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Learn to work Safe! |
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 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. Check with you're multimeter set to 600 DC volt setting and make sure the voltage has been discharged.
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Next up is the bias supply I'd like to point out that a solid and well built one will keep those expensive power tubes running safely and smoothly, lose a bias supply due to cheap underrated components or poor workmanship and you will FRY your power tubes to a crisp! For the bias supply caps I use Sprague "ATOM" THE BEST electrolytic caps you can buy. They are the two large black component seen here. These are 10uf each and rated at 150 volts each! The blue trim pot is used to fine tune the amount of negative bias voltage, it's rated at .5 watts at 250 Volts NOTE: Some are only rated at .25 watts at 200 Volts. I only use the higher rated and better one! I'm a stickler for quality! By the way, this is a cermet trimmer and has a nice tight feel to it, notice how it sits, its very easy to adjust. Let me point out some other areas you need to know about like R30 its mounted on the board to the right. This is the bias range resistor if you can not get the power tubes to the recommended idle current between 30-40mA then you will have to change this resistor either up or down.
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V1 and V2 are the preamp tubes having 4 amplifying staes in all. V3 is the phase splitter and considered part of the power amp section.
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Power tubes can get tricky so let me point out a few things. PIN 6 IS A TERMINAL - internally the tube has no connection (NC). I have chosen to keep the conventional resistor arrangement on these sockets just like Marshall does.
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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 (2 watt) resistor 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 gets 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 60 milivolts or bellow then you can bias the amp to where ever you are going to bias it (using the adjustable bias trim pot). Between 30 and 40 millivolts. If the current rises up to above 60 millivolts you may not have enough negative bias voltage and the bias range resistor (R30) may have to be a smaller value. OR If the current rises up to only 25 millivolts, then you have to much negative bias voltage and the bias range resistor (R30) will have to be a larger value.
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The control pots can be very confusing but first, there seems to be a little confusion on the web about the terminal numbering so here it is and terminal 3 is the wiper. R5 on all the older non-master amps was always board mounted but not with this model. Its the low input resistor. The stock amp also had R5 paralleled with a 470p ceramic cap and most people remove these because the amp gets too bright sounding. Not part of the control pots but to the upper right you will see A (green wire) and B (brown wire) this is the negative bias voltage. They connect to pins 5 at the power tubes after going through the 5.6k grid stopprs.
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I have to thank Mark Huss for his help on this one! Another example of how wiring from just a schematic can get confusing without a good layout. These jacks are wired differently than the older 4 input amps and are unique to the 2204 and 2203 master volume model (head or combo). But what I like about them is that you can go from high gain hard rock/metal on the high input jack (3 gain stages) or to warm bluesy tones with the low input jack (2 gain stages).
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