Ampeg SVT 3 Pro mosfet bias

[Randall]

Customer amp is a bit fuzzy sounding at lower volumes, wants new tubes and for me to check the Mosfet biasing and adjust if necessary. A search shows this is a common issue with these amps. Does anyone know how to do this?

42601H15[1].PDF

[g1]

http://music-electronics-forum.com/t16172/#post133300 (post #6)

[Randall]

I saw that earlier, but I'm looking for someone to confirm the procedure. I don't have a way to measure wall current. And is a 10K signal right?

[Leo_Gnardo]

I saw that earlier, but I'm looking for someone to confirm the procedure. I don't have a way to measure wall current. And is a 10K signal right?

Randall you could use one of your meters to monitor 120V supply current, no? Dial up slow with your variac so you don't pop your meter's fuse with a turn on surge. I don't look for a specific power draw, just monitor the AC current to make sure it doesn't start climbing to some unexpected high level. I do monitor the output waveform with a scope, while driving a speaker with whatever load resistor in parallel adds up to 4 ohms, and use a low test frequency. After all it is a bass amp. Say 100 Hz. You'll hear a fuzzy overtone and see it on the scope. Dial that trim pot like you're a brain surgeon, just a tiny bit at a time. Overshoot the mark and you'll see your ammeter climb. When you hear the fuzz fade, and the waveform looks clean, you've hit it right.

Might be an idea to buy yourself a clamp-on ammeter. They don't cost a fortune, you can probably get a good one under $100. Monitoring current in situations like this got so much easier once I got the clamper in my toolbox. Just yesterday it helped confirm a bad PT in a kit amp.

[Enzo]

Does your meter have an AC current range, specifically the 10A range?

Current meters go in series with the current. So to check mains draw you can:
1. Remove the main fuse or breaker and clip your ammeter in its place. Not recommended because if there is a fault, you are relying on the meter fuse.

2. Leave the power switch off, but clip your ammeter across it to complete the circuit. Now when you plug into the wall, the current meter serves as the ON switch, and reads the current draw.

3. DUmmy up a meter circuit. Got a bulb limiter? Pull the bulb and clip your ammeter to the bulb socket terminals. Now the meter takes place of bulb in series with mains.

4. Or temporarily wire up a socket with your meter in series.

[Randall]

Clipping across the power switch... of course! I wish I had thought of that!

[Randall]

Great suggestions! I used my ammeter in the bulb limiter, powered from a variac. Worked like a champ, I used a 1K signal. I didn't find the pot to be too finicky.