5e8-a tweed twin bias circuit questions

[Blueshound]

Just purchased a 5e8-a tweed twin clone. The amp does not have a bias pot, instead it is wired exactly like the schematic and Fender layout. The 56k resistor is in a really tight spot; just curious, what effect would changing the 3.3k resistor that comes before the diode have?

 

[zenas]

I built a 5e7 which is similar, a few less tubes but similar. When I did it I just put in a bias pot and that's what I'd do with your amp if I was working on it. Hoffman Amps web site has several bias pot arrangements to look at.

 

[HotBluePlates]

... what effect would changing the 3.3k resistor that comes before the diode have?

If you lower the 3.3kΩ the bias voltage becomes more-negative (less idle plate current). If you raise the 3.3kΩ the bias voltage becomes less-negative (more plate current).

Just purchased a 5e8-a tweed twin clone. The amp does not have a bias pot, instead it is wired exactly like the schematic and Fender layout. The 56k resistor is in a really tight spot ...

When I built a 5F4 Super clone using a Fnder-style eyelet board, I made the mistake of copying Fender's bias circuit. Later, I cut out the existing 56kΩ resistor, and replaced it with a 50kΩ trimpot in series with a 27kΩ resistor. I used a trimmer like the one below, and glued the small end opposite the screw to the fiberboard.

 

[zenas]

I did my 5e7 with a 10k pot and circuit like what the blackfaces had. Mostly because I've normally got those pots handy.

 

[Bandit127]

Swap the 56K for a 33k and add a 20k trimpot. I use a small, standard rotary and it has worked fine in a couple of builds. Despite my initial worries, it has fine enough adjustment and it is stable over time.