Adjustable Bias for the Blues Junior
For rev. A/B/C/D (green) circuit board, 1995 through mid-2001

Before you do another thing with your Blues Junior, get the bias under control! The way it came from Fender, it's running the output tubes way too hot.

Bias is a negative voltage that's applied to the grid of the tube that controls the amount of electrons--the plate current--that hit the plate. If too many hit it, the plate overheats. It can actually turn red and even melt. But long before the plates melt, excessive plate current shortens tube life, overheats your output transformer, and makes your amp sound bad. The maximum rated plate dissipation for an EL84 is 12W, so for class AB (the way the tubes interact with the output transformer) operation, it should dissipate approximately 70 percent of that, or 8.4W, at idle. The stock Blues Junior dissipates nearly 12W per tube at idle, meaning that as soon as you turn the volume up, you're exceeding the tube's design limits. I have no idea why Fender chose to set the amp up this way--they certainly know as much about tube circuits and operating parameters as anyone. If you fix their error, however, your amp will sound better and last longer. I've measured the temperature difference between a the output tubes of a stock-biased Blues Junior and one that's properly adjusted, and the difference can be as great as 100°F.

A member of the Fender Discussion Pages, Luca_r (from the Venice, Italy area), did an adjustable bias mod for the Blues Junior that I liked very much. This mod is based on his fine work.

This mod requires access to the back of the circuit board. See the reverb mod page for some suggestions on the best way to loosen the board. I don't remove it from the chassis, I just tilt it out.

Below is the stock Blues Junior bias supply. The connection C- goes to R21 and R22, which then go to the grids of the power tubes. R37 and R31 form a voltage divider, which sets the bias voltage. On a stock BJr, it's about 10 volts, which results in over 40 milliamps of plate current. Given the approximate 330V plate voltage, that comes out to around 13 watts per tube. That may sound like a good thing, but it's well over the limit for an EL84. Also, in class AB circuits (sorry, I'm not going to explain that further) the power tubes take turns pushing the signal through the output transformer. They're supposed to "rest" for part of the cycle, and plate dissipation is typically set for 70  or 75 percent of the maximum allowable, which is 12 watts. So each tube should be dissipating 8 or 9 watts, not 13!

This is Luca_r's modified bias supply. It changes the values of the voltage divider and adds two 50K trimmer potentiometers (trimpots). Two pots allow each tube to be biased separately, so matched pairs are unnecessary. It also becomes possible to intentionally unbalance the bias by a few milliamps, which sounds better to many guitarists. It's necessary to change the voltage divider resistors so that the total resistance from the -26.5V test point to ground remains roughly the same as the stock amp.

	I implemented the mod with two  trimpots, and made all of the connections from the back of the board for a clean look. Here's what it looks like. The two blue circular objects are the trimpots, and the white center is slotted for a small screwdriver. These particular trimpots are 4-turn pots, meaning that it takes four full revolutions of the adjustment screw  to adjust the pot through its full 50K range. This allows very accurate adjustment of the bias voltage. This adjustment is not hypercritical, but it's easier than with single-turn pots, which can be touchy and difficult to set exactly. Here's a close-up of the trimpot, a Bourns type 3339P. They're available from mouser.com and other electronics suppliers. At around $4.50 each, they're more expensive than open-frame trimpots, but the convenience and reliability outweigh the additional cost. I placed the trimpots in the large open area to the right of the power supply capacitors. This is the bias supply area. You'll notice that the 5K  replacement for R31 is no longer in the original position. I drilled a new hole for it (using a number 55 drill) to connect to the trimpots. Update: I'm now using a Bourns 25-turn trimpot for this mod. It's more accurate and less expensive.
	Here's the view from the back. You have to cut traces in two places. The first cut is made on the trace between R31 and R21 and 22. The second cut is made between R21 and R22 to separate them, so that they can be adjusted individually. Cuts are made with an X-Acto knife, but scrape side-to-side to remove the trace so a slip doesn't damage any other traces. Check with an ohmmeter to make sure there's no connection. The outer legs of the trimpots are wired together, and the wipers (center connection) go to R21 and R22. The light blue wire runs from the left side of the trimpots to the hole where R31 used to connect to the capacitor and to R37. See below. Remember to replace R37 with a 27K or 28K resistor.
	This diagram gives you a close-up of the wiring of the trimpots on the back of the board. You have to drill seven small holes in the circuit board, 3 each for the legs of the trimpots and one for the new location of  one leg of R31. I used a #55 drill, but Luca_r drilled his by just wearing through with a small jeweler's screwdriver. Tiny drills are fragile; don't press too hard. When you connect the leads together or leads to wires, make sure you have a tight mechanical connection. Don't trust the solder alone to hold them. I made a loop at the end of each wire where it attached to a component lead and crimped it, then soldered..

