What you should know about buying a tube amp

I am often asked for some opinions about what people should buy in the tube amp world.

Here are my thoughts.

First, the basic platform must be roadable-that is, rugged and durable, able to survive temperature extremes, odd voltages in dodgy ginmills, and one that can withstand being bounced around in the trunk of a car for hours.

Second, the amp must-MUST!-be free of unnecessarily complicated circuitry and gimmicks, because those doodads exponentially increase the likelihood of maintenance issues, breakdowns, and, for the technician, countless hours spent in chasing down intermittent faults. Some of the design features overwork the key components, which are the tubes.

For the most part this rules out Mesa Boogie amps and similar analogues such as Bugera, B52 and many others. They are complicated to work on, service information is closely held and not available to the trade, and that limits the number of people who can or will work on them. I have always thought that vertical information silos depend on key people to be available when you need them. On the other side, broad based systems of general knowledge do not depend on key people and are thus more likely to be available when you need them. I recently had a Dual Rectifier Road King II in the shop with failing muting jfets of which there are 15 or so. By my count it had 33 knobs and 22 relays.

This increases potential failure points in a geometric fashion. If the failure point is A, you have one potential failure point, but if you have five potential failure points you have AxBxCxDxE potential for failure of a critical component. The quality of the components used can only compensate to a certain degree.

This is directly connected to Bayes' Theorem, which applies to probabilities. You can read more about it here.

My math may be a little bit lacking but I think you can see that complication leads to failure at inopportune times.

This is not to say that Boogies are bad, or evil. They are fine in the studio if that's what you like to do.

This in general rules out all amps that have more than two channels and use channel switching via relays, optical couplers like Vactrols, and the like. They're troublesome.

What you can do here, when considering an amp for acquisition, is go to the best most respected amp tech you know and buy them a coffee. Ask them if whatever you're considering is a good buy from the standpoint of serviceability and roadability. You may be surprised at what you learn in a face to face encounter with a real person.

Third-printed circuits. Are they inherently evil? The answer is no, if they're properly built with good components and proper engineering. They're a fact of life in this day and age, but they do require that the technician who works on them have a deft touch. They're less user serviceable than amps of an earlier era. They're also less damage and shock tolerant than traditional methods of hand wiring. Also, when troubleshooting down to board level faults, you may find the amplifier becomes uneconomic to repair.

Printed circuits have been around for a while now. The earliest one I have actually seen was in a Motorola table radio and it had a copyright date of 1956. At that time my father was working on a wave soldering project for Weston, so we've had them for at least sixty years. I'm informed that the first printed circuit boards with plated through holes was made in 1947.

Now, having said that, it is worth asking whether power tube and rectifier tube sockets are mounted directly to the circuit board. They generate heat-lots of heat, and the boards cannot withstand that forever. What happens is that the substrate gets carbonized to some extent, and as we all know carbon is what resistors are made from. That means that you start getting stray voltages and current leakage here and there. Ask the Marshall folks about conductive circuit boards in their DSL amps.

Also, circuit boards for guitar and bass amps are not generally available in the aftermarket, with very few exceptions. That means if the worst happens and a board needs replacement you may be out of luck. Occasionally some boards may be available in the secondary market or from friends in the trade but it is not something that can be relied on.

What this means is that you should spend all the money you are capable of getting your hands on and buying a hand wired amp built the old, artisan way.

Educate yourself on the methods by which your chosen piece of gear is built. Not all hand wired amplifiers are created equal. Learn about maintainability if you intend to take it on the road. Learn about after sale service and what you can expect from the builder.

Does that mean you should buy from a homebuilder who will assemble a kit for you? Probably not, unless you have seen examples of their work in the wild, and if the critics are satisfied.

All of this takes you back to the beginning. By far the best values for the money in the amp world are silverface Fender amps, some of the older Traynors, and some boutique-y amps such as Tungsten, Victoria, and Kendrick.

They're hand wired on eyelet boards and not overly complicated. With ordinary care they will last a lifetime of playing.

And last, do you like the way they sound? Do they speak to your style of music? Ultimately that should be the deciding factor.

