Monday, December 19, 2011

Repurposing A Courtesy Outlet

When installing a grounded three prong power cord on a Fender amplifier the question always arises "What in heck am I going to do with the courtesy outlet, which is itself ungrounded?"

It occurred to me a while ago that the courtesy outlet occupies the same footprint that a garden variety octal socket does. As I was in the process of replacing the two conductor cord on an AC568 Bassman, I also found that the power tube sockets needed replacement. It was a good opportunity to remove the hybrid cathode bias setup that Fender briefly used, and convert the bias balance to a simple bias level adjust system.

The only reason I can see that the hybrid system was used would be to ensure that if you lost your bias supply the amp wouldn't go into power tube melt down mode. It didn't last at Fender.

Checking the bias on a Bassman or similar amp configured as a head is impossible without removing the chassis, which can be problematic if changing tubes on the fly or between sets at a gig.

I took a replacement socket and installed two 1 0hm 1w 5 per cent resistors as shown. I wired each to the cathodes of the power tubes, and downstream I grounded the cathodes.

Now, setting the multimeter to millivolts will give cathode current in milliamps, and this can be read while the chassis is still installed in the head cabinet. In addition, each individual power tube cathode current can be read. Knowing the plate voltage allows the operator a safe range in which to operate, or alternatively to choose a spare set of power tubes for the road bag before leaving home.

You could bring out plate voltage to one contact but that could be a little hazardous with the number of little kids with sticky fingers that seem to always be underfoot. Better to leave it inside where it's safe.

In case you were wondering those screen resistors are Dale 2w metal film from Mouser Electronics. They meet the need and occupy a lot less real estate.


  1. My name is Aaron. I was brought here by your posts on Vintage Amp Forum about your Bantam Bass. I also own a Bantam Bass and love it but it needs more volume. I am currently researching how to install a larger OT. I have done a cap job, new tubes, speaker. I am hoping to pick your brain as the info on the net doesn't address real questions I have. Are there any circuit changes to accommodate the larger OT?Would a Super Reverb OT be as good? Mostly looking at the primary impedance(2000 vs 4000).Would the Twin's OT want to see a different secondary as the Twin has 4-6L6 instead of the two? Also, the use of the courtesy outlet for bias measurement is brilliant. Thank you.

  2. My Bantam has a Twin Reverb output transformer in it and there's plenty of real estate for it. Remember that because you're only using two power tubes the amp wants to see an 8 ohm speaker load. I had a Jensen P15L in it but I have recently installed a JBL D-140 and it sounds divine. I bought it this way and the circuit board seems to be pretty much stock. If you drop me an email at rwluedemanATmchsiDOTcom I'll send you the complete file. Great amps, highly underrated and cheap to buy.