Sherwood 84-22

I acquired a Sherwood 84-22 a few years ago in a state of disrepair-deshabille might be more like it-and some stinkiness about which more anon, mostly because I liked its looks.

The Sherwood dates from the late forties and was probably sold by Montgomery Wards. It was clear from the beginning that it had been made by Danelectro back when they were in Red Bank and before they'd discovered the uses of Homasote as cabinet material.

The circuit is interesting, as it uses the filament of the first preamp tube as a cathode resistor, in conjunction with a resistance on the ground side-which keeps the filament above ground and dumps the detritus into the center tap of the filament winding. It also runs the 12SJ7 pentode pretty hot which makes for some crunchy sounds-not the typical late forties thing.

Along with the Rola field coil speaker, with modest voltages provided by the iron from the House of Freed, it's got its own groove that owes little to anything else of the era.

The overhaul resulted in some head scratching because the stinkiness got worse and finished up with a cloud of evil smelling smoke and lots of distortion. After puzzling over it I realized I had a similar amp in the collection and on opening it up, it was easy to see where the last owner had bodged the circuit which accounted for the stinkiness and lousy vibrato.

Of course, experimenting with odd circuit ideas was something that happened a lot in the House of Nat Daniel. I've seen several variations on this theme, including one which uses a power tube to modulate the field coil current for a Leslie type vibrato effect, and one that uses a potentiometer to adjust the preamp tube filament voltage for yet more cool Danelectro edginess.

The circuit is the same as the Silvertone 1303, and I've seen it in amps from Wabash, Noble, and Danelectro. As part of a cooperative project I dug up an original copy of the schematic and it was redrawn by Sean Weatherford of Bean Amplifiers, so that was probably the last one the seller got five bucks for.

It is now the world's property, and it would make a great home brew project. Why make yet another tweed Deluxe? This'll get you there in style, plus vibrato to boot.

I've got an 84-23 which is the same amp with an extra 8 inch PM speaker, and it's undergoing a refit as we speak.

How's it sound? Sharp, edgy, and full of fight. It's not smooth at all like a Deluxe but edgy and in your face like you'd expect a guy from Jersey to be.

Vox AC30CC2 Normal Channel Shutdown Hissy Fit

I've got a Vox AC30CC2 named Sophia Loren that is presently up for sale. This may be a cautionary tale about Resistentialism-which is the philosophical notion that things are out to get us. It also supports the notion that things have feelings and ways of punishing us.

I mean, you have to know that this amp has never been abused or scorned-it has never left my non smoking house either. It has, in a word, been spoiled rotten.

So a guy came to look at the amp and the normal channel quit working right in the middle of my sales pitch. Of course he more or less figured out at that point he was a Fender guy at heart, and so am I, which is why Sophia Loren was up for sale anyway. She was acting more like Roseann Barr at that point.

I got a little time today to dig into things and schlepped the chassis out and down to the laboratory. I started checking voltages and the preamp voltages seemed OK. Then a little grid disturbance testing led me to the first preamp tube's normal channel grid which produced no noise. Then, as if by magic the normal channel sprung to life. So, I tightened up the tube socket pins and retested and things were still on again, off again.

Looking through the holes in the vertical preamp chassis I could see that not only was pin seven-the one at issue-on the first preamp tube a lousy solder job, but pin seven on the second preamp tube was equally dodgy. I was able to reach through the holes and resolder the connections, and things are back to normal.

I know now that it's not right to threaten eviction when a needy and dependent lover is on the receiving end.

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.