When you've completed all of the wiring, test with an ohmmeter to make sure there are no shorts, that R21 and R22 are no longer connected, and that the trimpots are operating correctly. Before you power the amp up, use your ohmmeter to measure the resistance of each half of the primary side of the output transformer from the center tap. The center tap is a red wire plugged into CP2, just over the "CAUTION" sign.The primary connections are brown and blue, plugged into CP1 and CP3. Each side should measure around 100 ohms. Write down the resistance for each side.

Remove the EL84s from their sockets and turn the amp on. Test the voltage at the top of R21 and R22, and set each to -13V with the trimpots. This will give you a good starting point for your bias adjustment. If you don't have a negative voltage that you can adjust at R21 and R22, go back and troubleshoot your wiring.

Proper Biasing
Turn the amp off, reinsert the EL84s, and power it back up. Now you can bias it properly. There are many ways to bias an amp, and many of them give erroneous results. Some require you to install test points or additional resistors, while others require you to use an oscilloscope. None of these are necessary. Randall Aiken of Aiken Amplification has the best dissertation on all of the right and wrong ways to measure and set bias at his site, in an article called The Last Word on Biasing. Required reading!

 Using the output transformer resistance measurements you made earlier, you can calculate the exact amount of plate current from the voltage drop across each side of the winding. If your plate voltage is 330V (it may be a little more or less in your BJr) and you're running at 70 percent dissipation, 8.4W, the maximum plate current is 25ma (.025a). Multiply that by the resistance of each side of the primary, and you have the target voltage drop for that output tube. In my amp, the primaries measure 96.4 and 97.3 ohms respectively, which gives target voltage drops of 2.45 and 2.48V.

If you would like a simple spreadsheet that calculates the target voltage based on your amp's plate voltage and output transformer resistance, here's an Excel spreadsheet you can download:

EL84 Bias Calculator

Once you do this, the amp will sound different--brighter, clearer. Then it's time to do the tone stack mods! You'll also find that the power tubes run noticeably cooler. Tubes and components drift with time and temperature, so you'll want to check your readings again after you've used the amp for a while, but don't get crazy about it. Remember, too, that a slight imbalance of 5-7ma can actually improve the sound (distortion is a good thing in guitar amps). You can also use any unmatched tubes, including different brands, to get a distinctive tone. Just adjust the bias to the proper target voltage, and it'll work great.

And if this is just too difficult...
You can bring any BJr much closer to its proper operating point by changing the resistors in the bias voltage divider. This isn't nearly as good as having adjustable bias, but it's way better than the factory values. The stock bias resistors in a green board BJr are R31 and R37, 22K and 33K respectively. (In the cream board, they're designated R52 and R51, also 22K and 33K respectively.) Leave the 33K alone and change the 22K resistor changed to 27K or 30K. The 27K should bring your C- voltage up to around -12.5V from the -10V factory setting, and will cause the tubes to dissipate somewhere between 8 and 9 watts at idle. The 30K resistor in place of the 22K will give you around -13V, which will give you clean, incisive tone. Going too cold makes the amp sound a little cold and gritty, but with as little as-1.9 or-2V voltage drop across the transformer, you can have an extended clean range--very respectable jazz tone.

Obviously, there are a lot of BJrs out there cranking away at 11+ watts idle dissipation. They're not bursting into flames or melting their vinyl covering or anything. But they'll last longer and sound better if they run a bit cooler. I've seen many older stock Blues Juniors with heat damage to the circuit board that holds the tubes, and the expansion and contraction caused by the high heat causes solder joints to crack.

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