Bogen CHB-35A question

[pdavis68]

I have a Bogen CHB35A that I plan on repurposing as a guitar amp at some point. I'll probably redo a lot of it, but I'd like to understand it first and I'm stuck on the 1st stage preamp. I've attached the schematic.

The 6EU7s appear to be fixed bias, in that the cathodes are grounded, but the grids don't appear to be lowered below 0V. What's up with that? How does that work?

The 12AX7s are interesting as well in that the cathodes are connected to the 240V via 220K resistor. I mean, I guess it's just a voltage divider, but I've never seen the cathode raised above ground that way. Anything different in terms of how it operates there?

 

[Enzo]

The 6EU7s are wired for what is called "grid leak bias." Look it up, there will be explanations online. Essentially the electrons from the cathode, as they stream past the grid will sometimes stick to it, and it builds up a small negative charge. This biases the tube.

Voltage divider from the 240v B+ should put a hair over .5v on the cathode. Just a way of making a stable voltage there.

 

[pdavis68]

Thanks Enzo. Is there any advantage to doing the voltage divider over just using a resistor to raise the voltage? Does it obviate the need for the bypass cap or do you still get the NFB without the cap?

 

[Enzo]

Oh, I am not into it that way. Here is a great place to start, Merlin knows his stuff and discusses things in general rather than some specific change to some specific model amp.

How to design valve guitar amplifiers