Biasing The Vox AC50

Every now and again you tumble across something that stands head and shoulders above the rest, and I had that experience the other day with a JMI produced Vox AC50, circa 1965.

It came in to have the bias tweaked as the owner had had a power tube quit and replaced them. Then, the sound disappeared.

Being no slouch, he also knew enough to have the installation finaled out with a bias setting and a quick visual inspection to make sure nothing had been damaged. The problem consisted of a blown B+ (or HT if you prefer) fuse. Good work by the design engineers to fuse this important circuit, and many a Fender could have been saved from major repairs if they had incorporated this idea early on.

In any event, this amp has a hybrid bias feature that was universally scorned when Fender tried it for a brief moment back in 1969-1970, and that is a fixed bias supply and a whacking great cathode resistor for each power tube.

I've yet to figure out just why this feature was used and discarded because it does work pretty well and it also offers some protection if the fixed bias supply quits on the job. You won't have a melt down.

In any event biasing the AC50 is a relatively simple task as the factory recommended setting is measuring 2.2 volts across the big cathode resistors and adjusting the individual bias level controls to suit.
It's noted right on the schematic there.

This is without a doubt one of the best clean sounding big amps I've ever heard. It's got all the sparkle of a classic Fender, and the midrange is more thoroughly developed as well. I missed the chance to try my Gretsch through it but I bet I woulda found what defined George Harrison's distinctive tone.

Fact is, I may just build one this winter. Stay tuned.

Is Your Item Still Available? A Record Of Correspondence With A Craigslist Scammer

I'd forgotten all about this.

Read from the bottom up. This was a lot of fun.

what are you trying to do to me? I thought you were my friend, steven. Then I find that your other email account jemjim20@gmail.com is out of service?

I didn’t hear back from you I sent you this message so I looked in my email directory and found your other email address
Well, I’ve changed my mind but I want to know why you want all my personal details ;like my account numbers address and all. Y’see I found this guitar in a closet in a house that was being demolished it’s older than 1995 and I just know that the Lord Jesus musta give it to me for a reason because I’m mostly homeless and to tell the truth I broke into the house and I was sleeping in there until the cops ran me off. I do have a laptop but no permanent address but what I do is sit in a McDonalds and use their wifi. I can meet you in downtown Des Moines, if you hand over the cash I’ll go and get the guitar from where I have it stashed and I have prayed a lot and I figure jesus and buddha will be ok with me doing this even though the name inside the guitar case reads Gene Vincent and the Blue Caps. Now who on earth woulda worn a blue cap?
My real name is randy.
I have ttell you that the stratt was not the only guitar I found in that closet the other one was something called a rickenbacker which if you ask me is the stupidest name for a guitar I ever heard sounds like chewbacca in the movies. I go to a lot of movies because I can sleep there and its warm sometimes you can find sole leftover popcorn under the seat I’ll let you have it cheep because I really need to get out of the hooverville I stay in down by the river and get into a hotel for a while until my disability checkk comes in the lady at the post office is very nice to me and holds my mail as you can sea I am a little learning disabled mostly because Mrs. Lobasco my third grade teacher hit me over the head with a copy of the websters dictionary and she went o prison fer it but I figure she’s really my ma and I need money to go visit her at the prison in mitchellville.
peace and love randy lobasco.
Well, see, it’s just that I don’t have a permanent address because I’m more or less homeless. I also don’t have any credit since my bankruptcy so I can’t get a google pay account. The time I spent in prison didn’t help either and those tattoos really turn people off. I don’t know why you need my address just pick an address in des moines and your mover should be able to come up with a proce
From: Steven Edwards
Sent: Monday, December 17, 2012 11:55 AM
To: Robert Luedeman
Subject: Re: 1995 Fender Strat American - $735
I need your Name, Address, City, State, Zip code for the mover to calculate the cost of the shipment
On Mon, Dec 17, 2012 at 5:57 AM, Robert Luedeman <rwluedeman@mchsi.com> wrote:
Well, I’ve changed my mind but I want to know why you want all my personal details ;like my account numbers address and all. Y’see I found this guitar in a closet in a house that was being demolished it’s older than 1995 and I just know that the Lord Jesus musta give it to me for a reason because I’m mostly homeless and to tell the truth I broke into the house and I was sleeping in there until the cops ran me off. I do have a laptop but no permanent address but what I do is sit in a McDonalds and use their wifi. I can meet you in downtown Des Moines, if you hand over the cash I’ll go and get the guitar from where I have it stashed and I have prayed a lot and I figure jesus and buddha will be ok with me doing this even though the name inside the guitar case reads Gene Vincent and the Blue Caps. Now who on earth woulda worn a blue cap?
My real name is randy.
From: Steven Edwards
Sent: Thursday, December 13, 2012 10:19 PM
To: Robert Luedeman
Subject: Re: 1995 Fender Strat American - $735
Hello,
Am sorry i can only make the payment through Google Checkout, Don’t worry about the shipment, the item will be picked up by Private shipping company. And e-mail me back the pickup address such as Name,Address,City, State, Zip-code, Phone Number with your Google Checkout email account...
Thanks
On Fri, Dec 14, 2012 at 5:17 AM, Robert Luedeman <rwluedeman@mchsi.com> wrote:
I’ve got to see a guy locally about trading for some gear I want. If that doesn’t get done you can have it but here is how it’ll work. You pay me via paypal plus an estimated shipping cost. I withdraw the money from paypal and when it is deposited in my bank then I ship, and not before. Any excess shipping gets refunded to you. I’ll email you tomorrow night.
From: Steven Edwards
Sent: Thursday, December 13, 2012 7:34 PM
To: Robert Luedeman
Subject: Re: 1995 Fender Strat American - $735
I understand the present condition as started in the advert and i'm okay with the asking price, i will not be able to come to look at it due to the long distant and the easiest way for me to make the payment is through Google Checkout checkout.google.com and I'll take care of the pickup and delivery when the payment has been made by my mover, provide me your name, address, city, state, zip code with your phone number for the mover to calculate the cost of the shipment and your google checkout email account for me to make the payment.
Thanks
Steven Edwards
123 Beloit road
Marquette Heights IL 61554
On Thu, Dec 13, 2012 at 7:57 PM, Robert Luedeman <rwluedeman@mchsi.com> wrote:
Yes, it is.
From: Steven Edwards
Sent: Thursday, December 13, 2012 10:49 AM
To: k48pt-3443517076@sale.craigslist.org
Subject: 1995 Fender Strat American - $735

