Preamp Tube Overdrive : Which Triodes are producing it?

[tbonesullivan]

So, as always I'm rolling tubes like a fiend, trying to find the "best" mix, and having access to the schematics definitely helps in this.  The question is, in a preamp design that has 5 triodes in the signal path before the phase inverter, which ones are the ones producing the overdrive?  I was looking at the schematics and manual for my Stiletto, and it has the following signal path for the gain channels:

V2A: Input Driver Stage followed by Gain control

V1A: Second Gain Stage

V2B: Third Gain Stage

V3A: Fourth Gain Stage

v3B: Fifth Gain Stage (cathode Follower before tone stack).

So, looking at this, I would assume that Gain Stages 2 3 and 4 are the triodes being driven into clipping. Am I reading this wrong? I would also assume that the cathode follower is not being driven into clipping.

[tbonesullivan]

Yeah, I forgot about V1B. That's input driver for the Tight Clean, Crunch, and Tight gain (per mesas manual) and V2A is for the Fat Clean and Fluid Drive modes.

Basically, I've got two Tungsram ECC83 used tubes that test as new, so I'm trying to figure out where to put them in the signal chain. They are supposed to have a nice overdrive sound, and are compared favorably to the Mullard and Brimar by some. I'm trying to get it more "British" sounding, if you will. Maybe a bit smoother clipping sound with more "meat" to it. This is why I'm trying to figure out which tubes are the ones actually being driven into clipping. I have some old production tubes that I love for clean but they don't break up well at all, like CBS Hytron.

Currently I have a Chinese "Mullard Style" tube in V1, but I have heard it's better for "moderate" gain, and since one triode of it is definitely part of the cascade, I'd like to swap it out. I have a Mesa Russian 2 EHX tube in V2, and an NOS JAN GE 12AX7WA in V3.  V4 I just have a Mesa-branded JJ, and a Sovtek 12AX7LPS for the Phase inverter.

Due to the design of the combo, it's impossible to change tubes without pulling the chassis, unless you have baby hands. So, tube rolling can be a bit arduous. I also picked up some S.E.D. EL34s from Mesa, and was going to replace the Mullard Reissue EL34s currently in there. Those I'm going to throw in the Rivera, where I can actually control the Bias level.

[tbonesullivan]

Also, the ACE is a stage II Stiletto. They made a bunch of changes to the component values. I think the schematic can be found in the second post in this thread:

https://music-electronics-forum.com/showthread.php?t=39976

Also, I had someone tell me that the cathode follower driving the tone stack, which mesa calls stage 5, probably isn't being overdriven very much.

[tbonesullivan]

Yeah, I put the GE there, but I'm not sure that position gets the best money out of it. I may throw in a Beijing China tube into that slot instead, behind the two tungsrams. I can't put in anything from the Sovtek plant there or the Cathode to Heater voltage will fry them.

[Dutchman]

17 hours ago, tbonesullivan said:

I can't put in anything from the Sovtek plant there or the Cathode to Heater voltage will fry them.

I think those tubes have a spiral filament. I believe this was to create a quiter tube but they can’t take the abuse a NOS tube can. 

[tbonesullivan]

9 hours ago, Dutchman said:

I think those tubes have a spiral filament. I believe this was to create a quiter tube but they can’t take the abuse a NOS tube can. 

Yep. I mean, the 12AX7WA/B/C that have standard filaments can.  And these cathode follower spots often put 200+ volts on the cathode. The RCA and Philips specs for limiting cathode to heater voltage are 200 and 180 V respectively. Heater voltages are 6.3V or 12.6V.

In the Stiletto, V3B has 245V on the cathode, and V4B has 210V. While V4B is just pushing it a little, V3b is significantly over.