I own two of these amps, one in its original combo form, and another that was re-cased as a separate head.
It is important that you understand a strange part of the layout of this amp. The preamp tubes from left to right (if viewing the amp from the front) go in this order: V1, V3, V2 , as they relate to their labeling on the schematic. They DO NOT go V1,V2,V3.
In other words, the 12AU7 tube is correctly placed in the third socket from the left. The first and second sockets from the left are specified for a 7025 or 12AX7A (pretty much the same tubes as each other...the 7025 is reportedly more "hi-fi", but they are the same gain factor).
I don't know why the sockets are arranged this way, but what I am saying here is true.
Realize that if you put the 12AU7 where a 12AX7A is supposed to be, and a 12AX7A where the 12AU7 is supposed to be, the gain structure of the preamp is VERY different.
A 12AX7A tube has more than five times the gain factor as a 12AU7 tube, as can be seen depicted in these charts:
Fuzzy's Dead Mouse - 12AX7 Family Tube Substitution Guitar amp tweak - change the gain factor of your pre-amp
If you happen to have these two tubes switched with each other (as simple logic would lead you to do), then you will have about five times too much gain for the five-band EQ section, and about one-fifth the gain specified for the V3 socket (the SECOND from the left) of the preamp.
If you have this wrong and then make it right the preamp will have dramatically more gain (as it is supposed to!).
On my amps, I rarely have the Volume (which is the Gain) turned up higher than 7 or 8 on the dial. I get a mildly warm overdrive beginning around 5 (when using a passive bass like a regular P or J). A higher output bass, like a G&L L2000e that I have, will get more overdrive at a given Volume setting. I never need to turn the Master Volume Output knob (on the far right) past 6 or 7, even when using a Bag End 4x10 cab that is rated to take 800 Watts (it is a pretty efficient cab, however).
A possible tube swap would be to try a 5751 tube in either V1 and/or V3. These have slightly less gain than a 12AX7A, and may be more to your liking. Just give it a try. It is never harmful to your amp to replace a preamp tube with another tube of lesser gain. Rolling (switching between brands and gain factors) of preamp tubes is kind of fun in these amps, because they are sensitive to subtle changes (more so than in something like an Ampeg V4, in my opinion/experience).
You probably realize that no matter what, these amps are not going to grind/fuzz like an Ampeg, and they are not meant to. What they are is basically the ultimate Fender Bassman. I have read several comments here on TB where someone says that they love the tone of their old Fender Bassman, but wish it could go louder and keep up with a loud drummer and/or loud guitar player. The solution to that problem is the Fender Studio Bass.
In the combo, consider replacing the original EVM-15B (relabeled as a Fender) with a more modern and efficient (and lighter weight) driver like an Eminence Kappalite 3015. The highs may not be quite as sweet, but you will get quite a bit more volume at any given setting of the Gain and Master Vol. Output.
Fantastic Amp!
