Mark IV EQ on positive voltage?

[GainFreak]

Hi,

How can I modify Mark IV EQ to run on positive voltage? Should I just replace all PNP transisitors with NPN (BC550) and NPN with PNP (BC560C)? Will any rebiasing be necessary?

[loudthud]

That should work fine. Be sure to reverse any electrolytics and make sure any ground wires get moved to the proper rail. I'm not familiar with the transistor numbers you quoted, just stick with something about the same beta and voltage rating as the originals.

[GainFreak]

That should work fine. Be sure to reverse any electrolytics and make sure any ground wires get moved to the proper rail. I'm not familiar with the transistor numbers you quoted, just stick with something about the same beta and voltage rating as the originals.

Thanks.
BC550C is a low noise transistor with beta ~500. The original MPSA63 however is a darlington transistor so I guess I should replace it with BC516 not 560.

[loudthud]

The MPSA63 and BC516 are a pretty good match. I did note however that collector and emitter are not in the same locations. That design is very tolerant of transistor selection so I wouldn't anticipate any biasing problems. IIRC the original circuit came from a project published in Popular Electronics in the late 60's.

[GainFreak]

Mesa obviously likes this circuit beacuse I can see it in many of their schematics and it's in the sound path at all times which technically makes their amps somewhat hybrid