Samick Motherlode

December 2014
more... Ask Amp Man

Firing Up A Vox

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I have a question about an old Vox AC50 Super Twin head. I’ve had it a long time and never fired it up. I want to sell it, and need to hook it up to a speaker to make sure it works. The amp has two XLR 3 pin connectors and I need to know how to tap off of the original wiring to go to a regular 1/4” phono plug speaker receptacle. Also, is there anything else I need to be aware of?

–E. J. Lazar


E.J.,
First let me say that if this head has not been fired up for “a long time”, especially considering its age, there’s a good possibility that the filter capacitors may be dried out. You should consider having it checked out by a knowledgeable tech so that he can at least power the amp up slowly using a variac in an attempt to re-form the caps. They may still need replacement (along with the amp needing a thorough cleaning), but it may save you the surprise of the caps “venting” when you fire it up. This is generally evidenced by a loud hissing sound or very significant “pop!” followed by large plumes of white smoke and the fuse blowing. While this might look very cool on stage during your last song, it’s really not something you want happening in the confines of your home. That said, here’s how to make an adaptor for your AC50.

Purchase an XLR connector from your local music store or electronic shop (or cannibalize a mike cable). Make sure it is of the proper orientation (i.e. male/ female) to mate with the connector on the amp. Using a length of 2-conductor wire, connect one of the wires to both pin 1 and pin 2 in the connector (the two horizontal pins, they should be numbered). This should be the common connection. Connect the other wire to pin 3 (the bottom of the triangle if you will). This should be the hot or “+” connection. On the other end of the cable, connect whatever ¼” configuration you wish, plug or jack.

The stock wiring of the head is 8 ohms, so try to use an 8-ohm speaker load. No big concern if you’re just going to test the amp for functionality, but for any extended period of raucous high-volume rock-n-rolling, the impedance really should be matched.

Hopefully that will “get it sold” for ya’.



I have a Lab Series L5 with an Accutronics two spring long replacement reverb tank. Can a reverb be installed incorrectly? Physically the reverb tank appears to be symmetrical and it is impossible to discern input from output or if it makes a difference. The leads from the amp are molded red and black plugs, which suggests an order to the assembly. Switching the leads did not change the sound. Can any harm be caused if reversed? Thanks in advance for any information that will ease my mind or allow me to assemble this little baby correctly.

–James S.


James,
OK, I don’t usually do this, but I’m going to answer a solid-state amp question for ya’, only because this amp was favored by a couple of well-known guitarists.

To answer your first question, yes, a reverb tank can definitely be installed incorrectly, electrically as well as sometimes physically. As far as the tank being “symmetrical”, that’s incorrect. There definitely are specific “inputs” and “outputs” on most all reverb tanks, and yes it makes a difference. You mentioned that this is an Accutronics tank. The input and output connections are almost always marked on these tanks, most likely embossed next to their respective jacks. Physically, the tank should be oriented so that the output of the tank is farthest from the AC mains transformer. This generally minimizes any hum induced into the tank by the transformer.

The fact that you hear no discernable difference either way you hook it up leads me to believe that it may be an incorrect tank. According to the schematic, the input impedance of the tank should be 8 ohms (with an isolated ground connection), and the output impedance 2250 ohms. This, surprisingly, is a typical set of impedances for a tube-driven reverb system such as a Twin Reverb. My guess is that at some point, someone installed a tank with what would be typical solidstate input and output impedances. Without the part number of the tank I cannot say for sure, but my guess is find the correct tank and you’ll have a properly operating reverb. According to the specs, a 4AB3C1B Accutronics tank should work fine.

Now play on, and make Holdsworth and Tabor happy!




Jeff Bober
jeffb@budda.com
www.budda.com
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