What The Mouse Done Ate: Reviving A Gibson BR6

This Gibson BR6, circa 1947 followed me home a few years ago from an auction in Mount Union, Iowa for forty bucks. When I got it home and opened it up I found a lot of gnawed wire and caps and one very dead mouse. No Bromo Seltzer was gonna  save his skinny ass.

I started the task of rewiring it and then for some reason put it aside until this week. All I really had left to do was to finish the power supply and the phase inverter which took a few hours of thinking how I was going to engineer it. The solution amounted to mounting some of the components on terminal strips. I glued a pair back to back and mounted them on the power transformer studs which is where the phase inverter resistors went..

I had decided to build it as a BR6F circuit, which used a pentode preamp tube, mostly because I like pentodes but there's another reason. The volume control in the original BR6 circuit is ahead of the first preamp tube grid which makes for a noisy and feedback prone amp, because it's running wide open all the time. You have to attenuate the signal with your guitar which cuts out a lot of the tone. Maybe lap steels are different but that layout isn't any good for guitar. I had the same problem on an old Flotatone I have, and in that case rewired it as a 5C3 Deluxe. It solved the noise and feedback problems so I knew it was a good fix. The BR6F is a much improved circuit.

I also installed a 250 ohm cathode resistor bypassed by a 22 uf electrolytic, which qualifies as a modification but is an improvement to my way of thinking. It probably would sound a bit more raw without the capacitor but I'm all about clean tone. It's good the way it is.

The wiring is not as pretty as it could be but remember, I started this project a number of years ago. 

Surprise! It fired right up at the first flick of the switch and now purrs like a kitten, which is a first for me It sounds pretty good within its limits and will make a good addition to the fleet or some trade bait.

I spent a couple of happy hours yesterday putting this amp through its paces and it's the equal, if not the better of any Valco of similar age and configuration. Aside from the Ruby 5AR4 blowing its wad while I was watching, it's a good performer.

Series String Amps: Demystifying the Widowmaker

NOTE: Some folks have suggested that this post may not state the facts accurately. In particular one correspondent suggests that an isolation transformer only insulates the circuit from the line, and then you become the ground path. Another suggests that the same result could be achieved with a GFCI plug. I'm going to get to the bottom of this, so stay tuned and be careful with these damned things.

Here's a circuit for a series string amp and in all respects it is typical of the breed although you sometimes see these with four, fiver, and sometimes more tubes. The idea's the same.

In a never ending quest for ever less expensive radios, RCA released model schematics for what are known as All American Five radios.

By putting the filaments in series and adding a suitable resistance, a power supply of a sort could be cobbled up without buying a power transformer. The typical offering in a guitar amp works on the same principle. Get as close as you can to 120v with the filaments in series, add in enough resistance to make up the difference, a couple of good stout electrolytic capacitors and use the chassis as the return path and Bob's yer uncle as the Brits say.

Well. In a radio it's usually sitting on a shelf inside a plastic or wood cabinet and you only touch a plastic knob so what's the worry? The Arvin folks, known then, as now for their metal stamping expertise released a line of metal case radios using this technology but did a baked enamel finish that is a pretty good insulator. Sometimes I cringe when I see people who have stripped and chrome plated these sets but nevermind. The subject's guitar amps.

The evil here consists of two things-the chassis as a return path for line voltage, and the guitarist or harp player's intimate connection to that return path through the guitar or mic cable. Standing barefoot on a cement floor or wearing sweaty leather shoes only makes it more dangerous. You can get the whole 120v of AC through your hands or mouth and it CAN kill you.

There is one, and only one, way of making this safe to use and that is through the use of an isolation transformer. That is a device which ISOLATES the circuit thus described from line voltage and thus saves you from being the ground return path. There is NO other way. A three prong cord will not make you safe.

You can obtain isolation transformers pretty easily from a number of sources or, you can make your own using a pair of Radio Shack door bell transformers back-to-back and accomplish the same goal.

Having said all that, these can be really fun amps to play with, but be advised: They're not really safe in their unmodified state.

UPDATE: One of my colleagues has posed the question of using a GFCI self contained cord with these amps. I myself have not used them, and the idea seems to be OK, but I think the difference is that with a GFCI setup you're interrupting a potentially dangerous problem, and depending on a relay to do it. I think the difference is that relays do not always do what you want when you want, and the problem is always there-a hot chassis connected to one side of the line. With a suitable isolation transformer, you eliminate the hot chassis problem completely and totally forever.

Thanks to Clark Huckaby for the use of his schematic. Just sharin' the wealth and keepin' it real, folks.The isolation transformer setup is the work of Dennis Poirier. A great fellow.

