Ampeg SVT 3 Pro Bass Amp Catastrophic Failure & Repair

This Ampeg SVT 3 Pro Bass Amp came in after experiencing a catastrophic failure. A failed repair attempt by someone else led to a twist of folly. I walk through the diagnosis and repair approach that led to a successful repair. Love Your Tone!

[@hugeshows]

Thanks for this video. I had one of these lingering around my bench for months after having rebuilt the whole power section. I was still getting too much dissipation in the 330 ohm resistors and couldn't figure out why. This video prompted me to replace the 13v zener diodes and that was the final piece of the puzzle. Q3 and Q4 were fine on my example, as well as the standard diodes, but everything else failed very similarly to your unit. Cheers!

[@markprice1308]

GREAT VIDEO, love your technique , great example on why to use the variac and what to expect as voltage is being applied im glad i stumbled on to your channel , im restoring an old ampeg myself cant wait to see more of your troublshooting concepts

[@proampsolutions]

Glad you enjoyed it! Love Your Tone!

[@tjsogmc]

I bought a 3 Pro that was broken and cutting off. It would pass sound, but any volume above 2 or heavy plucking on the strings would result in an overload shutdown. I played around with it as a learning tool, but eventually gave up and bought a whole new power board. Even with the new power board from Ampeg, the exact same problem existed (yes, I swapped out the tubes and nothing changed). So I give up. It's sitting on my "to do" shelf for some future time when I have the motivation to look at it again.

[@proampsolutions]

Thanks for sharing!

[@franksanta9826]

I'm Frank, I'm looking for some information to repair an ampeg svt iii Non Pro, it doesn't have a mosfet, it has a BJT (MJ150022_23) Not much information on the bias adjustment on this model, have you had a chance to repair any of these? Could you say some adjustment value? Thank you so much. Kind regards

[@ToneCraftStudio]

Do you know how the MOSFETs are matched? If only 4 transistors on one side need to have same parameters or both sides the same parameters altogether? Bias circuit is common for all of them.

[@rocketsauce5067]

Just by chance have you ever worked on a Univox u-1246 bass 50 watt tube head.It's a four hole plexi copy from the looks.I don't know circuits but from what I read that the output transformer is weird and has 750 volts plate voltage and it just fries power tubes.

[@proampsolutions]

It has been a while, but that one is memorable. It uses half wave voltage doubling that does not regulate very well. The plates should be running 670V +/- and the screens at 335V +/- which limits the current to reasonable amounts. If I remember right, it also uses a cathodyne phase inverter. You must use robust 6L6GC-STR's in that thing. It is an interesting bass amp, but not very powerful in that use case.

[@GL64]

So what was the actual cause of all that failure?

[@proampsolutions]

The owner was not sure, I highly suspect the output was run below 4 ohms for a period of time, perhaps running two 4 ohm loads in parallel. I am also not sure if there was further damage by the person who attempted to work on it before it was brought to Pro Amp Solutions. Never the less, we got to the bottom of it and got it in good working order again. We deliver results! Love Your Tone!

[@StevenCullen-t8u]

MOFSET is that a new type transistor?

[@proampsolutions]

The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which determines the conductivity of the device.