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.

Ampeg Gemini 1 Full Wave Bridge

An Ampeg Gemini 1 showed up a little while ago with the owner complaining that it tripped the ground fault interrupter in his residence and had voltage on the faceplate. After removing the so called death cap-a likely suspect-and isolating and checking the power transformer for leakage, the only thing left that had a path to ground was the full wave, no center tap diode bridge.

Although the diodes checked ok with a voltmeter, 1N4007 replacement diodes are inexpensive, and the originals were probably 50 years old. Not willing to bet the ranch for 15 cents worth of diodes I replaced them. Looking at the original bridge, it had a quartet of old school diodes identified by yellow and red paint dots, not standard markings, some of which were worn off.

The above drawing with the lines properly oriented will build you a proper bridge. You can even take a chunk of large sized heat shrink tubing and use it as an insulator with a red marking for polarity.

Weirdorama From The House Of National Dobro

We give you, without comment, the National Dobro Model 31.

Saving Money The Old Fashioned Way

Ever since my last Ampeg fiasco I've been looking for an amp-any amp-with a line in line out feature that was cheap. Did I say cheap? Above all cheap.

The Ampeg mess began with a VT22 that was bought on the cheap by a local fellow, I suspect because nobody could repair it down where he got it from. After fielding the numerous curves it threw my way-no filament voltage on the phase inverter, bad tubes, one of which died on the operating table, a number of burned resistors, sick electrolytics etc, and testing every god damned component on the preamp and power supply boards, I was in the 9th inning and wondering how I could declare defeat gracefully.

By the by, there are four different schematics for the VT22, they're all somewhat different so if you are working on one, it is best that you figure out whether you have the right one for your project.

In a last desperate bid I decided I needed to test the preamp and power amp sections separately and I borrowed an amp-a Blackstar head-which had a line in/line out feature. This allows you to take the preamp signal of one amp and run it into the power amp of the other, and vice versa-thus isolating a problem to one section or the other of the corpse on your workbench.

As it happened both sections worked well enough but they were not talking to each other, and the problem that caused all this emotion and wailing was a bad ground on one of the line out jacks.

So much for the Ampeg. It's gone.

Now the quest began for an amp-any amp, but cheap, with line in line out, with no results. On a whim I strolled back to where I keep my junkers and found a Randall RH50 tube head that I'd bought for spare change a few years ago, the kid said the volume was low, and there were the line in line out jacks that I'd wanted. I'd stripped the tubes out of it and had tried with no luck to dump it as a project amp and then forgot I had it.

A set of tubes later, and I had no filament voltage-which was a good thing because it took me right to the filament fuse inside. One fuse and a plateful of junk tubes from my stash of pulls  later I was in business with a functioning piece of test equipment that doesn't sound half bad.

Of course the volume was low. It never worked at all.

Kid, it woulda cost you ten bucks to fix and if you want to buy it back you know where to find me.

How To Profit From Planned Obsolescence In The Computer Age.

I've got an elderly Dell desktop workstation named Edgar in the shop that I use for schematics and some basic internet browsing, and lately it has been running painfully (excruciating, really) slowly.

So I decided to reload the operating system, not without a few hiccups, so here's the score on how to avoid paying Microsoft-again. In my case I did not want to cough up a hundred bucks or so for a copy of Windows 7. In a philosophical sense, calling something a "workstation" when it's used in the office for fooling around on the internet is something of a puzzle but nevermind,

As a point of information Microsoft says it no longer supports XP but there are ways, my friend. Inside many cash registers and ATM machines resides a computer running XP, and there is a registry hack that is supposed to convince Microsoft that your computer is a cash register in a bar or an ATM at a casino. Either way I haven't bothered monkeying with it yet.

When Microsoft comes up with the message that says "You're screwed. Buy a new computer with Windows 8 and stop with the whining willya?" just ignore it. Let google be your accomplice.

The only caveat here is availability of drivers for equally elderly devices. The update to Windows 7 compelled me to obtain a new scanner to replace an equally good one for which drivers could not be obtained. Thanks again for adding to the growing mountain of e-waste, Acer and Microsoft. 

As it happens I'm ahead of the game all over this because we already got a couple of hundred dollars in a class action settlement here in Iowa-didn't you know it, Microsoft was overcharging us all?

