Biasing A Mesa Triple Rectifier.

I recently got a Mesa Triple Rectifier head in for a retube and general checkover. A little clarification is in order if the gibberish I have seen in researching this issue is any index.

Bias voltage is a negative potential applied to the grid of a tube, in this case power tubes,

A "fixed bias" amplifier only means that there is an external bias supply for the power tubes, and this can be obtained by means of a dedicated winding on the power transformer suitably rectified and adjusted, or by taking a little bit from one side of the power transformer high voltage and doing the same thing here-which means you have high tension in places where maybe it ought not to be. But nevermind. Fixed bias can be and often is adjusted with a potentiometer or, as in Mesa's case, with a fixed resistance value.

The other way is by cathode bias which means that a suitable resistance is placed in the cathode string of the power tube or tubes and this creates the desired potential. About this we need say no more.

One thing to do here is to measure the bias current and plate voltages in both tube rectified and silicon rectified modes before changing out the tubes unless there is other damage that prevents this. It gives you a basis for comparison, and there is a pretty wide spread between the two sets of values. We do not want to exceed the rated value in the silicon rectified mode, in case someone wants to use that and flips the switch. The tube rectified position will just have to be what it is going to be, and it will be significantly lower.

So....what to do?

You can install an adjustment pot in place of the 82k bias resistor which has the green test lead attached to it in the upper image, adjust and be happy, but you will have to remove that resistor and unless you remove the circuit board to get access to the underside you can damage the traces on removal and not know it.

You can also, as I did, take a different approach. I took some test leads and paralleled the bias resistor. The other end of  the resistor is the black wire on the 6L6/EL34 switch. Then, selecting a suitable resistance between 20k and 100k you can look for your sweet spot.

In my case a 39k resistor in parallel produced about 30 ma in silicon rectifier mode-which is good-and about 10 ma in tube rectified mode, which is not so great but will have to do.

I got out my magnifier, trimmed the 39k resistor and turned the ends into hooks and soldered it in.

Close enough but as I mused over the subject I got to thinking about the rather wide spread in bias values between tube rectified and solid state rectified it occurred to me that I did have three nice new Shuguang 5AR4 rectifier tubes that were worth trying in place of the 5U4GB Sovtek/Mesa tubes installed.

Well. Wonder of wonders. The 5AR4 rectified plate voltage was close to solid state, and the bias spread was only about 7 ma. It seems that the idea was good but the execution of the original was flawed. Now, I could get both plate voltage and bias schemes within spitting distance, stay out of crossover distortion country, and have the best of both worlds. I highly recommended it to the owner.

Going North For A Traynor.

Here are a few snapshots of a recently acquired Traynor YBA-4 Bassmaster. At least I think they call it a Bassmaster. Never mind. It is what you see, and it originally belonged to the Mound View Independent School District. That mighta been a cool school to go to if they had stuff like this.

I'd been looking for one of these for a while (my love of fifteen inch speakers) when I located one on the Minneapolis Craigslist. A price was agreed to and I committed to picking it up. Serves me right, I shoulda known better because it was actually up north of Brainerd, which is a long way from Des Moines. But I'd already agreed to it and it seemed like it would be a good road trip. I flew solo, which left the right seat vacant-a good thing because it did rain some.

Brainerd is about 375 miles from here, a couple hundred miles south of the border and maybe 75 miles west of Duluth. I met the young man who owned it, cash was exchanged and home I went, flying blind through a heavy fog. The Garmin GPS did a splendid job because I'd have never found the place otherwise.

Once I got home all it needed was a single preamp and a single power tube. I decided to replace the electrolytics in it and install 6L6GCs instead of the usual EL34s because I just don't like the way the EL34s sound. They're too Marshall-y (if that's a word), and I've just never liked the Marshall grind.

It's a drop in fit, but it's a good idea to remove the 47k bias resistor and replace it with a 22k resistor and a 50k trip pot so you can adjust the bias a little to suit the different power tubes.

Now. Pay attention here.

On the other hand if your amp has 6L6GCs and you want EL34s for some unknown reason that totally escapes me, this CANNOT be done without some surgery so beware, and if you can't read a schematic or wield a soldering iron leave it to someone who can. Aside from that the EL34 draws a lot more heater current and that's not a great thing.

One thing that's sure to please is that the circuit is almost identical to a 5F6A Bassman or a JTM45. There are some minor component differences in value, but the major difference is that the rectifiers are solid state and the power transformer and diode bridge is full wave with no center tap. Traynor calls the middle and presence controls the low and high range expander but they're pretty much the same thing.

The goal here is to make it as much a 5F6A  as is possible. I think a choke instead of the first dropping resistor a sag resistor modification, and conforming the rest of the component values ought to do the job.

As it is it is incredibly loud and the work I've done so far has it sounding really clean and bright. It's got a solid bottom end that makes for some good rockabilly plonking. It might even be worth investing in a few pedals, too.

Defeating the Microprocessor: Going Forward By Going Backwards And Ditching The Moose.

It does seem as if every millenial smart phone addicted design engineer in the world just cannot live without incorporating microprocessors in places where they do not belong because there's no reason for it.

What does this have to do with guitar amps? Well, not much, but it is technical.

My first experience with this was the infamous jazz board failure in my Maytag refrigerator a while ago.

Or maybe this stuff is now filtering down into every area of life because it's cheaper to make? I dunno. Maybe the Nanny State can't settle al Qaeda's hash but they sure know how to make life difficult for the rest of us with this energy saving nonsense.

Fast forward to about eight months ago. I was awakened from my stupor by a loud bang! as the gearbox in our Amana washer gave up the ghost after one too many overloads. It couldn't be economically repaired.

So down we went to Sears Roebuck and selected an Energy Star rated super environmental water savin' washer. That was a bust. It took forever to wash a load of clothes, they came out dirty, and it cost a lot of money.

Three weeks later She Who Must Be Obeyed had had enough of that, so, being chumps, we went back and got one considerably less expensive, with a real impeller, and thought we had it made. Wrong.

It has a digital mechanism in place of the old standby clockwork switching apparatus, and when it was running it sounded like a moose in the throes of moose coitus or maybe the moose was gettin' it on with a bale of hay-who knows? It has been the source of endless rude jests around here, and if you have time on your hands you can have a lotta laffs with this. I wish I'd recorded it.

Last weekend it did what microprocessors are real good at-it crashed, and those three LEDs sneered balefully at me as the whole apparatus came to a grinding halt like a machine version of Jimmy Hoffa. Patience had worn thin, and I was tasked to go and find a Speed Queen washer with an old fashioned clockwork switching mechanism.

Luckily enough, they are still in production although time is no doubt limited and they'll soon be gone. It's not a lot different than the Speed Queen we bought in Ohio in 1983 that lasted until 2005 but was getting cranky in its dotage.

It came yesterday afternoon, and we ran a test load of six very large bath towels, It was nice and quiet, got through the process in a timely manner and without all the moaning and humping.

Perfect.