--
Is your item is still available for sale
Steven Edwards

Music Man Driver Transistors

While I was away, a Music Man amp was sitting on the work bench waiting until I got home. After rehabilitating the bias network and power supply electrolytics and installing a pair of  2N6292 driver transistors the results were discouraging. Measuring across the 3.9 ohm emitter resistors revealed a significant split in voltages, no improvement from what was inside which was a mismatched pair where one had been replaced.

Then, I uncovered the updated service bulletin number 7 which indicates that the 2N6488 is the best replacement around for the 2N6292. And so it is, being matched within 2 mv. An adjustment of the bias pot finishes the job.

Note that with the use of the two stepped plastic washers and the mica insulator wafer in the 4724 installation kit, the driver transistor is insulated from ground and uses the chassis as a heat sink. Make sure that you use a good grade of thermal grease. also known as heat sink compound, bird poo and other unflattering names.

The 534-4724 Keystone mounting kits are available from Mouser Electronics as are the improved driver transistors.

The complete file of MusicMan service bulletins are available here including biasing instructions for all models including the earlier amps that used a 12AX7 driver tube.

Here's SB7 with a smallish shop drawing showing the positioning of the insulator wafer and stepped plastic or nylon insulating washers.

Thanks to the folks at Ernie Ball for maintaining this resource for the user and repair community.

Bastard Bassman

This started out life as a Bassman 50 that I acquired with a little horse trading. I may have mentioned it before, but I acquired a Teisco guitar at an auction for a cool fifty bucks-that's the benefit of superior knowledge-and, having no love for it I offered it for trade on Craigslist, the "why pay more?" store.

A friend of mine came up with a Sound City 120 in need of repair and we swapped.

I promptly offered the Sound City for trade for a Bassman and I acquired a Bassman 50 head in short order. After the usual tidying up inside, all was good. I then turned it into a tube rectified job with a socket and a Super Reverb power transformer-you need the 5v winding which the original PT does not have. Ultimately it will become a 6G6 Bassman but for now it is what it is.

About six weeks ago I ordered a combo cabinet for the Bassman head from J.D. Newell, the only place to shop for amp cabinets. It arrived yesterday, and I stuffed the hole with an Eminence from a defunct Tech 21 Power Engine that I acquired. It occurs to me that I also have a nice 12" Peavey Black Widow that I may try out and see if it fits.

The result is what you see here.  The total cost of the entire project amounts to $412.50-$52.50 for the guitar, $50 for shipping the Sound City to Texas, $220 for the cabinet, $50 for the blackface faceplate, and $40 for shipping the cabinet here. The power transformer, tube socket, and high class Mullard rectifier were all things I had here.

