Ampeg SVP Pro Preamp Question?

[zac10669]

I recently bought a used SVP Pro Preamp (the older US Made one w/the blue logo)

It arrived with all 5 of the preamp tubes loaded with 12ax7s. I know the schematics call for four 12ax7s and one 12au7 (I believe in the#4 position), and I think the 12au7 is supposed to power the Drive knob.

Question(s):

What potential issues are being created given the 12au7 was replaced with the 12ax7. Potential impact to performance and/or damage?

Secondly, As I was "twisting knobs" for this first time last night trying to get the sound dialed in, I noticed that when using the Drive Knob, there was little to no difference in tone/output even when I turned the Drive knob all the way up?

Any perspective on the 2 questions above would be greatly appreciated.

Thank You!

 

[ChrisB2]

I don't know about the tube question, but my drive knob only effects the tone in conjunction with the gain knob, and how hard I dig in. Turn the gain up until the peak LED flashes, then turn drive up to desired overdrive, up to full clockwise based on how hard you're playing. This is the procedure in my manual. If this doesn't get it to overdrive, then yeah it may be the tube...

 

[bgavin]

I saved this in my notes. It was from a player who did a lot of experimentation with his SVP-Pro tube layout:

V1 - JJ Tesla ECC83S (12ax7)
V2 - " " "
V3 - " " "
V4 - JJ Tesla ECC82 (12au7)
V5 - ElectroHarmonix 12au7a/ECC82EH

 

[zac10669]

Thanks Guys!