Schematic of Input circuit in 3 Ch Recto.
(Note: The 2.2M load resistor is in parallel to the voicing circuit and gain pot, which are serial to each other. This has some major ramifications regarding anode resistor values, as the output impedance of V1 will change when the resistor is changed. When my health improves a little, I will update this article with graphs and more information.)
Nov. 2016: While the objective facts of this article are good, one thing I didn’t address was the harmonic distortion content and I think my initial assessment about the early distortion was wrong. I absolutely do not recommend this as an improvement to the dirty channels.
As the resistance goes lower, the harmonic distortion increases. This isn’t clipping, but non-linear tears in the waveform. If the resistance is lowered to 150k or 100k, it would go a long way toward adding character to the clean channel. After tests and simulations, I don’t think it will have a major impact on the dirty channels, aside from a further decrease in clarity when the gain control is past 2:00.
As far as the fizzy nature of some of the 3 Channel Rectos, V2b has the biggest impact on it in the preamp and the power supply filtering is another source. You can test the latter by beginning at Silicon/Bold power options and working down to Tube/Spongy. As the power decreases, clarity decreases from the midrange and the harmonic overtones increase.
So, this article represents a good idea for improving Channel 1, but other assertions I drew were incorrect. I apologize. I’m leaving the article up for critique and transparency.
Some owners or users of Mesa Dual and Triple Rectifiers make complaints about the harshness of the clipping at higher settings on the gain control. The opinion is that getting super-sustained sound is hindered by grinding distortion (which for me is part of the draw). Some users also complain about the clean channel not being clean enough.
There is a way to affect both of these issues with the change of a single resistor. The input stage has a 220k resistor on the anode. Compared to a 100k resistor found in most vintage amps, 220k presents an increase in voltage gain, while reducing harmonic distortion. This mod reduces the value of the resistor to 150k.
Almost all iterations of the Pre-Reborn Dual Rectifier and Triple Rectifier have the same input stage, with one main difference. Early 2 Channel versions do not have the 100 ohm cathode bypass limiting resistor, which is present in the 3 Channel version. Later 2 Channels have LDRs with resistance at about 100 ohms, which is essentially the same thing as the resistor being present in the 3 Channel version. The following content applies to any Rectifier with LDRs or resistors from the cathode bypass cap to ground.