Dual Rec 12AU7 vs. 12AX7 in v's 4 & 5

[Jorn218]

I was wondering if there is a need to have a 12AX7 in preamp slots 4 & 5? I ask this because in the Mesa Bass 400+ it is suggested to install a 12AU7 in the driver socket. If I am not mistaken, the driver is the same as a phase inverter? My understanding is that the 12AU7 has lower gain than a 12AX7, but will handle much more plate voltage.

I am not so concerned with the driver section as I am with trying one of these lower gain tubes in the effects loop section. It may be interesting in that a possible change would be being able to increase the channel volume more without clipping the effects loop. Also reducing amp noise overall.

If you were to use a 12AU7 in the phase inverter/driver socket could the tonal response of the amp be better? A drawback would be less voltage hitting the power tubes leading to a later saturation stage of the power tubes.

Any thoughts from you guys on this?

 

[Third Age Amps]

It may not hurt anything, but the PI circuit is made for 12AX7's ... Try a 12AT7 in the PI and a 12AU7 in the loop and it should be better. To optimize the PI for 12AT7/AU7, you would need to change a couple of values of the LTP, so it may sound a bit brittle with the stock circuitry. Give it a go and let us know how it sounds!

 

[Jorn218]

Yeah I will. I have a gig tomorrow so I wont be doing this til next week. My idea stems from a noise problem I am having I described in another thread. I have traced it down to the effects loop section. When I turn up the loop so I can use the amp at real volume, the noise floor is really bad.

The Marshall I own that my other guitarist uses is modded for high gain and the amount of usable gain in that amp is more than what the Dual Rec has. Seriously, it will laugh at any sort of gain pedal placed in front of it and spit your cable out the input then kick you in the Jimmy. Talking Pantera level distortion. Well the noise floor on that amp is dead silent compared to the Dual Rec.

Thinking that a 12AX7 is a little too much, or the tube installed is too hot a ax7 for the loop section. Was thinking a lower gain tube might work better. Seeming that the lower gain tubes in a AT7 or AU7 still pass through a decent if not more current than an AX7 before clipping, this might be perfect for a section of the amp not needing a necessarily gainey tube.

 

[Jorn218]

Meh an at nor au 7 in v4&5 sounds like poo. The noise floor is lowered considerably, but so does the signal. The amp's output volume will not get louder than bedroom levels. This I do not understand as an au7 can flow more current than an ax7. Ill just find a low gain ax7 and be done with it.