Here is how I upgraded a friend GA15.
It's here.
It's in french but I can answer to your questions.
There are three clips, the first was done before the mods (amp in stock), the second during the tweak and the last after all the mods were done.
Now the amp sound realy nice.
There were some mistakes on the amp conception.
The power supply had not well thinked and caused some hum.
The tube voltage was totaly out of the datasheets.
The resistor cathode BIAS value wasn't correct.
The préamp component values were realy bad and there was no gain pot.
The amp just had three EQ pots and a master pot.
And it was totaly impossible to have some préamp or power amp overdrive.
![Wink :wink:](./images/smilies/icon_wink.gif)