Koko Taylor

Fixing Stuff, or, How I learned To Stop Worrying And Love The Transistor

A journeyman musician of my acquaintance left a Music Man guitar amplifier here for repairs and I finally got around to digging into it last weekend. After some routine maintenance type repairs that any 30 year old amplifier would need, I figured I couldn't avoid troubleshooting any more.

The procedure for setting the power tube bias voltage level is to measure the voltage developed across the two resistors in the picture and adjust the level with the potentiometer that is in the tip of the picture.

Doing this revealed that there was no voltage being developed on one tube. Releasing the circuit board from captivity revealed that one of the bias transistors had a broken lead.

At least I had the common sense to order up some integrated circuits and bias transistors while I was dithering over what to do.

It works great.

KittyHawk M5 Gut Shots: The Area 51 Report

Seeing as Sean Huston's Blues Deville is going out on the road here, I decided it was time to start refurbishing the Kittyhawk M5 you see here.

This amplifier is some kind of ultimate orphan, as very little is known about them and the German manufacturer is long since out of business, as far as I know. I do have a partial schematic and I can probably figure out the rest. Some voltage mapping will be in order, and then eliminating the patented plastic "DestructoPots".

If the power transformer is capable of it, it might be fun to plug in a couple of the new JJ 6V6S tubes, as we hear they're more robust and capable of delivering an improvement in wattage.

If you've got any information or documentation on this amp, please send me an email.

We Make What We Make When We Make It And We Can Call It Whatever We Like

We are, of course, referring to the newly released Vox AC15C, and thereby hangs a tale.

The star-crossed history of the AC series of amps from what we'll call the Vox people is pretty well documented in several nice coffee table books, the most recent by Jim Elyea for which I've yet to scrape up the C-note that it'll cost to get it here.

When the Jennings empire folded after an ill advised toxic marriage to the Thomas Organ Co. in the US-a classic case of brand dilution that many a business school MBA ought to take note of-the manufacture of the flagship model AC30 passed through several owners with a gradual decline in build quality until it was naught but a shell of its former self.

When the Korg people picked up Vox in the mid nineties, they set about rehabilitating the brand, using the signature AC15 and AC30 as leaders for a line of forgettable modeling amps that are suited for lesser tasks, but the AC15 and AC30 retained much of what people bought them for.

When Korg reached the ultimate decision to move production out of the UK, they moved it to a factory in China owned by something known as the IAG group-a vertically integrated facility that actually makes a lot of the components that were used in the AC15 and AC30 Custom Classics as they were known. Although built on a printed circuit and incorporating solid state reverb drive and vibrato, they held true to the original Dick Denney inspired dual chassis build.

According to Denney and Peterson's "The Vox Story" Denney built the prototype AC15 on a standard box chassis but realized that they couldn't take a pounding on the road. Ordinarily the answer would be to build on a steel chassis but because the amplifier is high gain, that presented a risk of undesired hum and oscillation. Denney's solution was to put the power supply on a steel chassis and the higher gain stages on an aluminum chassis attached at a right angle to the power supply chassis-all of which served to reduce crosstalk and made the resulting chassis robust and easily serviceable. Air circulation and overheating was always a problem however.

Apparently there was some sort of a falling out between Korg and the IAG Group, as evidenced by the departure of Steve Grindrod, the designer of the Custom Classics, to employ with Wharfedale, an arm of the IAG Group, and the departure of AC15 and AC30 production to an as yet undetermined production facility.

The new amps are being made with MDF cabinets rather than the Baltic birch plywood of the IAG built amps, no more Wharfedale speakers (fancy that), but what's notable is the layout and construction of the chassis, which appears to be a much more production oriented, reduced labor setup. The two chassis approach is gone, perhaps for good.

Which brings us to the ultimate question: When is an AC15 not an AC15 even though the people who make it and are entitled to the name call it so?

We haven't yet seen gut shots of the new improved AC30C, but if what we see here is a portent, it's going to be quite different.

Photos courtesy of ampaholics and oddjobpeters, whose PM bounced.

Sorry, man, they said you don't exist.

Big Joe Has Come and Gone

Did you ever have one of those moments when you're looking for something but you don't really know what it is, and then you hear it? Guitarists do this all the time, and they call it looking for the sound inside your head.

Me and Big Joe-we're like that.

Joe Turner was born in Kansas City in 1911, which would figure large in his musical evolution. Kansas City was a juke joint, barbecue, boogie woogie piano town like no other that has ever been.

Folks from Chicago may cavil and protest, but before Chicago was, Kansas City is. Los Angeles and New York City never were, and the only city that compares is New Orleans.

The music reflects the city-and while New Orleans is all weddings, parades, and funerals, Kansas City was all business when it came to music.

Joe was never a singer of country blues as many other bluesmen were, because his roots were urban and fixed in the time of pre war big bands and boogie woogie.

His blues wasn't hip and urbane like T-Bone Walker's was, or tongue in cheek like Louis Jordan, but its grit and power is unmistakable, like the roar of the daily nonstop 747 from Los Angeles to Paris that struggles off the runway, rattling windows for miles around.

A big man at 6'2" and north of 300 pounds, his music projects like some primal, unstoppable force that drags you kicking and screaming to your feet.

By the time rock and roll came around in the fifties, Joe wasn't a teenager any more and although he had some songs that made it onto jukeboxes, other folks like Bill Haley mostly made the money.

Bill was no spring chicken either, but a galvanized cowboy singer and deejay who knew a good thing when he heard it, and ran with it.

Joe was, in the end a product of the prewar era and his time had come and gone-at least for a time, until an new generation of people like me rediscovered him in the back shelves of dusty used record shops in out of the way places.

