Unstacking A Terror Bass And Looking For Tin Whiskers

An Orange Terror Bass amp came in with a very strange complaint. The owner indicated that the treble control was inoperative.

At first I chalked it up to drama but when I got it back to the shop a couple of things became obvious-the treble control did, indeed not work and had no noticeable effect, and second of all the true genius of Leo Fender's designs.

For those of you who are unfamiliar with this amp it is a hybrid with a tube preamp that's (allegedly) the same as their AD200 bass amp, and a 500w Class D solid state power amplifier. I still haven't figured out how they do this Class D business or pack all that power into a lunchbox amp but nevermind. They say it, I believe it, and that settles it-more or less.

The tone stack in the tube preamp is a direct copy of the same section in the 5F6A Bassman as a gander here will show, with a few minor component value changes. Here's the Bassman.

Well. The similarities are there, and this combination of high and low pass filters does a very nice job. Particularly when you figure that this design is better than sixty years old, it speaks to the genius of the designer-one Clarence Leonidas Fender-that has not been surpassed.

It's homage in a way that Leo's designs live on in the newest amps-or relatively new amps.

So. After consulting with the Brain Trust and studying the schematic, an order was placed with Digikey for three pots of the correct size, value and termination points. No small thing when you start shopping for potentiometers I may say. Digikey's on line catalog is excellent because the graphics are clear and sharp, they let you see exactly what you're ordering, and they're only two days away, being in western Minnesota.

Also ordered at the same time were a pair of .047uf capacitors and a 470 pf ceramic disk capacitor to replace the 120pf capacitor as I figured it was not a bad time to revoice the amp for slightly better bass response. These had to be relatively small to fit in the space allotted to them, but at least the board is keyed to the schematic diagram so at least I knew what to remove and replace.

For that task I used my handy dandy ZD915 solder removing machine. It came from Marlin P. Jones Associates, and there is a good youtube review of this piece of gear on the EEVblog. It goes by the trade name Rhino in Oz, but it's the same piece of gear. It ran me about $120 US, and I ordered plenty of spare tips, filters, and a spare gun which was only $20.  I do not know how I got along without it, as the manual solder suckers I'd been using could lift pads on Fender boards quite easily with their recoil. Not a good thing.

Keep the ZD915 clean and you can use it on leaded solder or lead free as you wish. It helps to put a dab of leaded solder on the connector before you use the sucker-that or some flux is fine, but either makes the lead free solder's exit from the scene much more easy and a lot less drama to boot.



After reassembly everything worked swimmingly and it went back to its owner today.  I will likely autopsy the pots to see what went haywire. Some of my colleagues suspect that it was a case of lead free solder growing tin whiskers and shorting the treble pot out. I will do an autopsy with my 12x lamp and see what I find. Failure analysis is always fun stuff.

Tin whiskers are real, not fanciful, and they are microscopic fibers that grow out of the tin used in lead free solder. Some folks choose to believe they're not real, but I have seen microscopic images of them and have heard reports from other technicians who should know they're a real phenomenon in the age of RoHS compliant electronic products.

You can read all about them here

Straight Outta Linden

Mr. Brown, my Pawnshop Excelsior returned from its sojourn the other day. I guess the person who had borrowed it got tired of me whingeing and being a crying brat about it, but it was in a studio where they do some great work, and these guys are friends of mine so it wasn't like I needed it, mind you. Plus they do send me some interesting projects to work on now and then.

Thanks to Brandon and Micah-they're grand fellows and well connected.

Somehow or other the funky Gretsch footswitch took a powder but I had the makings of an Ampeg style footswitch here. If you recall, I detailed how to install a footswitch jack in this amplifier, although my simple but elegant solution to it was met with a certain amount of what passes for internet "commentary" but never mind.

The footswitch consists of about ten feet of zip cord-a large roll of which I acquired out of a defunct radio shop in Williams, Iowa, a right angle Amphenol jack, an on-off pushbutton switch as found in most of these things and a doorstop, which is a plastic or hard rubber wedge. You could make a wooden switch housing if you like.

I chawed out one of the ribs in the doorstop with my trusty Swiss Army knife, drilled a hole of the right size although here a note is important.

Do not do this with the doorstop resting on your leg. The results will be painful.

The other end of the zip cord is soldered to the switch terminals and then it is mounted in the doorstop. Attach the jack to the other  end in the usual manner.

If you've installed a jack in your Excelsior or other fine amplifier, then the last thing to do is plug in and play.

Come to think of it, I have a beautiful old Jensen P15LL that would be just the ticket for this amp to take some of the brightness out of it.

The Avalanche Project

A while ago I had a Deluxe Reverb Reissue come in with a blown power transformer, and of course they are made of unobtainium right about now.

The reasons why the power transformer failed were pretty simple. There is a small 22 ohm resistor in the bias supply and that failed open. With the loss of bias voltage, the power tubes red plated and died, bad smells came out and the power transformer primaries expired-no doubt it was the thermal fuse within, which is unrepairable.

 However, I did have an old style Vibrolux Reverb power transformer that's a drop in fit and it was a pretty simple thing to fix up all the burned stuff, add a couple of spade connectors , plug everything together and power it up. It worked fine, too, but I had the germ of an idea.

Now that I had a power transformer that was rated at 180 ma instead of 120 ma, what if I installed a more substantial output transformer as well? Then, I'd have a 35 watt amp in a 22 watt package with a pair of 5881 power tubes.

It also presented the opportunity to do some tinkering to make it all work together, upgrade that ridiculous 22 ohm grain of wheat resistor, and gave me a chance to try out my prototype vibrato roach (optocoupler) which I am calling the Doodle Bug.

There was the Surf Bug and the Black Widow roaches but they're out of production now and the maker of the Surf Bug has also passed off this mortal coil, more's the pity.

But there's the Doodle Bug.

 So Doodle Bug number one has been installed and it's a winner. Smoother than the original, it's more like a tube modulated vibrato than an optocoupler.

So the transformers went in in pretty quick order although the output transformer must be moved aft about half an inch to clear the speaker frame. It's tight but there's about 1/4 inch clearance which is plenty enough. You do have to drill a new hole for the mounting screw, and it helps to angle it a little bit because it comes up against the grommet where the wires go through.

I'd previously installed F&T power supply capacitors for tone and reliability. The bias supply resistor required some adjustment to increase the range to a usable level.

This amp has a 5A filament fuse which is OK for 6V6s but a little light for 5881s so I uprated it to an 8A fuse which should be fine given that most Deluxes didn't have a filament fuse to begin with and the Supersonic 22, which uses 6V6s has a 10A filament fuse. To be on the safe side I cleaned off the traces on the underside of the board where the fuse holder is attached and laid in a more thorough bead of solder. This was all because the 5A fuse failed after about three hours of service.

It's got a lot more guts than the DRRI, that's for sure.

The amp was donated by Matt Wellendorf, a local guitarist and also a great pal. I provided the brain power for this project.

I used Marvel Electric transformers which are plenty good and at a fair price. Currently the output transformer is a 4 ohm number  but the amp can stand the mismatch as all Fenders do. For a high end deal I'd probably install a Classic Tone Multi tap output, but the Marvel iron is good enough for proof of concept and some reliability testing. I see where Marvel is offering an 8 ohm output of this configuration which would allow a closer impedance match.

If this is something you'd like to try you can get in touch with me at all the usual places. At this point with all the parts added in about $350 should do it. If you figure that some routine maintenance and improvement should cost you $150 or so, it could be a good time to upgrade your 97 pound weakling.