Replacing a Selenium Rectifier On A 5D6-A Bassman

 This vintage Bassman came here a few weeks ago for an overhaul and the first thing I noticed was that it still had the nonadjustable bias with selenium rectifier setup of yore. After a general refurbishing, I ordered some small trimmer pots from Mouser. Do not order fake Bourns pots on ebay folks. I took a small pair of pieces of vulcanized fiberboard and mounted the components that duplicated a later adjustable Bassman bias setup, and dropped it into the slot where the selenium rectifier used to live.

There are a couple reasons for doing this. Selenium is nasty, toxic stuff for starters. The selenium rectifiers are pretty unreliable as diodes and they don't have nearly the capacity that modern day silicon diodes have. Plus, you get to fine tune the bias level and get all that early fifties goodness that Leo built for you.

Ripoff Alert: Eight O'Clock Coffee

Editor's Note: This is a little out of the usual amp realm but we need coffee to fix amps, right?

I was at the grocery this morning shopping as I do every Sunday and I went down the aisle to grab a bag of 8 O'Clock coffee.

The first thing I notice is they're out of the red label stuff so I grab a brown label-the price is always the same.

Then I notice the price is two dollars less.

Then I notice the bag is marked Value Pack, and it is 1 pound 6 ounces instead of  2 pounds 4 ounces.

Doing the math, the net price of the product in the older package is $0.35 per ounce and the net price of the product in the new "Value Pack" is $0.68 per ounce.  Thus, they manage to double the price of the product and call it a value. For whom?

Truth in advertising would be better served if the new Value Pack was marked "Bend Over Pal Pack."

Shame on you,  8 O'Clock. Do you really think we're that stupid?

Transforming a Marshall JCM2000 DSL 100 watt head

Like many others of its kin, the Marshall JCM2000 DSL 100w head suffers from a poorly made power transformer-in this case, a Dagnall TXMA 00061. The pictures show a flaw in the wrap, and this was where the smoke was coming out. The good news is that Dagnall is no longer in existence. The bad news is that until recently a replacement power transformer was hard to find and tough on the wallet-the only replacement item was a Mercury Magnetics Fat Stack that comes in at about $325.00 or more. The last time I did this I was able to locate a used serviceable Dagnall.

Just this year Magnetic Components, a/k/a Classic Tone. began offering power transformers for these amps, and I obtained one. Of course, the wiring color codes are different, and you have to make up some of your own connectors because the Classic Tone does not come with spade connectors installed.

The first thing I did was make a map of the old connectors on the Dagnall and where they went to and on what board-main, or power inlet. Then, I took the hookup diagram that Classic Tone provides and started doing the connections. It's simple enough if you can read a schematic. If you get a little confused it's a good idea to draw it out for yourself.  It's also a good idea to save some of the wiring you remove, because you're going to have to do a little splicing to wire up the bias circuit with the right size spade connectors. Some of the connectors are a bit smaller than our standard, but you can take your new ones, squeeze them a bit and re-flatten them out, making sure they're a good fit on the lug. You can see what the finished installation looks like in the photos.

The Classic Tone has more metal in the core, and it's about halfway between the original installation and the Mercury Magnetics Fat Stack.

Next, removing the old power transformer you can mount the new one so that the green and green/yellow leads point toward the filament circuit lugs on the main board. After wiring up the power input per the schematic for 120v (primaries in parallel) you can proceed to wire up the filament circuit and the B+ (ok, ok, high tension) circuit, which runs through the standby switch. At this point I would stop, and power up the amp with no tubes in it. You can then check for 6.3v on the filament circuit-I check the preamp tube sockets with an old RCA cleartop tube. At this point, make a chart and measure your filament, plate (pin 3)  and screen (pin 4)  voltages and note them. You should see something resembling operating voltages.

Now, you'll have to splice the small plugs onto your bias lugs on the main board. The old ones are white-black-white, and the new ones will be white-blue-white.

Once you've done that you can power up again and measure your bias voltages on pin 5 of the power tubes and you should see something around 45 volts DC-remember, it's rectified.

If this is all good, you're ready to police your wiring job, set up to check bias per the Marshall method using the Molex connector on the back of the tube pan, and adjust as required. It's always a good idea to start out low and slow until everything's had a chance to settle down and you're sure you aren't going to blow any fuses. Then you can adjust up to around 80 mv per side using the pots provided.

What The Mouse Done Ate: Reviving A Gibson BR6

This Gibson BR6, circa 1947 followed me home a few years ago from an auction in Mount Union, Iowa for forty bucks. When I got it home and opened it up I found a lot of gnawed wire and caps and one very dead mouse. No Bromo Seltzer was gonna  save his skinny ass.

I started the task of rewiring it and then for some reason put it aside until this week. All I really had left to do was to finish the power supply and the phase inverter which took a few hours of thinking how I was going to engineer it. The solution amounted to mounting some of the components on terminal strips. I glued a pair back to back and mounted them on the power transformer studs which is where the phase inverter resistors went..

I had decided to build it as a BR6F circuit, which used a pentode preamp tube, mostly because I like pentodes but there's another reason. The volume control in the original BR6 circuit is ahead of the first preamp tube grid which makes for a noisy and feedback prone amp, because it's running wide open all the time. You have to attenuate the signal with your guitar which cuts out a lot of the tone. Maybe lap steels are different but that layout isn't any good for guitar. I had the same problem on an old Flotatone I have, and in that case rewired it as a 5C3 Deluxe. It solved the noise and feedback problems so I knew it was a good fix. The BR6F is a much improved circuit.

I also installed a 250 ohm cathode resistor bypassed by a 22 uf electrolytic, which qualifies as a modification but is an improvement to my way of thinking. It probably would sound a bit more raw without the capacitor but I'm all about clean tone. It's good the way it is.

