The 12AX7 has gain factor of 100, while the 12AU7 is 19. I own a Mesa 400+, and this is also a common mod for that amp. So using a 12AU7 is a common way to clean up the tone of amps. To get the pedal up to unity gain required the drive control to be cranked up, which made the pedal distort more than I wanted. So in most situations the pedal has lower output than is needed if you don't want to over drive the 12AX7. The problem with the pedal is the output is not buffered. But mostly it's used to fatten up the tone, which is what it was designed for.

I'm not using it for distortion, and usually have it set crystal clear. This pedal was meant for bass and keyboards, or for guitar players looking for cleaner tube tones. The 100k series resistance reduces the level of the signal going to the tube, so the tube is not driven as hard. The BlueTube was marketed as a 'tube enhancer', not an overdrive. If you Google it, a lot of people have hum problems with these pedals. I haven't checked mine against the schematics, but my pedal is a one of the early ones when they first came on the market in '89. Also, the schematics by JD Sleep are said to have errors, or are different. You probably have the later revision schematics. I will try to replace it with others, for example 22nF.

In the schematics that I have and in the prototype that we are experiencing, this 47nF is substitute by 10nF and seems to go really well. It seems to me that there are numerous errors in these two schematics that you have shown: in both, between the pins 6 and 7 oh the Op-Amp, in parallel with the 500K pot is indicated a 12pF capacitor alone (in my opinion it is 120pF.) on the RT-901 one of the two 10K resistors seem badly positioned (the “horizontal” resistor should not be connected as a grid stopper, between grid and node, but between the node and capacitor 47nF). In my prototype, 10K + 10K combination makes the 12AU7 voicing as a deep and hot, but light (smooth) overdrive… than I do not think so interesting to experience what would happen with 100K + 10K! It seems an excessive difference however (1/10 VS ½ Vpp), between the two situations, even if on the RT-903 is used 12AX7. On the'RT-901, as usual, the voltage divider consists of two 10K resistors, then the swing that pilot the first grid has half the amplitude of the dispensed from Op-Amp. On the'RT-903 the divider 100K + 10K makes sure that on the grid triode arrivals the first one-tenth of the swing voltage delivered from Op-Amp (the second resistor 10K acts as a grid-stopper).

Therefore you prefer the sound of the TL072, I understand you correctly? I find the TL0s tend to sound brighter.”….

Bk butler tube driver schematic