Danelectro DS100 Schematic

Folks, here's something that I found in the back of the Neptune Bound book by Tulloch. Most of us get by with the schematic for the Silvertone 1485 amp which is similar but not identical.

The DS100 uses a split chassis instead of having everything on one hunk of metal.This schematic will prove useful if you are working on one and trying to figure out where the two octal connectors-the one between the chasses and the one to the six speakers-go to. Aside from the fact that they call the same circuit a tremolo on one amp and a vibrato on the other, the DS100 and the 1485 are pretty similar.

This picture is of the very amp I'm working on, and it came from Springfield, Illinois. You can see more pictures here . I actually got a phone call from this fellow when my friend Justin arrived on my doorstep with this amp, which looks an awful lot like an Easter Island stone head.

The excavation is going to begin shortly, and I can't wait to see what's inside.

Christmas Card From Iowa

Merry Christmas from our house to yours. We've come to the end of one year and getting ready to start another. Our best to you all, and for my loyal customers I hope that the work I have offered has been up to your expectations. Music is about making people happy, but on a deeper level it's about expression and giving voice, and saying things that need to be said.

If I have contributed to that in any way, it has been a good year for you and me.

Installing a Vibrato Footpedal On A Fender Pawnshop Excelsior

 I recently saw a post on a chatboard wondering whether there was a mod available to install a vibrato footpedal on the Pawnshop Excelsior.

It seemed like a good idea because the on-off switch can be a little out of reach when you're playing. As I've already voided the warranty on my Excelsior in numerous ways (tone control and red pilot light) I figured this would be a good contribution to the group.

The first thing you have to have is a footswitch like the Gretsch-ed footswitch that I got from the irrepressible Dirk Newton, who a lot of you know if you're in central Iowa.

The first step was to open it up and see what we had. There are a blue and a green wire that go to the switch mounted on the back of the vibrato control. Desolder these. The next thing to do is get a plastic Cliff type jack that is not grounded to the chassis. They're inexpensive and easy to find.

Next drill a pilot hole through the chassis and open it up with a step drill to mount the jack, but don't mount it just yet.  It's best to start your pilot hole from inside so as to allow sufficient clearance and avoid damaging anything like your circuit board.

Next, solder your blue and green wires to the pins that connect to the contacts that get lifted when you put a phone plug in the jack. Mount the jack next to your 500k tone pot like mine has. I had to use a smaller pot than I had initially, so this required a few minutes to hunt thru my junk box and find a smaller 500k pot.. If you are using an Alpha pot this should cause no interference problems.

At this point you should power up your amp, plug in your footpedal  and make sure that your footpedal hookup is operating properly. You can temporarily insulate the jack with a shop rag because there's voltage down there. Then you can power down, clip the pins on the jack that you're not going to use to get them out of your way, mount the jack permanently, make sure nothing's touching anything, recheck everything for proper function, and reinstall the cover.

Now, your footpedal controls the vibrato, and if the footpedal is not connected the vibrato won't work.

New Life For an Ampeg Big M

Every now and then you come across something you decide you just have to have, and that was the case with this Ampeg M-15 a/k/a Big M.  According to Hopkins and Moore, the Big M was a 20w version of the M-12 Mercury with 6L6 power tubes and a fifteen inch speaker from the house of Jensen, introduced in 1959. This one's a little later because it has the checked blue covering rather than the earlier random flair that my Mercury is covered in. I had a chance to preview one and decided I had to have it.

On the general subject of Ampeg combo amps of this era, they seem to be a bargain right now and about as low as the price may get. I've acquired this one, a random flair M12, and a J12D Jet. Once you get around the odd tube choices Ampeg sometimes made with some of their amps like the Jet (6BK11s, 7591s and 7868s) they're solid, reliable amps. I'm awaiting the arrival of a modestly priced Jet II reissue, but more about that later.  Now, if Ampeg hadn't smeared all that rubber cement on the phenolic circuit board in this amp to keep the components from vibrating-but I digress.

Not too long after that, this one showed up on fleabay at a relatively modest price because the power transformer was deceased, and it had a substantial dent in the control panel. As it happened, I had a new Stancor PC8410 power transformer that would fill the bill. I'd acquired the transformer at an auction and it was sitting in a tray of junk from the garage, obviously new and never hooked up. It cost me all of a dollar for the whole tray which included a World War One German army belt buckle as well.  Pounding out the dents in the control panel was fairly simple as was replacing the power transformer. While I was in there I installed a Twin Reverb output transformer on a lark to see if more iron was good. The cement bodied cathode resistor also fell apart in my hands so that got replaced.