I don't know why more people don't do this with orphan amp heads.

Stemming the Flood: Repairing A Leaky Pinion Seal In A Ford Ranger.

I realize this is way distant from my usual material but it did happen on the way back from snagging the Traynor amp I described.

The following applies only to a Ford 8.8 open rear end, although I am keeping my eyes open for a Traction Lok unit. I'd lile to have limited slip this winter. If you don't know what rear you have check the driver's side door for a build code label or check the tag on the rear end itself. They look different from the seven inch rear anyway.

It was hot, sweaty, and I was sort of flogging the truck, and when I arrived in Louisville the Ranger burped out about a cupful of gear oil from the rear end pinion seal and dirtied up my sister's driveway. A trip to an auto parts emporium got me gear oil and a tool to remove the fill plug, and I put in 12 ounces of gear oil.

I should also mention that I'd changed the oil and filter and used Castrol 5W30 and when good and hot the oil pressure gauge would flicker when taking off from a stop. Some STP cured that but I'm going back to Valvoline for good.

On the trip home, at my next stop I put in 8 ounces of gear oil, then four ounces, and then the level maintained itself. However, it did make a mess in my driveway as well when I got home.

The critical point here is that the torque on the pinion nut crushes a sleeve which sets the preload for the differential gearset, and that is why care must be taken with disassembly and reassembly unless your idea of a fine day is removing and setting up a rear end.

These notes apply only to a 2wd 2002 Ford Ranger with the 8.8 rear end, 4.10 rear gears and an open rear end.

You've got to know what rear you're working on, and there are plenty of resources after you have gotten the code off the builder's data plate in the door jamb and/or the tag affixed to the rear end.

The first job is to mark with a punch and a hammer the driveshaft flange and the pinion flange and unbolt the driveshaft and get it out of the way. I used a hammer and a punch. The driveshaft is held in with bolts that take a twelve point 12mm box wrench. Then push the driveshaft out of your way.

The next task is to clean everything off with Brakleen or other suitable degreasers. Then, index mark the shaft, the nut, and the flange with something like paper white, lacquer, or anything. This is critical, because if you don't have these references you're screwed. Really. .

My impact wrench didn't do the job so I used a 1-1/16 socket, a breaker bar and a cheater pipe to remove the pinion nut which was the  handle from my engine hoist. When the nut loosens, then take a ratchet and turn the nut off while counting the turns. In my case it was 16-1/2 turns.

Then, using a gear puller pull the flange and check the sealing surface for wear and grooves. Clean with scotchbrite if it is needed.

With a small hammer and a chisel or a screwdriver remove the old seal. Clean the area with solvent and remove the sealer if the seal was installed with it. Dress any scratches or dings with a small half round file.

One can then carefully install your new seal, first packing it with grease and wiping a little rtv on the outside.

Start it with a small hammer and then finish with a block of wood, making sure you don't damage the sealing surface or distort the seal itself.  Make sure it's bottomed out on its mating surface.

Buy the best seal you can, either National, from the Ford dealer or CR if they sell in your area. Don't go cheap.

Reassemble in reverse order, counting the turns of the pinion nut until it bottoms out. Then, you can reef the nut down with your breaker bar and cheater pipe. All your index marks should now line up perfectly. Then you can reinstall your driveshaft.

If you've done this correctly you will have preserved your preload inside the differential.

You can now remove the cover to drain what's left in the differential and reinstall the cover, torquing the bolts to about 20 foot pounds in a star pattern.

The NAPA folks sell a gasket for this task which is more trustworthy than a mere bead of RTV. Reinstall the cover, gasket and a thin coat of sealer, bolt it up, and then let it sit for a good 24 hours cure time.

Refill the differential with about 2.7 quarts of  a good grade of 80W-90W hypoid gear oil. A limited slip rear requires anti friction additive to the gear oil.

Take a road test to make sure you didn't damage the seal on installation and have to do it all over again
.

And Now For Something Completely Different

Here's something that I acquired a few years ago and hadn't pulled it out until recently.

I have no idea who made it at all and it could be a homebuilt.

The speaker's a Magnavox field coil job made for Crosley, and the tube lineup is 6J5-6J5-6J5-6V6-6V6-80. The 80 rectifier and the field coil dates this to likely prewar or possibly just post war.

It probably pushes 6 or 8 watts at full tilt and it's got an interesting neon bulb that flickers when it goes into clipping. If I had to take a guess it is probably a converted record player amp.

Mystery Traynor Gut Shots