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.