improvement of ac30 layout

[tomcat]

Hi all,

I used the Hoffmann layout to replace the pcb of my Vox ac30tbx and it worked out great. I had to change 2 things however to get an optimal result:

1. In the existing layout the long tail resistor (47k) and the cap of the phase inverter are grounded to the ground buss that goes to the power transformer ground. As a result I got an annoying hum/buzz in the amp even when the volume controls for both channels were on zero. As the long tail resistor is a potential input for this kind of phase inverter and the transformer ground buss collects a lot of high voltage ripple this is not too surprising.
So I cut out the connection on the board which leads to the ground buss and grounded the phase inverter via the preamp grou :think1:nd buss. As a result the amp is very low noise now. I would propose to change this in your layout.

2. With respect to the chassis of the existing AC30tbx I found the use of 1 turret board quite impracticable, so I separated the board into two boards: one for the preamp section and one for the power amp section. The valves of the preamp including the phase inverter are now located between the two parts of the board which leads to much shorter connection leads compared to the single board solution. A hint towards users who want to exchange the pcb of the AC30tbx with a Hoffmann turret board could be helpful.

Thanks again for the wealth of helpful information on this forum.