5e8-a tweed twin bias circuit questions

Blueshound

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Messages
413
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

Member
Messages
8,768
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

Member
Messages
10,691
... 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

Member
Messages
8,768
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

Member
Messages
113
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.

 




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