The wiring is not as pretty as it could be but remember, I started this project a number of years ago. 

Surprise! It fired right up at the first flick of the switch and now purrs like a kitten, which is a first for me It sounds pretty good within its limits and will make a good addition to the fleet or some trade bait.

I spent a couple of happy hours yesterday putting this amp through its paces and it's the equal, if not the better of any Valco of similar age and configuration. Aside from the Ruby 5AR4 blowing its wad while I was watching, it's a good performer.

Series String Amps: Demystifying the Widowmaker

NOTE: Some folks have suggested that this post may not state the facts accurately. In particular one correspondent suggests that an isolation transformer only insulates the circuit from the line, and then you become the ground path. Another suggests that the same result could be achieved with a GFCI plug. I'm going to get to the bottom of this, so stay tuned and be careful with these damned things.

Here's a circuit for a series string amp and in all respects it is typical of the breed although you sometimes see these with four, fiver, and sometimes more tubes. The idea's the same.

In a never ending quest for ever less expensive radios, RCA released model schematics for what are known as All American Five radios.

By putting the filaments in series and adding a suitable resistance, a power supply of a sort could be cobbled up without buying a power transformer. The typical offering in a guitar amp works on the same principle. Get as close as you can to 120v with the filaments in series, add in enough resistance to make up the difference, a couple of good stout electrolytic capacitors and use the chassis as the return path and Bob's yer uncle as the Brits say.

Well. In a radio it's usually sitting on a shelf inside a plastic or wood cabinet and you only touch a plastic knob so what's the worry? The Arvin folks, known then, as now for their metal stamping expertise released a line of metal case radios using this technology but did a baked enamel finish that is a pretty good insulator. Sometimes I cringe when I see people who have stripped and chrome plated these sets but nevermind. The subject's guitar amps.

The evil here consists of two things-the chassis as a return path for line voltage, and the guitarist or harp player's intimate connection to that return path through the guitar or mic cable. Standing barefoot on a cement floor or wearing sweaty leather shoes only makes it more dangerous. You can get the whole 120v of AC through your hands or mouth and it CAN kill you.

There is one, and only one, way of making this safe to use and that is through the use of an isolation transformer. That is a device which ISOLATES the circuit thus described from line voltage and thus saves you from being the ground return path. There is NO other way. A three prong cord will not make you safe.

You can obtain isolation transformers pretty easily from a number of sources or, you can make your own using a pair of Radio Shack door bell transformers back-to-back and accomplish the same goal.

Having said all that, these can be really fun amps to play with, but be advised: They're not really safe in their unmodified state.

UPDATE: One of my colleagues has posed the question of using a GFCI self contained cord with these amps. I myself have not used them, and the idea seems to be OK, but I think the difference is that with a GFCI setup you're interrupting a potentially dangerous problem, and depending on a relay to do it. I think the difference is that relays do not always do what you want when you want, and the problem is always there-a hot chassis connected to one side of the line. With a suitable isolation transformer, you eliminate the hot chassis problem completely and totally forever.

Thanks to Clark Huckaby for the use of his schematic. Just sharin' the wealth and keepin' it real, folks.The isolation transformer setup is the work of Dennis Poirier. A great fellow.

Building a Solid Workbench Addendum

One of the things everyone should have in their shop is a good solid work bench. I was getting by with a $40 Chinese made plastic topped banquet table that I got at Menard's, but soon after I put it together the overhang end started to gradually fold and collapse. Last week I knew something had given way because the end of my bookshelf was sagging badly.  The corner was folded under and fractured, and total collapse was imminent.

I built a workbench for the garage a month ago using the plans I found here and decided that it could be adapted to my particular space considerations. Although the plans call for rather tall legs and a long top, I made mine with 30 inch high legs and a 60 inch surface. You can adjust the dimensions any way you like to fit your particular needs and whether you plan to work standing up or sitting down.

For this kind of work it is better to attach the top with flush surface wood screws which I did. Then, if you like, you can attach a sheet of tempered masonite for hardness.

The biggest cost items are the Simpson Strong Tie RTC24 corner braces, which run nearly six dollars each. That puts you at about $55 with braces, hardware and a tube of construction adhesive. The Simpson people have been making structural connectors for wood structures since the 1950s and their products are good stuff. When you're done, you will have a very strong and rigid workbench or work table.

It takes a couple of hours to cut your lumber and put the bench together.

Here's a picture of the bench in use.

Fender Excelsior, You Fathead

These are pics of my recently acquired Fender Pawnshop Excelsior. Like others I have undertaken a few minor modifications to make the amp more to my liking.

The first was adding a tone control to replace the two position bright-dark switch. A 500k pot is installed and wired just like the switch with the existing wiring. You can widen the switch hole a little with a small file or two, and then use a fender washer to dress up the area and cover the screw holes. Be careful not to scratch the surface of the panel with your file-I did, and I'll probably strip this down and have it electroless nickel plated before long. A trio of vintage chicken head knobs dressed things up a little.

I don't know about you but I don;t like orange pilot lights. I snipped the wires to this eyesore and installed a typical Fender type lamp socket with a red jewel and a #47 bulb. I piggybacked off the wires for the second 12AX7 preamp tube socket which read a nice 6.7 volts. Plate voltage runs about 370 on these amps by the way. At the same time I added a pair of JJ 6V6 power tubes and a Sylvania JAN 12AX7 in the first position.

How's it sound? The tone control gives a little more flexibility and the tube upgrades are useful. Plus Leo would have approved of the red jewel.

Plus you get to see my newly reconstructed shop complete with killer work bench I built Sunday.

My apologies to the ghost of Jean Shepherd-if you spent any time connected to a radio in Jersey youy get the joke.