Carvin Bass Amp tube question

[DrNomis_44]

I just downloaded a pdf of the Carvin BX500 Revision F schematic, I'm not sure if it is the exact revision you have, but it should be close enough, anyway, just been studying the tube/valve circuit section, and the first thing I've noticed is that both of the Triodes in the 12AX7 tube (V1A and V1B) are wired-up with the plate/anodes going directly to the +HT supply, which I'm guessing is maybe at anywhere from +125V to +150V, given that there's a 22uF/160V cap (C8) going to ground that's acting as a supply bypass cap.

Okay, so if the plate/anode of both of the 12AX7 Triode stages (V1A and V1B) are going directly to the +HT supply via no resistors, that means each Triode stage is configured as what's called a "Common Anode/Plate Amplifier" or "Cathode Follower" stage, now a Triode configured as a Common Anode Amplifier, or Cathode Follower stage, produces a voltage-gain of slightly less than one, but, it also provides a low output impedance (which just means that the output can supply more current to several inputs without loading-effects degrading the signal).

I had another look at the Carvin BX500 Rev. F schematic to see where the 12AX7 tube/valve (V1) was getting it's input from, and it looks to me like the input signal is coming from the master volume pot, the signal first gets sent to a diode-clipper network which is made up of two blue Leds (D52 and D53), and what looks like a bridge-rectifier formed by 5 other diodes (D55, D57, D58, D59, and D60) which I'm assuming are standard power diodes, I've seen a similar diode-clipper network used in a few Marshall amplifiers (the Marshall Silver Jubilee, the JCM800, and the JCM900), and also a tube/valve pedal using two 12AX7 tube/valves (the Mesa/Boogie V-Twin), that diode-clipper network is most commonly used to generate distortion by clipping the signal more or less symmetrically, I think that in the case of the Carvin BX500, that 12AX7 tube/valve (V1) is being used to buffer, or provide a low output impedance for the diode-clipper network according to my analysis of the Carvin BX500 Rev. F schematic, since both triode sections of the 12AX7 tube/valve aren't contributing much voltage-gain I don't think it actually has much effect on the tone, other than maybe introducing a subtle brightness or maybe adding a very slight amount of the so-called "Tube/Valve Warmth", also, since the 12AX7 (V1) isn't contributing much gain you should be able to use any 12AX7, ECC83, 7025, or any of the other 9-pin equivalents of the 12AX7 twin-triode tube/valve and they all should work normally, providing that they all use the same pin connections.

There's also a Fet (Field-Effect Transistor) Q102, which is used to switch in/out the signal from the 12AX7 tube/valve.

I should also add that since there are two separate paths that the signal can take, starting from pin 7 of the op amp IC A7.B, that means that if the 12AX7 tube/valve (V1) is removed from it's socket, the signal still has an alternative path it can take to the input of the power amp section, which means that whether the 12AX7 tube/valve (V1) is in it's socket or not, there won't be any difference in the sound other than no clipping/distortion from the diode-clipper network.


That is,unless my analysis of the Carvin BX500 Rev. F circuit is incorrect, which I hope not, but if it is then I apologize in advance.