sovtek mig 100u need to add some gain

[Jingletone]

This might be confusing without a correct scematic. Let me explain it by which pin on the tube the component connects to. Each 12ax7 has two gain stages. One is pins 123 and the second is pins 678. Pins 4,5 and 9 are your heaters they have 6.3 volts ac on them. If you follow the wire from the high input jack to the tube it either connects to pin 2 or pin 7. Which ever one it connects to is the first gain stage. Pin 1 and 3 are the plates they have high dc voltage and tube output ac voltage on them. Pins 2 and 7 are the grids or tube inputs. Pins 3 and 8 are the two cathodes they set the bias and effect the way the tube operates.
The 33k he is referencing is the grid stopper, it is 68k stock and is usually located right on the tube socket. This will as a small amount of gain.
There are two main ways to increase gain. The first is to change the cathode resistor to a smaller value, this will add gain and the second is to add a capacitor across it to boost certain frequencies and increase gain dramaticaly. For the sound you are after normally i would connect a 2.2k resistor bypassed by a 1uf capacitor from pin 3 or 8 ( depending on which one you determined is the first gain stage) to ground. ( if im guessing which amp yours is you already have a 2.7k bypassed by .47 so this might not do much)
To make the tube break up and add smooth distortion I would also change the 100k resistor going to first gain stage plate ( pin 1 or 3) to a 220k this lowers the dc plate voltage and lowers the headroom on the gain stage. I would add a 500pf ceramic disc across it to filter occilation and high highs...
Ok, now there is a .022 or .047uf bypass cap also connected to the plate. It filters the dc voltage and lets just the ac which is your amplified signal pass through to the preamp gain pot. Inbetween the two is a resister and cap that makes a filter and atenuates the gain stage. You probaly dont need to change this the stock value is fine.
So now we move on to the second gain stage, the wire from the middle tab on the gain knob goes to the second gain stage grid ( if input was on 7 this will go to pin 2)
The BIGGEST GAIN BOOST is on the second gain stage cathode (pin 3 or stock it is like 10k unbypassed, meaning no capacitor across the resistor. Change the resistor to a 2.7k and put a .47 .68 or 1uf cap across it in parallel to ground.
Leave the 100k on the plate it will sound good at stock votlage.
You are pretty much done... I would unsolder pin 2 of v2 and put a 220k grid stopper on the tube socket. One end on pin two other side of the resistor attach the wire you just unsoldered.