Disabling the Master Volume and Pull Boost On A Twin Reverb

There's a generation of hand wired Fender amps starting from about 1974 or 1975 that had a master volume and a pull boost added. These are generally despised as tone sucking turds, and although I wouldn't go that far, they can be easily done away with while you're reverting your Fender amp to an earlier and tastier configuration such as the AB763.

Enter the Quad Reverb. This was a 1975 issue Quad Reverb which is nothing more than the venerable Twin Reverb circuit in a 4x12" combo cabinet. In terms of wretched excess it is exceeded only by the Super Six and the early Peavey Vintage 6x10"-which I've only seen in pictures.

Well, nevermind. I acquired this Quad Reverb in a trade for some amp work I did for a guy, he got it from the widow of a friend in exchange for some house painting so we all made out good. The widow got her painting, my bass player pal got his Ampeg spruced up and I got the Quad Reverb.

And that's where things stood for about five years until last weekend when I asked myself what in heck I was doing letting this amp sit.

I'd recapped it way back when, so that was OK. The capacitor board was dangling by the wires and the cake pan was long gone. I decided it needed to be reverted to the original AB763 configuration and that involved changing it from bias balance to bias level, removing the master volume and boost switch, working my way down the circuit board and changing out everything that was non conforming and finally replacing most of the blue Paktron capacitors with Mallory 150s and Mojo Dijons. I finished it off with an earlier Twin Reverb faceplate to cover the hole and polished it up.

There are plenty of resources on the internet to guide you through this process. Suffice it to say, here are the steps and the links to the steps.

1) convert bias balance to bias level. http://www.premierguitar.com/articles/20025-ask-amp-man-better-biasing-for-silverface-twin-reverbs

You may need to tailor the 15k or 27k resistor a bit to give you the range you need to bias your power tubes correctly. In my case an 8.2k did the trick.

2) remove and discard master volume and pull boost. Props to Mike Pascale for this.

3) Retrogress to AB763 configuration. You can either use this link http://www.premierguitar.com/articles/19916-ask-amp-man-twinterventionrescuing-rotten-twin-reverbs

or do as I did and take the board layout and work your way from right to left and conforming your circuit board to what's on the original schematic. There will be a couple of orphan parts left over from removing the pull boost which takes its drive off the reverb circuit but if you leave them I won't tell.

Removing riveted in tube sockets on the Thunderclunker

Quite a number of amps have tube sockets that are riveted in. I can think of a lot of Ampegs for one, and a lot of Valco made amps, the Thunderclunker included.

It's quite simple for an old aircraft mechanic like me, but for those of you who are not used to this, do the following.

Use a spring loaded punch to put a small divot in the bucked head of the rivet-that's the round side.

Drill with a smallish drill so as to get a good pilot hole started. Then finish it off with a larger drill. I can't remember the sizes offhand but you can either punch out the rivet stem or use the butt end of a drill to snap the head off.

For tube sockets it's easier to make a little map of what goes where and then go ahead and cut the pins off with a pair of surface cutters. Then, once the new socket is installed clip the wires, strip them a little and reinstall.

Note the burned plastic on the old sockets.

them one by one.

Reviving the Thunderclunker a/k/a Supro Thunderbolt Harmony 420

I've spent a couple of months on and off negotiating with a guy down south in Clarke County for a defunct Harmony 420, a/k/a the Thunderclunker. My reasons for doing so were because with the exception of a treble and a bass control it's the same as a Supro Thunderbolt of the later, no rectifier tube variety. There are both kinds, y'know.

A little research in the schematic library revealed that the Harmony 420, a couple of Airline amps, and the Supro are all pretty much identical to wit, two 12AX7s and a pair of 6L6GCs, cathode biased, nominally 35w (which I don't believe for a minute) and a Jensen C15P ceramic fifteen inch speaker.

UPDATE: I put this amp on the dummy load and it produced 37w maxed out. It was clean on the scope right up to 25w, which suggests that a nice JBL speaker would not be a bad thing. I have a couple in need of repair sooooo........

The amp was completely dead with no glass and no sign of life, so a complete-and I mean complete-rebuild was in order.

First to go were the tube sockets (burned and cracked), the Mallory can capacitor, and the taped in output transformer, the two way switch, neon lamp, fuse holder, dropping resistors and power supply diodes which got replaced with 1N4007s. The old ones tested OK but for about ten cents apiece I wasn't going to scrimp.

 This is more or less what I found inside. It became a complete gut job of the power section.

Here's the pile of crap I removed.

And here's the finished product. The output transformer is a replacement for a Fender Vibrolux which I had. and the power transformer is original. About the only thing saved in the power section were the terminal strips. The fuse holder was broken so that went. The three way switch wasn't needed with the three conductor grounded cord, and I dumped the neon pilot light for a 6.3 volt Fender type lamp with a cool old vintage red jewel.

I measured up the finished voltages with a full complement of tubes and they are AC: 178 & 158 for a total of 336v, B+: 405vdc, Vscr 395, Vp 399 vdc, first preamp tube pin 1 244vdc, second preamp tube pin 1 219 vdc, pin 6 181 vdc, all of which are at reasonably healthy levels.

At the first power up all was well for a minute or two and then a heavy hum originating in the power section commenced to build to intolerable levels. After a minute's thought I remembered rassling with a Deluxe I used to have that did the same thing. It was as easy as isolating the transformer filament center tap and clipping a couple of 100 ohm 1/2 w resistors to the ends of the filament strings to beat some sense into this thing.

After some decent glassware was installed, the results were pretty serviceable with  the original speaker doing its chores.

Remember that this amp is of the later type with a full wave bridge so there's no transformer high voltage center tap, despite what you may see on the schematics that are floating around. I'm thinking that obtaining a a replacement power transformer for the solid state rectified amps might be a chore, if one was to need one.

Here's a layout of the power section I did to keep track of things while taking frequent coffee breaks and doing domestic chores.

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.