In any event this computer started out with parts from my Dell computer that went to Goodwill a few years ago (memory sticks) parts harvested from a defunct TiVO recorder (hard drive and DVD drive), parts from Newegg and the now defunct CompUSA chain (internal hard drive, Western Digital standalone hard drive and DVD drive) a case with a motherboard, power supply, Pentium IV 2.8 processor and a floppy disc drive from Iowa State University surplus sales, and a Viewsonic 21 inch CRT monitor, all of which got me to about $150 over the course of six or 7 years. I still retained the OS disk from my Dell that went to Goodwill.

I also have a large box where I store all my spare cables, parts, software and miscellaneous stuff I've acquired over the years. It pays to save the stuff.

So, I redid the boot sequence as required, started a clean reload and all went well. They didn't even ask me for a product code so maybe they think I'm Iowa State University or maybe they don't care anymore. Either way you take your pleasure as you find it.  Installing the Linksys software for the wireless receiver took a couple of tries with different CDs before it took hold when I used an earlier version 4.0 of the software.

The big problem was that my OS disk was a very primitive installation, and I had to next install service pack 3. To make a very long story very short, you can get it from Microsoft for free (it's the version for network administrators) and put it on a DVD yourself. No other third party source will work properly as I found out.

Once you get a good working copy of service pack three, you can load it and then proceed with retrieving at least 133 updates, which took a couple of hours. I loaded the usual stuff for viewing images and documents-Adobe Acrobat reader and MS Office 2003 complete with product code, thanks to a former employer who shall remain nameless because they're world class asshats.

This morning I installed a nice flat panel display courtesy of Kevin Neal's studio, which predates USB ports but I did have an RGB cable ready to go-remember that big box of obsolete crap?

I may see if I can up the memory some more. Right now it's 2gb but it is cheap because it's obsolete.

Cheap and Cheerful: The Peavey Delta Blues 115

Those who know me know my weakness for anything with a fifteen inch speaker and a nice set of tubes. So I had a Delta Blues 2x10 here for some repairs last week and when repairs -of which more anon-were completed I liked the sound enough to go and hunt one down in the fifteen inch configuration, which arrived today. The idea of spring reverb, vibrato, and a 15 inch speaker was one I couldn't pass up for $380 all told including shipping. Quite a value considering that they're about six hundred or so new.

A little more experimenting with it should tell me whether it's a keeper or not.

The Delta Blues has the same chassis more or less as the Classic 30, and thereby hangs a tale. They're known for needing to have the tube socket pins resoldered because they're kind of weak. This is a bit tedious because of the way that the three (yes, three) circuit boards are connected with bare wire and folded into a U-shaped configuration. Here's what I mean.

As I recall this one had some problems with filament voltage not getting to the right place, and a little rework because the solder joints with the connecting links getting loose because of the amount of flexing I had to do to make the needed repairs.

The new-ish 1x15 amp arrived in very clean condition with a cover and some unnecessarily heavy duty casters which I will save for another project. Some of these amps are known for acoustic feedback through the reverb tank, but this is not one of them. It does have a very slight hum, and if I ever get the notion to take it apart it'll get some nice new F&T electrolytics. In the meantime I shall try some different preamp glassware.

Stay tuned.

UPDATE: The date of manufacture of this amp appears to be circa 2004, and the background hum was accountable to a brand new preamp tube. Mr. Hickok said no, so in the trash it went. . If you're so inclined, give homage to one Job Barnart, who patented the Hickok tube tester mutual conductance circuit in 1935 and founded an industry.

UPDATE 2: Wouldn't you know it? A week after this arrived I stumbled over a 1971 non master volume Twin Reverb for a great price so the Peavey is gone-at a profit too.

The Panther Tank And The Engl 530

From the fine folks that brought you the Panther tank-a great gun and frontal armor but cursed with a bad transmission, a gas guzzling engine that had the life span of a mayfly, and final drives that would puke if you looked at them crosseyed-where was I?

Oh. I remember. A friend of mine brought in an inoperative Engl 530 tube preamp-a nice piece of gear, but one in which the product support borders somewhere between weak and nonexistent.

The power transformer looks like a brick, but without all the endearing qualities of the said brick. The 6.3v filament circuit that runs the two 12AX7s is defunct. No voltage is forthcoming.