One thing is important to note when replacing the can cap. Most Ampegs of this era have the Mallory can cap insulated from the chassis with a bakelite washer and an internal ground wire going off somewhere. If that wire is broken the can is live, and if you touch the can and the chassis at the same time you get to be the ground.  It is not an enjoyable experience as I found out a few dozen amps ago.

The results were flat and disappointing, and there wasn't nearly enough volume for me. I spent about two weeks trying to figure out how to convert the amp to fixed bias, got it done and it made not a lick of difference so out it went into the junk box.

I figured since the amp is not collector grade and some parts had already been changed I might as well just clean up all the messes and dump all the capacitors and resistors, even though the savants will tell you that shotgunning an amp is the worst possible thing a person could do. In some cases it is justified, I own the amp and nobody'll ever know unless I tell them. So there. Building things back to print is a good way of starting your troubleshooting journey if the amp you've been working on has been bodged like this one.

 I left the vibrato alone because that was working well and at this point in life I leave things that work well enough alone. Everything else got replaced, including the can cap which got swapped for a JJ can cap-inexpensive and effective, too. A new set of tubes, mostly Russian stuff that I had lying around filled up all the empty holes.  While I was excavating from one side of the board to the other I did find that some previous bodger had installed a 560k plate load resistor on one side of the phase inverter instead of the specified 120k.That was something of a revelation. The circuit, all things being equal is pretty simple once you've spent as much time as I did on this amp.

Could that one resistor have made all the difference? Perhaps. It's reasonably tolerable in the volume department now, and I've spent a happy hour or two putting it through its paces. When you're working with the Joe Piazza schematic as I was, it's worth noting that the tube numbering is incorrect. The first preamp tube is V2, the vibrato tube is V1 and the phase inverter is V3.

A Case of Mesa Laryngitis

I had a Mesa 5:50 Express here recently with a bad case of laryngitis. It could barely muster a croak, but otherwise seemed in good physical condition. After verifying that it was not under warranty and that I would not therefore void the owner's warranty I put my mind in Boogie mode.

What's that, you say? Well, when working on a Mesa you've got to get used to the idea that it may be a little crowded inside, but that there is a procedure to get to where you want to be. That means patience, quiet, no distractions, and technical information readily to hand. That's a Boogie frame of mind-you've simply got to get in that zone or you could be wasting a lot of time. When you're 64 like me, you become a little more conscious about time and using it efficiently.

One of the marvels of our digital information rich age is this: if you have a consumer product-doesn't matter what it is, either-you can be sure that some person somewhere has had the same problem and has written about it or posted about it on a chat board somewhere. That, after all, is how I fought my Maytag built Amana refrigerator to a standstill despite not knowing a thing about refrigerators.

So, after digging around I located a schematic for this amp, and an excerpt containing the master controls and muting circuit which you see above. It's patented by the way, and you can pull up a copy of patent number 6,621,907 which describes  "A mute circuit for momentarily inhibiting signal travel in the signal path having an input terminal and an output terminal in response to a operation of a coil operated relay."

And that's exactly what it does. The circuit  momentarily mutes the signal when a relay is switching-in this case a channel switching relay.

I also found out through further research that the J175 JFET transistor used in these things is a frequent source of problems leading to amp laryngitis.

After removing the chassis, testing the tubes, looking for cooked resistors, and measuring voltages throughout I figured the muting circuit JFET was a component of interest. I ordered half a dozen figuring I'd need them some day or other.

The disassembly process is a little involved. You have to get access to the underside of the circuit board and this is done by removing the pots and most of the switches on the front panel. Of course you have to number the pots with a sharpie marker as well as where they go on reassembly. Then you have to tackle the circuit board. It uses plastic standoffs with spring tabs that have to be squeezed while the board is being lifted a little. Patience, patience. You'll get there without breaking anything. The muting JFET was permanently marked as such on the circuit board. It's best to have a fine soldering tip and a solder sucker readily to hand after you've snipped the leads. Doing this and pushing the stubs through with your by now asbestos fingertip avoids damage to the pads on the underside of the board.  Soldering in the JFET is just as easy after you've cleaned the holes out, but I recommend letting the JFET stand tall as there's less possibility for heat damage  with long legs, Daddy.

After reassembly the Mesa sprang to life, ready to go out and make people happy. Then, and only then, is it time to police the wiring job and push the board all the way down on its standoffs.

