diy fender pro jr. tube amp board layout.

Hand Wired Pro Jr. Layout CAUTION:
AMPLIFIERS CONTAIN LETHAL VOLTAGES EVEN WHEN UNPLUGGED

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Pro Jr. Board Components

HIGH VOLTAGES CAN KILL YOU - ALWAYS WORK SAFE!

You have to understand how to safely work inside the chassis of a tube amp. Just reading this is not enough, but here are a few tips: Never work on a amplifier that is plugged into the wall socket, always unplug it first! Stand on a rubber mat of some sort and don't work bare foot, under the influence of alcohol or any other drug, legal or not for that matter. Here is another good tip: Only probe an amp with you're right hand and with you're left hand by you're side. So if you do happen to take a high voltage shock, the the voltage will not go across you're chest!

High Voltage is stored in the electrolitic capacitors and they need to be discharged!That's what I'm doing in this pic. These are those large round can's and there usually blue or black. The discharge tool is made of 600 Volt wire with a 3-5 watt resistor of 500 to 1000 ohms. Basically you clip one side to ground then clip the other end to the Pos + side of the capacitor. If I have two cans as seen here then there are actually 4 capacitors 2 in each can, so I will take the time and clamp the discharge tool on all 4. Some cans can have more that 2 as well. Check with you're multimeter set to 600 DC volt setting and make sure the voltage has been fully discharged.

Please read through these two Links:

How much voltage can kill?
A Must Read!

FULL BOARD LAYOUT

Some of these components are to scale and some are not. This is a general layout idea that I use you'r layout may be differrent for any number of reason, just double and tripple check all connections against all documents-original schematic, the board, and this web site. Carefull planning through out is needed to assure that all the parts will not interfer with each other, and in amp building space is a factor.

Pre - Amp Tubes

There is not alot to talk about here just a close up of how the pre-amp sockets wire to the board.

Power - Amp Tubes

The resistors on the tube socket is R20 & R23 its a 1.5k 1/2watt Pin 1 is a NC (no connect) its used for a terminal just to hold the resistor in place. On pin 3 is the 1 OHM 1% ressistor R-CB-1 and 2 used just to aid in biasing the power amp tubes-see the bias section for more details on this. To the left is V5-GZ34 rectifier tube socket. The two yellow wires are for the 5Volt fillament supply note if your PT has a CT with the 5Volt dont use it.

Volume & Tone Pots

About as easy as control pots can get. Until you wire it wrong and they work backwards compared to what you were expecting! Keep in mind you usually connect one of the outer lugs to ground either direct or through a resistor or a capacitor, the side you use will determine whether the pot will work clockwise or counter clock wise neither is wrong old fenders were one way newer ones are the other way. Soldered to the back of the pots is a grounding buss wire before you try and solder one on sand the area of the pots to get of the plating materal and use a 30-40watt Iron.

Filtering Section

The 2 resistors R-ACT, at top right attached to the jewl Stand for Ressistor Artificial Center tap, these are used to minimize the hum from the alternating current and to further minimize this hum we twist the heater wires very tight I use a drill to get this done nicely. If your PT has a CT for the 6.3Volt don't use it just use the resistors as shown. The setup I have here is just a suggestion and in your chassis you may wan't to set it up differnt to better fit your needs or your own style.

Biasing EL84 Power Tubes

CATHODE RESISTOR METHOD
This is the easy way to bias your amp and its also the safest (for you). There are two other ways to do it also but this is what I recommend. Before you do this bias metod you have to install a 1 ohm 1% (2-3 watt) resistors from the cathode to ground on each power tube.

Before You Power Up Your New Amp/Board Be Sure To:

With all tubes in their sockets, connect your multi meter across the 1 ohm power tube cathode resistor. Polarity does not matter so pay no attention to the positive or negative sign. Set your meter for DC millivolts and connect it across this 1 ohm resistor. Turn on the amp, and as you watch the power tube current, be ready to turn the standby switch off if the current (millivolts) rises too high. Turn on the standby switch keeping a close eye on the meter. Remember that with the cathode resistor method (millivolts converts directly to milliamps) So if the meter reads 20-40 milivolts then you can bias the amp to where ever you are going to bias it (using the adjustable bias trim pot).

If the current rises up to above 40 millivolts you may not have enough negative bias voltage and the bias range resistor (RB) may have to be a smaller value. OR If the current rises up to only 10-25 millivolts, then you have to much negative bias voltage and the bias range resistor (RB) will have to be a larger value.

Here is a tip and seen in the pic, if at first shot the bias is not correct and it problably wont be, I unsolder and remove RB then connect the two alligator clips (as seen) to a new value RB ressistor and power up and re-check you may have to do this a few times in increments to get it right. Note the Heat Sink used When working with any kind of silicon chips or even a diode you can't over heat them or they will be destroyed. When you unsolder the one side of resistor RB close to the diode put a heat sink on the diode just incase.

The figures above are based on a plate voltage of aprox. 250-300 volts. measured at pin 7 of the power tube. The formula you need to know for a precise figure is:
.7(watts)/actual pin 7 voltage=your bias setting.

Pro Jr. Tag Board

Can you say Boutique?

This board went out to Eric in Stockholm Sweden Thanks! its made with the very best of quality parts! And best of all you can very easily mod or repair these Old School Boards. If you ever had to work on a pcb board in a modern amp you know that its really a pain and its very easy to destroy the delicate copper traces found in all pcb amps. Not with this board, this is solid, just like the chassis you can purchase from me soon to come.

Pro Jr. Board Components

A jumper accros R18 will give you the sound of an old school amp if desired. R4 & R11 (cathodes) don't have the usual bypass caps I like this idea alot! C13 and C14 are larger than needed but will provide smother filtering. The blue trim pot is smooth and is actually an up grade because the Pro Jr. comes only fixed biased, you can't adjust it. With this board you can tailor the sound to your liking better, running the tubes a tad hotter for a greasier more brown sound!