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.