diy marshall jcm800 2204 2203 tube amp layout and mods.

Marshall Jcm800 2204

CAUTION: TUBE AMPLIFIERS CONTAIN LETHAL VOLTAGES
EVEN WHEN UNPLUGGED

Table of Contents

READ FIRST-DANGER HIGH VOLTAGE

New JCM800 2204 Layout, BOM, & Schematic in .pdf

DETAILS OF BOARD COMPONENTS

SAFETY AND FILTER CAPACITORS


	Back to top !!CAUTION!! HIGH VOLTAGES EVEN IF UNPLUGGED! TUBE / VALVE AMPS CAN STORE VERY LARGE VOLTAGES! THESE VOLTAGES CAN KILL YOU! 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. Check with you're multimeter set to 600 DC volt setting and make sure the voltage has been discharged. Please read through these two Links: How much voltage can kill? A Must Read! Back to top

2204 MASTER VOLUME BOARD

Back to top I have this board in my 2204 home brew, and as seen @ Ebay. at the bottom of this web site. This board measures 12" X 3" std. BUT I also have the same layout that measures 10.75" X 3" just ask. IF this is going into a Marshall chassis then there should be enough room for the 12" board.

The on board wire is Belden Teflon coated and rated at 600+ Volts. The large white resistors? These are rated at 5 watts at 500 Volts each, used to step down the B+ voltages from the power supply.

LEFT SIDE OF BOARD

In this pic from left to right you see the cathode resistors and bypass capacitor. The blue resistors are for the plates and are Metal Film to help keep noise as low as possible. The yellow coupling caps are Mallory 150's .022 uf . Mallory 150's are the choice favorite of a lot of amp builders including me. If you prefer something like "Orange Drops" 715p's just let me know because the parts are subject to availability at times. At the end of this picture you see an odd shaped brownish component this is a "Silver Mica" capacitor used for the treble control in the tone stack, the carbon comp resistor horizontal from that part is the slope resistor. These are some of the components used for V1-a and b. The first two stages of the 2204 amp board.

RIGHT SIDE OF BOARD

At the very end of this pic you will see larger than the normally used 1 Amp diodes. My boards use diodes rated at 3 amp's each and therefore provides safety and a more reliable amp like those 5 watt resistors do. But there is much more good stuff read on...

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.

FULL BOARD LAYOUT

Missing from this layout is the power transformer, primary 3amp fuse, power switch, standby switch, output transformer and speaker outputs. This is not for a novice, you have to have some background with tube amps and electrical skills!

PREAMP TUBE WIRE DIAGRAM

Seen above are the pre amp tubes V1, V2, and V3. V3 is the phase splitter and considered part of the power amp section. As you can see there is no connection to pins 4, 5, and 9 because these are the tube heaters and it is assumed that it's already hooked up if you are doing just a board replacement.

POWER TUBE WIRE DIAGRAM

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.

BIASING POWER TUBES BASICS

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.

POTS LEFT SIDE

The control pots can be very confusing so I have included three large pictures to help out the less experienced person. 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. Note the line across the back of the pots, this is usually 22 AWG buss wire soldered to the back of each pot. R5 on all the older non-master amps was always board mounted but not with this model. Its the low input resistor and I use 68k on mine instead of the stock 470k. The stock amp also had R5 paralleled with a 470p ceramic cap (C4 not shown) 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.

POTS RIGHT SIDE

This pic again gives a clear view of the wiring on the other end. But 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 ideal current between 30-40mA then you will have to change this resistor either up or down.

POTS TONE SECTION

In the upper left hand corner is C10, C11, and C12. These are the tone caps and affect how the Bass, Mid, and Treble respond. It's one thing to see a schematic and another to actually wire it. Some times this transition is not that easy, hope this helps.

CLIFF JACKS

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).

PRE AMP SCHEMATIC

This schematic is dated 1988 and is what my board layout is based on. All labeled components from this schematic can be directly linked to my board layout. Now there are a few mods that added a few extra items in my board layout that are not on this schematic those are defined by a "00" on my board layout. I actually added one to the above schematic see R00. So on my layout if you see R00 or C00 this is a mod that I added (R=resistor and C=capacitor). None of the mods affect the board layout in any way and are easy to add or remove. Let me know if there is a particular set up that interests you.

1988 2204 Preamp Schematic Here

POWER AMP SCHEMATIC

This is a continuation of the main schematic used but of the power amp section. The long tail type phase splitter can also be seen here along with the, bias and negative feed back circuit. Notice R30, this is the bias range resistor that may need adjusting when biasing your new board. On my home brew I can switch my negative feed back on or off and so far I seem to like it off.

1988 2204 Poweramp Schematic