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.

New Arrivals and A Project

A couple of new amps arrived here at mi ranchito for stays of indeterminate duration. One's a silver face Deluxe Reverb with the volume boost about which little need be said except that nobody ever uses the volume boost. It needs a little love in the form of a cleanup and a new vibrato roach which will be ordered this morning.

It remains to be seen whether I'll form a bond with it, although the Dragon Lady said "Oh. You can keep that if you like." Which was a big surprise, and it raises questions about whether I've got some kinda Stockholm Syndrome thing going on here but whatever-I'll take de job.

The second is far more interesting but enigmatic, and when I heard it had a 15 inch speaker I was hooked.

It's the Maestro M216 RVT pictured at left, and it's some sort of weirdo production from the Gibson works back in 1963 which was when it was made. The tube lineup is 5Y3-6V6-6V6-6EU7-6C4-6EU7-6EU7.

I've acquired it in a trade for a home brew consisting of a Motorola HS621 chassis mounted in an old Masco speaker cabinet with a 70w Eminence topping it off. It sounds surprisingly good and it appealed to the previous owner of the Maestro. The deal's done, and all that is needed to seal the deal is for me to hand over a couple of Franklins.

The circuit is unique and resembles only the GA19RVT Falcon in the Gibson Master Service Manual, but it is voiced differently-allegedly for accordion. It's single channel with swampy reverb and vibrato, but the amp just doesn't have enough volume. I spent most of Sunday afternoon trying different things to see if I could raise the volume level to acceptable levels-changing resistance values in strategic locations, installing a larger output transformer from a Deluxe Reverb, a solid state rectifier module and bypassing certain components to see whether the mystery could be unraveled.

All was in vain-even though I did find a resting place for one of my 1957 vintage Jensen P15R speakers. A general cleanup and chrome polish was had, and I subbed some Dakaware stove knobs because I just don't like those red pointers any. The knobs came from a tube tester I scrapped a while ago.

So, it seems I must study circuit design and topography to unravel this mystery. One clue that I have uncovered is that if the volume control is dimed, there is next to no white noise-which tells me something's rotten in Denmark.

Resuscitating A Silvertone 1396 Twin Twelve

I'm a sucker for projects, so when a friend brought me a Silvertone 1396 combo that had been in a barn for twenty years or so after being stripped of its speakers and grillcloth I was on board.

The owner represented that it had worked at that time but I find that belief problematic. There was no discernible preamp action, which I later discovered was because the cathode resistor upon which all in the preamp department depends had gone open. It fell apart in my hands, actually.

A preliminary power up with some borrowed speakers showed that the audio section and power supply was functional although needing a complete suite of new tubes and a thorough going over.

Down below there are two separate output transformers, each of which feeds its own speaker. It thus satisfies the Daniel penchant for weirdness and complexity coupled with cheapness of build. But nevermind.

Not so the top chassis-and thereby hangs a tale.

The top chassis contains the controls, a smallish phenolic slab that mounts eight capacitors, four resistors, a tube socket, and a single 12AX7 tube.

Looking at the schematic you can see that the filaments are in series with the cathodes of the power tubes-a favorite Nat Daniel Red Bank era feature.

The upper chassis is a nice cozy place, and it looks as if a field mouse or two shacked up in there for a while, using the upper chassis as a latrine and a source for things to gnaw.

Another feature of this amp is the blue green Sangamo capacitors, all of which showed unacceptable leakage and got tossed. I didn't even need to fire up the capacitor tester I built a while ago, as my trusty IRT2 insulation tester showed enough leakage to make that irrelevant.

The upper board required a complete stripout of all the components, a thorough cleaning and a rebuild that included a new shielded Belton tube socket.

After a few fits and starts and a bit of rework, it's as good as it will ever be. The speaker chores are being taken care of by a pair of period vintage Quam Nichols alnico speakers that came out of a PA enclosure I forgot I owned. They looked almost new, and I doped the somewhat light duty cones for good measure.

Finishing up the upper chassis I bridged the channels for more volume.

How's it sound? Bright and articulate, but the vibrato is slow and dreamy and it will take you to Link Wray "Rumble" territory in a short time.

It's not as in your face as the current day run of cheap amps, but it definitely is one to put on Santa's list. They're not the most common amps in the world either, mostly because they weren't built to last fifty years as this one has.