I have this mental picture of Big Joe leaning against his brand new Hudson Terraplane looking sharp in his suit and snap brim fedora, thirties all the way.

More Judybox Revival

Sometimes you stumble over something, and as you follow the trail, an entire new world is revealed. People who dig quirky guitar amps are no different, and that led me to the Judybox-partially because it's named after my wife. I decided I had to have one.

The story's quirky. The amps were developed by Al Nelson down in Austin, Texas about six years ago or so, and that's where you'll find most of them. A pretty big splash was made at the 2006 NAMM show which was held in Austin that year, and from what I read on TGP and other forums, the wheels started to come off the project soon after. Deposits were taken and whether they were returned or anyone got amps is anyone's guess.

Suffice it to say there were and are a lot of unhappy people in the Austin area who'd ordered amps and didn't get them.

There are stories of illness and mismanagement, a transfer to shadowy investors, some or all of the assembly being done in China, and that's pretty much where the story ends, in 2007.

The registered address of the business, 8804 Chisholm Lane, looks a lot like a residence with a stable out back. It's owned by a gentleman named Stanley Phillips and has been in his hands since 1972 according to the Travis County Assessor.



Entity Information: JUDYBOX, INC.
8804 CHISHOLM LN
AUSTIN, TX 78748-6378
Status: NOT IN GOOD STANDING
Registered Agent: AL NELSON
8804 CHISHOLM LANE
AUSTIN, TX 78748
Registered Agent Resignation Date:
State of Formation: TX
File Number: 0800639464
SOS Registration Date: April 10, 2006
Taxpayer Number: 32019547028

Well. What's the story on this orphan?

It's cathode biased, two channels each with its own volume, bass and treble, and reasonably well equipped although looking like it was done in a bit of a hurry. There are no identifying marks on the transformers which look a bit Asian. There are extra taps on the power transformer that could prove interesting.

The tube layout is conventional: 5AR4, 6L6GC, 6L6GC, 12AX7, 12AX7, 12AX7. The voltages are relatively low, with a B+ of only 369v and 309vAC on the legs of the power transformer. Some of the fitments are a little cheesy-the pilot light and power inlet socket are pretty much low grade schmutz plastic, and shot, but overall it looks pretty decent.

Sound wise, this amp does clean in a big way, and it's got a nice deep bass tone that's right for single coil pickups, particularly P90s. The original speaker's been replaced with a Celestion Super 65 that gets the job done well.

Where the story ends, I don't know. If you've got some information feel free to post a comment.

UPDATE 4-7-11
Since I wrote this piece I've been doing the work required to get this amp in serviceable condition and here's what's been done so far..

I replaced the pilot lamp assembly with a military surplus item that the original looked like it had been copied from, got rid of all the ceramic resistors except the cathode resistor, replaced the electrolytics with suitable replacement values, and went up to the hardware store for enough hardware to cook up proper standoffs. This required modifying a few holes and drilling others, but now the board is level, well supported and not preloaded at all.

When you think about it, the screen resistors and grid stoppers do not need to be 5w ceramics, and the dropping resistors do not need to be 10w. 7w and 5w ceramics. Fender did just fine for the last sixty years without that stuff. Plus, smaller resistors make for a cleaner installation and that's the look I'm going for.

Even as we speak the UPS man is supposed to be bringing me a new power socket to complete the job. Then, I can reassemble it and survey the grounding.

One interesting thing I found was the control pots-they're all linear, made by Cosmos Tokyo and very nice looking stuff. I shall try and find a source.

Judybox Revival

I finished up the Judybox Revival amp and powered it up today. It got topped off with a nice set of vintage Dakaware chicken head knobs I'd been saving for a special occasion.

Here's what I did to it.

Replaced all the ceramic dropping resistors with a 3w metal film, a 2w metal film, and a 2w carbon comp.
Replaced the ceramic 5w screen and grid resistors with 2w Dale metal film and the grid resistors with 1/2 w carbon film from Rat Shack.
Replaced the pilot lamp assembly with a Fender item. This was broken
Replaced the power receptacle. This was broken as well.
Replaced the 40-22-10 e'lytics with 47-22-22 450v items
Replaced the three ceramic octal sockets with bakelite.
Installed 5 standoffs of the proper length to support the board level and not preloaded.
Revised the ground scheme to a star ground setup
Replaced the chassis screws with new stuff from the hardware store.
Installed a smooth plate Tele in the first position.

How's it sound? About the same-bright and sparkly with a lot of depth on the bottom end. Sounds a lot like an AB165 with the bass and treble dimed and the switch on bright.

The builder used Tocos (Tokyo Cosmos) pots-very good stuff-but they're all linear. I may change them out for audio taper, at least the volume controls. All the connects from the pots to the board are shielded wire. I didn't see a nfb resistor but maybe it's there somewhere. I'm thinking that the first two preamp tubes share a common filter capacitor, because if they did not there would be four and there are only three e-lytics. Both preamp tubes have small electrolytics on both cathodes.

I don't know whether I'm qualified as an expert-I'm in the middle of doing a schematic layout and a bill of materials and when I'm done I may be an expert in which case if you find this drop me a line.

I am in a position to make some recommendations in order of importance.
Ditch the power receptacle and pilot light for something good.
Install proper standoffs to support the board and level it out.
Dump all the ceramic resistors except the power tube cathode resistor. They look stupid.
Revise the grounding to a star grounding common-it was strictly from hunger.
Replace the octal sockets with bakelite, Beltons, or military Amphenols

If you've got any questions about this amp, drop me a line.