The Cowpoke Forty, or, How I Stopped Worrying And Learned To Make My Own Bassman

You know sometimes how you get an idea and you just can't shake it? Gotta scratch that itch?

That's what happened here. The first Bassman 50 just didn't have any muscle, so I decided if I couldn't have a real Fender Bassman I'd have to build one myself.

The first step was to acquire a chassis and I was lucky enough to find a new production chassis on fleabay, the why pay more store. I already had, in no particular order, a Super Reverb power transformer from Pacific-nice stuff!-a Marvel choke, and a Classic Tone output transformer.

In short order I cooked up a circuit board from the usual stuff, punched out an extra hole for a rectifier tube, and commenced.

After some off and on work the Cowpoke as I called it was finished but still needed some fine tuning in the bass channel. Other than a tube rectifier it's pretty much all AB165 Bassman, and it was wired with traditional cloth covered pushback wiring, Omron octal sockets, and a mix of yellow poly and brown chiclet capacitors from China Inc-they were cheap at about ten cents apiece and do the job just fine, thank you.

For glassware it has a Raytheon 5V4, a pair of vintage Sylvania STR387s, a JJ ECC81 and a trio of JJ ECC83s. After trying it out I had to rectify a couple of mistakes I'd made with the wrong value caps in the bass channel.

That was all finished up today, the Bassman face plates were installed and in it went to make nice with the Weber Silver Bell that's in the J.D. Newell cabinet that I acquired a while ago.

You do not have to keep a Bassman power transformer in stock if you will but keep a Super Reverb/Pro Reverb power transformer. Just cap off the rectifier tube 5v winding if you don't like it. Stock one instead of two.

It's a good piece of kit and I will be experimenting with it and getting familiar with its Bassman goodness in the next two weeks or so.

The pictures.

Getting back to the basics of troubleshooting. Blackstar content.

After the contretemps with the dual rectifier road king which has 33 knobs and 22 relays by actual count, I worked on a nice old Bassman on Sunday and Monday to level my alpha waves for the next migraine, which was a Blackstar HT60  combo with dead overdrive channels that I tackled today. The overdrive channels were noisy but took no signal and they expired completely on the bench with the occasional pfffft! pffffft!.

I remembered something about the clean channel being solid state and the overdrive channels being tube driven, so I started with a set of preamp tubes which changed nothing.

Getting back to basics I started with measuring plate voltages and wouldn't you know, there was very little voltage on pin 1 of V2 which receives its plate voltage through R41. The resistor tested OK with my extra high quality Harbor Fright multimeter although not so well with my ten buck analog multimeter.

But the voltage was going in but not coming out.

It was a simple enough matter to snip the resistor and sweat out the cut off stubs without removing the circuit board, check underneath with a mirror to see that all was well on the underside, and solder in a new resistor which I could do because the holes are plated through. Cleaning out the lead free solder from the holes and using some good Kester 44 did the job. To be on the safe side I resoldered pin 1 where it merges to the circuit board. It looked like it could use it.

The overdrive channels sprang to life with all the bowel loosening feces tone Blackstar is famous for, and after checking and adjusting the power tube bias, away it went. I kind of like the idea of a bias balance and level set which these amps have. I mean, I had to say something good about them, didn't I?

The point is, start  with the basics-swap out for a set of known good tubes and then some checking to see that voltage appears in all the right places. As it happened no fancy schmancy tooling or scopage was required to set this amp right, and it was not the first time I've seen such things happen.

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.

Measuring Your Voltages: Why To Do It.

A relatively new Blues Junior III came in the other day because it wasn't making any sound....not a peep.

The first thing I did after testing the power tubes was get out my trusty multimeter I got from Horror Freight and started measuring voltages on the power tubes. Lemme see, plate voltage OK. Bias voltage OK...where the hell's the screen voltage?

Dave's not here, man.

It seems that  R47 (I think) the 2.2k 2 watt resistor in the power supply string had failed open kinda like a self activated standby switch.

From there it was a couple of hours work to crack it open, get rid of the Illinois power supply capacitors and replace the suspect resistor.

It powered right up and all is good.

The point is, measuring voltages is the first thing you do. Had I not done so I'd probably still be throwing parts at it in a random fashion hoping that something would stick or give me a clue.