UPDATE: A confidential informant has told me that when replacing JFETs under warranty that they are not allowed to lift up the circuit board but to pull the JFET out through the top. I guess the boys at Mesa know what they're talking about, but as the legs of the JFET are bent over a bit before soldering in place, you run the risk of damaging the traces and pads by this rough treatment. It would be my bad luck to have this happen to me. Just my opinion, mind you.

Replacing a Selenium Rectifier On A 5D6-A Bassman

 This vintage Bassman came here a few weeks ago for an overhaul and the first thing I noticed was that it still had the nonadjustable bias with selenium rectifier setup of yore. After a general refurbishing, I ordered some small trimmer pots from Mouser. Do not order fake Bourns pots on ebay folks. I took a small pair of pieces of vulcanized fiberboard and mounted the components that duplicated a later adjustable Bassman bias setup, and dropped it into the slot where the selenium rectifier used to live.

There are a couple reasons for doing this. Selenium is nasty, toxic stuff for starters. The selenium rectifiers are pretty unreliable as diodes and they don't have nearly the capacity that modern day silicon diodes have. Plus, you get to fine tune the bias level and get all that early fifties goodness that Leo built for you.

Ripoff Alert: Eight O'Clock Coffee

Editor's Note: This is a little out of the usual amp realm but we need coffee to fix amps, right?

I was at the grocery this morning shopping as I do every Sunday and I went down the aisle to grab a bag of 8 O'Clock coffee.

The first thing I notice is they're out of the red label stuff so I grab a brown label-the price is always the same.

Then I notice the price is two dollars less.

Then I notice the bag is marked Value Pack, and it is 1 pound 6 ounces instead of  2 pounds 4 ounces.

Doing the math, the net price of the product in the older package is $0.35 per ounce and the net price of the product in the new "Value Pack" is $0.68 per ounce.  Thus, they manage to double the price of the product and call it a value. For whom?

Truth in advertising would be better served if the new Value Pack was marked "Bend Over Pal Pack."

Shame on you,  8 O'Clock. Do you really think we're that stupid?

Transforming a Marshall JCM2000 DSL 100 watt head

Like many others of its kin, the Marshall JCM2000 DSL 100w head suffers from a poorly made power transformer-in this case, a Dagnall TXMA 00061. The pictures show a flaw in the wrap, and this was where the smoke was coming out. The good news is that Dagnall is no longer in existence. The bad news is that until recently a replacement power transformer was hard to find and tough on the wallet-the only replacement item was a Mercury Magnetics Fat Stack that comes in at about $325.00 or more. The last time I did this I was able to locate a used serviceable Dagnall.

Just this year Magnetic Components, a/k/a Classic Tone. began offering power transformers for these amps, and I obtained one. Of course, the wiring color codes are different, and you have to make up some of your own connectors because the Classic Tone does not come with spade connectors installed.

The first thing I did was make a map of the old connectors on the Dagnall and where they went to and on what board-main, or power inlet. Then, I took the hookup diagram that Classic Tone provides and started doing the connections. It's simple enough if you can read a schematic. If you get a little confused it's a good idea to draw it out for yourself.  It's also a good idea to save some of the wiring you remove, because you're going to have to do a little splicing to wire up the bias circuit with the right size spade connectors. Some of the connectors are a bit smaller than our standard, but you can take your new ones, squeeze them a bit and re-flatten them out, making sure they're a good fit on the lug. You can see what the finished installation looks like in the photos.

The Classic Tone has more metal in the core, and it's about halfway between the original installation and the Mercury Magnetics Fat Stack.

Next, removing the old power transformer you can mount the new one so that the green and green/yellow leads point toward the filament circuit lugs on the main board. After wiring up the power input per the schematic for 120v (primaries in parallel) you can proceed to wire up the filament circuit and the B+ (ok, ok, high tension) circuit, which runs through the standby switch. At this point I would stop, and power up the amp with no tubes in it. You can then check for 6.3v on the filament circuit-I check the preamp tube sockets with an old RCA cleartop tube. At this point, make a chart and measure your filament, plate (pin 3)  and screen (pin 4)  voltages and note them. You should see something resembling operating voltages.

Now, you'll have to splice the small plugs onto your bias lugs on the main board. The old ones are white-black-white, and the new ones will be white-blue-white.

Once you've done that you can power up again and measure your bias voltages on pin 5 of the power tubes and you should see something around 45 volts DC-remember, it's rectified.

If this is all good, you're ready to police your wiring job, set up to check bias per the Marshall method using the Molex connector on the back of the tube pan, and adjust as required. It's always a good idea to start out low and slow until everything's had a chance to settle down and you're sure you aren't going to blow any fuses. Then you can adjust up to around 80 mv per side using the pots provided.

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.