to get the board out and do it right.

Hah! Good hunting expedition! Glad that was an easy find, as far as the identification and external patch to verify. Mechanically, none of the SVTx-PRO amps are a walk in the park for servicing the power amp end of things. The SVT5-Pro and SVT6-Pro are the worse, as the ONLY way you can work on/re-bias or repair the lower output stage is to remove that odd-shaped PCB. The SVT6-Pro is even worse, in that you have to remove the power transformer to even GET the PCB out. That earlier post this month shows what ya gotta do to set up for re-biasing that, external to the chassis.

Ampeg is now under the controls of Yamaha, having bought it from Loud Technology. There's a good handful of us here that service the Ampeg SVT series, though the SVT7-Pro & SVT8-Pro, not so much. All the others in the SVT line I service on a regular basis, being the one who maintains the rental inventory at CenterStaging, LLC in Burbank, CA. There's one or more MEF members who worked for Ampeg over the years.....and there's good archives in the forum database to aid in your understanding of all the models and their issues over time.