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.

Son of Judybox Mark II

I completed the second Son of Judybox, a/k/a Vamp tonight and at this point it is an unqualified success and ready for prime time.

If you've been following this saga you know that I was able to find a way to reuse the original Chinese made Ningbo Chaobo power and output transformers that the Champion 600 is equipped with to make this project a lot more affordable-at least $68.00 or so, depending on where you shop.

The problem was that the original power transformer had no 5v rectifier tube winding and the secondaries have no center tap. This was solved with a modified Graetz bridge and a 6X5 rectifier tube as indicated in the Yes You Can posting a few weeks ago, and a good solid 330v of DC and 6.4 filament volts was obtained with everything up and running. You could probably use a 6X4 as many Silvertones did, but it is a lot easier to wire up an octal socket-there's more room to work-and you have an octal socket sized hole ready made for the purpose already where you want it.

The schematics for this setup have been posted here, and the only change would be to add a pair of 100 ohm resistors, one on each leg of the filaments for noise reduction as the original power transformer filament winding is without a center tap either. I wired the filaments with two wires, where the original 5C1 uses the chassis for one side of the filament string.

Another useful thing to remember is to use a grounding jack for the input for noise reduction.

To summarize, then. The original speaker jack was removed and a fuse holder was installed after opening up the hole. Two holes were punched in the chassis to accommodate the two additional octal sockets. Care has to be taken to avoid interference between the 6V6 in the center and the speaker basket or magnet.

A piece of perf board did service for mounting the electrolytics and dropping resistors, and the rest of the components were wired point to point. The lead from the jacks to the preamp tube is very sensitive to interference, so I shielded it with about thirty turns of hookup wire wrapped around a pencil and slipped it over the lead and its capacitor.

At the same time I installed a 1 meg audio pot for the volume, a standard Fender pilot light assembly, and reused the original power switch, saving an extra three bucks there.

The last bit of the puzzle was getting the sound right. I first tried a Weber 6x9 inch ceramic speaker but it didn't get the job done.

Luckily as I was up in Minnesota hunting in a dead TV repair shop last week I uncovered a nice 1957 vintage 6" Rola radio speaker and that does the job just right.

Another good idea is to remove some of the aluminum tape shielding in the cabinet above where the fuse holder sits-otherwise you can get a few sparks if the amp is jounced pretty hard.

You can have tube rectification with all the sag and touch sensitivity that conveys and still retain the original iron which seems well up to the task. You save enough dough that you can go out and get yourself a big hefty output transformer and still have thirty bucks to buy some beers.

Fixing a Dead Standby Switch on a YCV40

A Traynor YCV40 arrived recently with a failed standby switch and it presented an interesting problem. Seems the switch actually controls a FET (field effect transistor) that controls the B+ voltage, and a shorted power tube will kill the FET.

The fix according to Yorkville was to replace the FET (Q8) a small 10v zener diode (D13) and an integrated circuit (U3). U3 is the little white square in the picture, Q8 is the transistor on the right and D13 is too small to see easily but if you start at U3 and count to the right it's the sixth device, to the right of five resistors. Make sure you check the part numbers on the FET and the IC because in this case the amp I was working on used a different device-note it has only six legs. Make sure you use some fresh heat sink compound under the FET.

Q8 is an IRF 830 FET and U3 is a 4M35 integrated circuit. Don't quite remember the part number on the zener but it's in Traynor's excellent service manual.

Also these amps were built with lead free solder so be prepared.

Champion 600 modified schematic Mark II

Here's the complete schematic for the low cost 5C1 retrofit using a donor Champion 600 and retaining the original equipment transformers. If you're building this make sure you position the 6V6 so as to have adequate clearance between the tube and the speaker basket.

ERRATA: I got the diode numbers screwed up here so just go ahead and substitute the omnipresent 1N4007 and double them up for safety's sake.

Yes, You Can and the Champion 600

Part of the Mark II Champion 600 to 5C1 modification program was intended to lower the price of the modifications substantially by reusing the original Ningbo Chaobo power and output transformers.
That presented a smallish problem because the power transformer was without a center tap or the more typical 5v rectifier winding.

The first objective was to wire up the filament circuit and see if the 6v filament winding had enough stamina to survive. This was done by comparing the unloaded and loaded voltages. Unloaded the winding produced 6.7 volts ac. Installing three tubes-a 6X5, a 6V6GT, and a 6SJ7-and powering up produced a voltage drop of only .3 volts, and it stabilized at 6.4v which was serviceable.

The second objective was to see if there was a way to use the transformer with the rectifier tube. A bit of looking around and chatting to people led me to a drawing of a hybrid power supply on diyAudio drawn by A. Ciuffoli, which is sometimes called a hybrid Graetz bridge.

It consists of tying the plates of the rectifier tube to ground by means of the diodes shown. I used the 1N5408 which is a bit heavier duty than the omnipresent 1N4007 diode, and then I doubled them up for good measure. Connecting pin 8 of the 6X5 tube to the load produced a good solid 360v of DC right where it is needed.

With all tubes installed and a load connected, I'm getting 330v-316v-273v on the B+ rail which is plenty for this job. By comparison, the original solid state rectifier setup in this amp produced 360v according to my notes, which puts this right on the money. According to the Sylvania technical manual, the voltage drop for this tube should be about 22v, which means I'm close enough for guv'mint work.

So. The proof of concept is good, much money will be saved, and you heard it here first.

Thanks to Mr. Ciuffoli, wherever he is, and the folks over at diyAudio who think hard about such matters.

ERRATA: I got the diode numbers screwed up here and on the schematic so just to be on the safe side use the omnipresent 1N4007 for the legs from the HV to ground and double them up as I did.