Over the last few weeks I have emailed Engl, the Weiss transformer people in Germany, and the alleged distributor of Engl parts in the US and what's uniform about all of these is that they do not respond to their email or phone calls. In fact they've treated the owner the same way.

A week or so later I managed to reach someone  at Weiss and they said no, they didn't have one in stock and they were having a hard time getting anyone over at Engl to respond to them. A (former) Engl repair center in California informed me that they'd stopped dealing with Engl because of similar parts hassles and the owner had two Engl amps deadlined for parts. Thanks for the good words, Jens. Velvet Distribution has not returned calls or emails at this date.

Maybe Engl could build a nicely designed product like the Panther, but it's no wonder why they couldn't move the ball across the goal line.

Donald Douglas said it best. When one of his design engineers came up with some super spiffy design for something, Douglas would impatiently say "Yes, yes, that's all very well. But how will it do in Peoria?"

That's something the Engl people ought to consider if they want to sell more than the odd amp or two here in the colonies.

From the Compression Ignition Department: Biasing a Diezel Hagen

It's rare that I have an amplifier in the shop that is more expensive than my 2002 Ford Ranger pickup, but this one arrived for a bias reset the other day.
Diezel is a smallish German builder of high end guitar amplifiers, and the Hagen is a 120w, 4 channel number with high gain characteristics that endear it to the metal crowd. That's not exactly my cup of tea (or stein of lager, for that matter).
The Hagen is often criticized for its numerous pots, but it's rather simple when you think it over-each channel has its own EQ, master volume and gain controls. Do your chosen setup on all four channels, flip the small toggle switch and you're right where you want to be.
Did I say it's well built? Everything about it is precise and well fitted, and the layout of components and wiring is organized with Teutonic thoroughness and attention to detail.
Unlike a lot of gear out there from various builders that's built down to a price, this amp is built up to a standard and it is user friendly. Top grade components abound, and the chassis is made of folded .060 steel with resistance welded end caps.

Setting the bias is a simple affair, if a little time consuming.

The KT77 power tubes are numbered T1, T2, T3 and T4. The fuses for the power tubes are  T1 and T4, and T2 and T3.
The instructions for setting the bias level are printed on the circuit board in English no less. So here goes.
Power up the amp with a suitable connected speaker load and let it warm up for fifteen or twenty minutes.  The volume and gain should be set at minimum. Then shut it down between steps of biasing
1. Remove the tube fuse for T1 and T4 on the back panel.
You will see the two bias adjust pots and three test points as shown above.
2. Using the pot, adjust to 70 ma between the MPA and MPG test points.
3. Shut down, replace the fuse, remove the T2 and T3 fuse and power up. After the amp stabilizes set to 70 ma between the MPB and MPG test points.
4. Repeat this procedure three times.

As you can see it takes a little time to let the amp stabilize and to get everything warmed up properly. There is a tendency for new tubes to drift a little.

I checked the plate voltage which measured out at 476v, and consulting the Weber bias calculator, 70 per cent dissipation looks like a few ma more than the factory limits called out, which means it's conservatively rated.

Remember, this information is purely informational and there's no warranty express or implied by you misusing it. If you don't know what you're doing here take your amp to a qualified technician for any adjustments.

Burned Up With Cement Resistors

One of the less endearing features of Fender's Deville and Deluxe series amps is the low voltage supply. Let me explain.
In order to operate the solid state devices these amps contain, it is necessary to supply -15 and +15 volts. Fender did, of course use a dedicated winding which is good because some amps like the Valveking piggy back their low voltage supply off the filament winding, and a shorted tube can wreak havoc on those solid state devices if it burps and backfires.
However, Fender chose a brute force solution to reduce the voltage down to the required level and that was the pair of cement resistors you see. In many cases such as this HR Deluxe, the circuit board can get some significant burning and in some cases the solder can melt.
I got this fix from Andy Fuchs, the New Jersey Amp Wizard, including the picture of the Arcol resistors.
It consists of getting a couple 15w 470 ohm Arcol resistors at about $3 each from Mouser. They are then mounted to the chassis with 4-40 screws and nuts, the original cement resistors are carefully removed so as not to damage the pads (easy to do), and the resistors are wired up as if they were on the circuit board.
After reassembling, these resistors transfer any heat generated right in the handy dandy heat sink of the chassis, and are barely warm to the touch.