Some random thoughts on the above mod.
Reading about transistor amplifiers, looking at LND150 curves and design notes, I made the wrong choice by reducing the Drain resistor size. If I have to say, the amplifier has more “fizzyfuzzy” to it with higher output pickups. The gain potentiometer kicks in later, but the tone is not “cleaner”.
The LND150 would be used here as a pure clean boost first stage, as much as possible, to avoid unpleasant sounds from the later stages’ gain structure. Audio amplifer: Boost the signal just enough to get the power amplifier to use it to drive the speakers, avoid distortion.
From what I read, in audio applications, one has to bias the LND150 in the middle of the “load line”, using a Source resistance about 1/10th of the Drain resistance. To increase gain, reduce Source. To add fidelity and decrease gain, increase Source. I have yet to probe the circuit around the FET, but, looking at the curves, I should now have an unsymmetrical signal shape, or, Distortion, coming out of the LND150.
The VOX designer already reduced signal input with a 68 kOhm “grid stopper” input resistor (equivalent to plugging into the “LOW” input in a traditional Fender style High-Low input amplifier – less signal into the first stage), as well as a small voltage divider before that. The Source (“Cathode”) resistor is split into two, with the smaller one fully bypassed (22 microFarad), also in an effort to reduce gain through the LND150 (not bypassing the total Source resistor leaves some negative feedback). The Source resistance is more than 1/10th of the Drain resistance, (3.5 kOhm and 230 kOhm), another attempt by the designer to put a limit on this stage’s gain (“add Fidelity and decrease Gain”?).
With this surface mount PCB, and the LND150, I can probably not reduce first stage gain by biasing, but I can remove the bypass capacitor. Reducing gain more than this will need bleeding off signal before the second stage. Not convinced that this PCB allows that.
Which all leaves me wondering about the second, very low gain LND150 after the Reverb circuit, feeding the splitter. That one is biased very cold (Source 48.5kOhm, Drain 230 kOhm. This setup looks like a “cold clipper”, distorting the signal, but not adding much “volume”, or producing “fuzz”. I shall have to probe and graph it out to (maybe) know, an oscilloscope would be so handy.
FWIW, compare the very clean, boring IBANEZ TSA15H circuit: The pre-amplifier is but one unstressed 12AX7. An initial, conservative first gain stage, into a plate-fed “lossy” tone and volume circuit, and on to a low gain second stage. That second half of the 12AX7, after the Tone and Volume controls, feeding the Splitter, is biased very cold too – Cathode 47 kOhm, Anode 100 kOhm. A “gain” of about 2? Hmm. The Ibanez amplifier really does not have any aggression. It has no Cathode Bypass Capacitor on the first stage either. A very low gain amplifier, very Hi-Fi in design, but for the second stage. That second stage probably distorts the signal (“cold clipper”?) to make the tone less sterile, but, that asymmetrical signal is not boosted by further stages, so avoids clipping and the “fuzz” tone one can get from that.
So, my AC10C1. Plan is to put the LND150 bias back to stock, and then look at removing the bypass capacitor. That should reduce initial gain without “colouring” the tone.
