JCM800 Superbass individual bias

[azatplayer]

Just had a 800 bass on my bench for a couple dodgy pots. Checked the bias while in there, and it has individual bias pots for each side.
Its a slight repositioning of components and the addition of one more part, another trimpot. Very easy mod to an existing marshall style bias circuit.
The thing was way out of whack, one side had 15ma, the other 21ma, both way low. Corrected it and the thing sounded great.
Question. Is the bass bias setup kosher? Its too easy to reconfigure an existing setup, but are they stable? Just asking if they have been reliable.

Im in the midst of a marshall type build, using a PPIMV as well, looks easy to run the dual bias trimpot feeds directly to the dual gang MV pots. As the pots replace the 220k grid leaks, will be very tidy.

http://www.drtube.com/schematics/marshall/1992.gif

[greenbottle]

Check out Kevin O'Connor's TUT vol 3 chapter 8. It deals with Marshall
800s. Not specificaly with bass amps but there would be very little difference lead series and bass series:- perhaps a few component value differences in the preamp.

You can get a copy from www.evatco.com.au in Queensland. Kevin's books are a bit expensive but then again good advise usually is.

Alan

[martin manning]

Is the bass bias setup kosher? Its too easy to reconfigure an existing setup, but are they stable? Just asking if they have been reliable.

I have owned a Marshall 4001 (another JCM 800) with this set-up for several years. It gets played almost daily and it works great. As a mod to an existing bias circuit where the bias voltage comes off the trimmer's wiper, it amounts to paralleling the element of a second trimmer across the existing one. In reality you need to replace the existing pot, and make both new pots twice the original's value to get the same voltage range and current flow through the divider.