Building a Solid Workbench Addendum

One of the things everyone should have in their shop is a good solid work bench. I was getting by with a $40 Chinese made plastic topped banquet table that I got at Menard's, but soon after I put it together the overhang end started to gradually fold and collapse. Last week I knew something had given way because the end of my bookshelf was sagging badly.  The corner was folded under and fractured, and total collapse was imminent.

I built a workbench for the garage a month ago using the plans I found here and decided that it could be adapted to my particular space considerations. Although the plans call for rather tall legs and a long top, I made mine with 30 inch high legs and a 60 inch surface. You can adjust the dimensions any way you like to fit your particular needs and whether you plan to work standing up or sitting down.

For this kind of work it is better to attach the top with flush surface wood screws which I did. Then, if you like, you can attach a sheet of tempered masonite for hardness.

The biggest cost items are the Simpson Strong Tie RTC24 corner braces, which run nearly six dollars each. That puts you at about $55 with braces, hardware and a tube of construction adhesive. The Simpson people have been making structural connectors for wood structures since the 1950s and their products are good stuff. When you're done, you will have a very strong and rigid workbench or work table.

It takes a couple of hours to cut your lumber and put the bench together.

Here's a picture of the bench in use.

Fender Excelsior, You Fathead

These are pics of my recently acquired Fender Pawnshop Excelsior. Like others I have undertaken a few minor modifications to make the amp more to my liking.

The first was adding a tone control to replace the two position bright-dark switch. A 500k pot is installed and wired just like the switch with the existing wiring. You can widen the switch hole a little with a small file or two, and then use a fender washer to dress up the area and cover the screw holes. Be careful not to scratch the surface of the panel with your file-I did, and I'll probably strip this down and have it electroless nickel plated before long. A trio of vintage chicken head knobs dressed things up a little.

I don't know about you but I don;t like orange pilot lights. I snipped the wires to this eyesore and installed a typical Fender type lamp socket with a red jewel and a #47 bulb. I piggybacked off the wires for the second 12AX7 preamp tube socket which read a nice 6.7 volts. Plate voltage runs about 370 on these amps by the way. At the same time I added a pair of JJ 6V6 power tubes and a Sylvania JAN 12AX7 in the first position.

How's it sound? The tone control gives a little more flexibility and the tube upgrades are useful. Plus Leo would have approved of the red jewel.

Plus you get to see my newly reconstructed shop complete with killer work bench I built Sunday.

My apologies to the ghost of Jean Shepherd-if you spent any time connected to a radio in Jersey youy get the joke.

Marshall Arts-Reviving a JVM210

This recent iteration of the 100w Marshall head arrived with no output. I surmised it had a blown power tube and thereby hangs a tale. A couple of tales, actually.

With today's move to more solid state devices and channel switching, well, you gotta get power from somewhere to run the little devils, and it's not always obvious to the eye when something in that line fails. I've also noticed that in the time it takes for a fuse to respond, a lot of damage can be done. Such was the case with a Peavey Valveking that I worked on a while ago.

Opening the chassis up I found a bulged power supply capacitor C49, a blown out bias resistor that had scorched a nearby capacitor and relay, a resistor that had turned from blue to brown R81, and I figured replacing those items would be required in any case and might fix things as well. In addition a new fuse and a set of new EL34s were installed.

The net result was zero. A quick check plugging a guitar into the effects return jack proved that the power section was working as advertised so the problem had to be somewhere else.

I was stumped.

As usual I happened to be trolling my favorite amp tech type chat boards and found a post by the ineffable Bill Machrone, a/k/a Billm, who's more or less the god of Blues Juniors. It was entitled

"How to make an audio test probe."

I was intrigued because I'd built something similar but a lot more elaborate than it needed to be. This was cooked up with a small amplifier from Radio Shack, a chunk of coax cable, a .01 capacitor, and a bit of heat shrink tubing. I knocked it out in about fifteen minutes. I already had a Leader audio signal generator from the last project-and it's a good one. Leader is some of the best test equipment i have ever seen at any price. This one cost about 25 bucks.

I fired up the amp and plugged in the signal generator. The first thing I noticed was that the tone didn't get past the first preamp tube .A quick voltage check revealed that the preamp tubes were not getting any high voltage although the phase inverter tube was doing fine. At this point I found it very useful to draw out the B+ circuit to see where I could intercept voltage, test for continuity and thus localize the problem. It was pretty quickly narrowed down to R82, a 15k 1w flameproof resistor that turned out to have about 100 megohms of resistance. A new resistor put voltage in all the right places, and luckily enough no other damage had been done.

The images show the parts that ended up being replaced and the signal generator and tracer rig.

