I own two of these amps, one in its original combo form, and another that was re-cased as a separate head, and that is "souped-up" with NOS Philips 7581A's in place of the specified 6L6GC power tubes.
It is important that anyone who owns one of these understands a strange part of the layout of the preamp tube sockets. 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.
link to schematic: [Invalid or Expired Link Removed]
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 one puts 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 is to try a 5751 tube in V1 and/or V3. These have slightly less gain than a 12AX7A, and may be more to ones liking. Just give it a try. It is never harmful to an 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).
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. And, these amps do indeed pair up nicely with a dirt box between one's bass and the input, for those that prefer more grit/grind/fuzz/crunch to their sound.
In the combo, one might 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.....and more solid lows as well.
Another great feature of this amp is the line level output, because it comes off the output transformer, and thus carries the tone of the power section as well as the preamp section (unlike many amps that have their line out between the pre and power sections). It is a hot enough output that it can feed a slave power amp, and drive as many more cabinets as one cares to chain. I have tried this amp driving a single 15 cab and then hooked-up a Crest CA9 to the line out driving an Acme Low B4. Hence, all of the tube tone and power tube sag, and volume to part the Red Sea. Holy Moses!
It is also cool to use the line output as a DI, by using a DI box such as a Countryman, (which is made to handle line-level inputs with no problem), and then send the balanced signal to a FOH/mix board. It allows one to DI their power tube tone, without needing to mic their cab.
There are only two down-sides to this amp as I see it. First, it only has an 8 Ohm tap on its output transformer. I think it would be safe to run a 4 Ohm load, but it would likely have a little less power, and might wear out the power tubes a bit faster. Second, it would be nice if it had an effects loop between the preamp and power section, and a tuner out. That is hardly a realistic complaint though, since these were made from about 1978 to 1981 (I think), and that kind of pre-dates the era when these features became commonplace on amp heads.
Fantastic Amp!
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