AGAweb Tech Tips
Wizard Amp Tips
|by Bill Kahle|
Just in case 'ya ever see one of these, this will save some aggravation:
The output as viewed on the chassis, (one power tube, a terminal strip, nfb tap & wiring, output jacks, and impedance selector), are too damn close to the turret-mounted preamp components and will cause a nasty, high F feedback when cranked up to typical rock volumes. It's a flaw that is acknowledged by the designer, and occurred for me on two different amps. The fix is rather simple by placing a grounded shield between the two sections. On both amps, I used a piece of approx. 3 x 5", 3/16" thick aluminum plate with about 1/4" bent to 90 deg. to allow for mounting. (Carefully remove a unused, press-in, chassis mounted stand off and mount the shield between the standoff hole and the closest output transformer bolt. Also, a little Loctite on the mounting hardware would be prudent since the shield is so close to the output section!) It will take care of the problem.
Also, since each preamp tube's wiring is tightly bundled to other elements, you may also experience preamp hum. I had good results by removing the ties and relocating a few grid wires.
All Wizards were designed for EL34's. There was a period when he used other tube types, ie., 5881WXT. If you wish to place '34's in one and the amp does not have 4 primary wires mounted on a terminal strip under the imp. selector, a rebias is all that is necessary. If the terminal strip is present, you must switch to the unused, lower dc resistance taps. I suspect that on those that had primary 4 wires, it's nothing more than a ultra-linear where he selected windings to better match plate impedance for the 5881WXT's.(?) BTW, regardless of which tube you use, get rid of this terminal strip! It will arc between the two inner plate taps,- almost (guaranteed) if you hit the front end with a signal while the output is open.
BTW, the build quality and tone from this amp is stellar! Since his stint with AC/DC has expired, Rick is back to building amps again.
I truly hope that my findings on these Wizard heads will save some tech. alot of aggravation! Afterall, we're all in this together... mmmmmm.....hhhhmmmm.