trinityamps.com

I built an sIII and absolutely love it. There's one thing I don't understand though. The signal for the normal channel is amplified through V1b, then it goes to the Tone and Volume pots. From there it's straight to the Phase Inverter with no gain make up. It seems like the signal would be pretty weak after the Tone and Volume pots. How does this work?

The tone network on the normal channel is not very lossy. So not much gain is losts to the tone control.

The TMB channel uses a typical three pot stack and is quite lossy. That is why the TMB has more stages and gain (well part of the reason).

_________________
http://pdfelectronics.com

Thanks, that's what I thought, I just don't recall seeing a single gain stage like that before. The normal channel seems to have plenty of gain and breaks up nicely, the TMB needs to be pushed more for the break up (has more headroom).

It's actually the standard Marshall 18W preamp. Although not common in most Marshalls, it is fairly typical for amps of the late 50s-early 60s Vox era.


In the sIII TMB variant, they have deliberately put in some measures to further throttle back the naturally available gain in the TMB channel. The regular 18W TMB, which the sIII derives from doesn't have those mods, and produces IMO a better level of gain. The actually clean headroom of the amp stays roughly the same, since that's more factor of how much clean power the power stage can push out before it starts to breakup.

_________________
Great tones come in small glass jars!

I looked at the Marshall 18 watt schematics on drtube.com and I don't see this circuit. Every iteration has each channel going through at least two gain stages (unless I'm missing something there).

Sorry about the previous post, I'm not trying to be a jerk. I'm just having a hard time understanding how this much gain is achieved with only one triode before the PI.

Triode driving a tone network with minimal lose, which drives the PI (has gain of it's own), which in turn drives the output stage.

Anyone of those stages can be clipping.

_________________
http://pdfelectronics.com

Understood. I guess I'm just over thinking it.

I've no idea what schematic you've been looking at then. The legendary classic Marshall 18W has a *single* gain stage in each preamp channel (although the two Normal inputs can run their individual gain stages in parallel, but not cascaded).

_________________
Great tones come in small glass jars!