AMPEG SVT-3pro

hey i don't have an svt3.. but i'm pretty sure that if you take your amp apart yourself it voids the warranty, which may be why you haven't gotten any responses... have you tried contacting Ampeg? Or maybe go to your local repair guy and ask him where the au7 belongs...

 

I agree with your findings - it does matter where you put the 12AU7. While 12AX7, 12AT7, and 12AU7 are pin-compatible (meaning they have the same basing and can be interchanged within reason) they don't have the same operating characteristics, like gain, for instance.

The 12AU7 has a much lower gain than the 12AX7, and since the 12AU7 is commonly used to drive power amp sections, putting a higher-gain 12AX7 in its place only serves to overload the power amp section, something you don't want to do (esp. with the Ampeg's MOSFET power amp). You want to introduce distortion by overdriving your preamp, not your power amp.

For reference, the SVT3 looks kinda like this:

preamp -> tone stack -> driver -> power amp

You *can* put a 12AU7 in the preamp stage if you want lower gain - no problem with that. But if you put a 12AX7 in the driver stage, you are upsetting the gain structure that Ampeg designed. You might not blow anything up at low volumes but if you push the amp really hard you'll end up with distortion and heat and blown cabs.

Some of the SVT3-Pros have 12AX7 in all positions. Ampeg simply modified the driver and/or power amp circuits to work with the higher-gain 12AX7, and I guess it makes life simpler for them.

I no longer have my SVT3-Pro, but it did have 4 12AX7 and a 12AU7 in the driver stage (the position closest to the back of the amp?)

Nightbass

 

I remember stumbling across a sitethat had scematics (sp?). I was looking around & couldn't find it. Did you take the tubes out & forget where thay go? I beleive (not 100% sure) that on the board near the "tube holder" (lack of a better term) it says what tube goes there.

 

Okay, I just opened mine up to check. The 3 12AX7's are in the fron of the amp, and the driver tubes are in the back. The 12AU7 is in the position closer to the front panel of the amp.

This is strange. The first tube is a Groove Tube 12AX7 (Ampeg advertizes that this amp is "Groove Tubed") The other 3 12AX7's are Sovtek and the the 12AU7 has no markings other than "12AU7". I like the labeling on the Groove Tubes, much easier to read. I wonder why does Ampeg only use one GT and the rest are straight from Sovtek or wherever?

 

Ampeg is sneaky like that.