The takehome for me was Bill's clear and concise explanation of what his signal tracer could do and how it could be used. It is HIGHLY recommended.

Shit Happens, Part 2.

If you've been following this blog any, you already know what I think about people who won't circulate their schematics and support the trade. It isn't good, either.

Notable exceptions to this trend are Fender, Peavey, Marshall, Traynor, Rivera, and for all I know others. If you know of an amp maker that supports the product with technical services and literature I would be glad to acknowledge that. After all, the motto we go by here is Joe Hammond's Maxim:  "That crow don't taste too bad if you use enough salt and pepper."

There are two reasons, I think, for the lack of support. The first is to steer repair traffic to so called "authorized service centers". The second is, I think, to make the product as cheaply as possible and thus disposable, using a skeleton crew to distribute and merchandise the product.

This creates difficulties for people like me, because without schematics and a technical support department, the range of repairs I can tackle on such products is limited, and the amount of time figuring something out is expanded. It also creates some resistance because everyone who buys one of these turkeys (not naming names here) and has a problem past when the warranty expires ends up getting hosed and has to dump the former amp at fire sale prices. That is, by definition, someone who is a) not happy and b) will remember who screwed him or her.

I was recently repairing a Vox AC30C2, which is the latest iteration of the venerable AC30 amp. It's quite different than any Vox that preceded it because it uses a single circuit board, has a handful of ICs, no tube rectifier and the cabinet is made from MDF. Without a schematic you can't figure out what's going on in a month of Sundays.

Here's some correspondence:

I have a customer with an AC30C2 that needs repairs. Is there any chance of getting a schematic diagram for this amplifier?

Thanks

Robert Luedeman

2008 66th Street

Windsor Heights, IA 50324

Here's the answer I got.

Dear Robert,

Thank you for your inquiry. Schematics are only available for authorized Service Centers. If you would like to become a VOX-authorized Service Center, please contact our Customer Service department at (631) 390-6800 or customerservice@korgusa.com.

Best regards,

xxxxx

Product Support Specialist

P:631.390.8737

F:631.390.8738

(Monday through Friday, 1pm to 5pm Eastern Time)

So there you have it folks.

And now, for something completely different

 

This is a little outside the ballywick of this blog but it may be relevant for someone, and electrons were involved.  I have a 1976 Kawasaki KZ750B1 vertical twin which followed me home one winter day back in 2010.

I didn't work on it at all in 2011 but this year I decided I was going to get it up and running and evaluate its mechanical condition prior to deciding on a modification plan. The battery was completely defunct so I obtained another and found that the starter was very weak.
I got a set of starter brushes and earlier in the week I decided to tackle an overhaul.

To remove the starter one has to remove the alternator cover to access the starter sprocket, remove the two mounting bolts and the battery lead, and slide it out to the right with a little help from a mallet and a soft drift.


But wait! If that's what you do, you'll never unscrew the two long screws that hold the starter together. All you'll do is strip the screw heads. So do this before you unbolt and remove the starter like you should have done in the first place.

So get yourself a proper sized phillips head bit and a socket to hold it, a couple adapters and a half inch breaker bar or a good healthy speed handle. Dab some coarse valve grinding compound on the bit, seat it properly in the screw head so it doesn't slip, lean into it and tighten it a little-that's right, tighten it. It'll break the hold of corrosion and then it can be easily unscrewed.

While you were reading this it should have occurred to you to use this old aircraft mechanic's trick on all the alternator cover screws. Otherwise you'll just wreck the screw heads with that idiotic impact driver you bought. If you've ever had to pull the wing planks on a Falcon 20 you'll never forget this trick, and you'll be glad someone taught it to you.

When disassembling the starter pay particular attention to the illustrated blowup from the Kawasaki manual and where your shims are located and take pictures if necessary. When that little item marked 22 in the picture falls out

DO NOT THROW IT AWAY!!!!

Clean the parts with white gas a/k/a Coleman fuel and blow them dry. Then you can test the armature.  Once you've tested and cleaned it with emery cloth and cleaned out the schmutz from the commutator grooves you can put new brushes in and assemble that end of the starter.

Putting the drive end together is a little dodgy. It only goes together one way, the body and housings are marked with index lines, and number 22 gets inserted in the appropriate place in the proper direction. Make sure and grease the planetary gears and sun gear.

Now, you can reef those screws down because without adequate torque on them the sun gear will slip and spin around. Put a screwdriver through one of the mounting holes on the commutator end housing and position it against something solid like the floor. Then use your disassembly rig to tighten the screws up.

Lastly, test it on a battery, and if it runs install it and be happy. You'll probably find that the starter clutch is shot, too, about which we shall speak more anon.