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JCM800 6V6 & Micro Guitar Tube Amps

By Rob Robinette

Have comments or corrections? Email rob at: robinette at comcast dot net


Design Philosophy

The Marshall 2104 Master Volume/JCM800 is my favorite high gain amp of all time. It is the sound of 80's rock-and-roll guitar but it's just too loud at 50 or 100 watts. These 20 watt 6V6 and 1 watt 12AU7 amps will allow you to shred at less than ear splitting volume and lay down studio licks without ear plugs. The preamp is all 2104/JCM800 but the power amp consists of the stock long tail pair phase inverter feeding either a pair of 6V6 or a single 12AU7 power tube in true push-pull. The layouts are designed for use in a standard Princeton Reverb chassis and cab which allows the use of a 10 or 12 inch speaker. Many people gig with Princeton Reverbs so here's a shredding machine in a Princeton Reverb package.

The first JCM800 6V6 has been built by Marcus Albrecht.

Marcus Albrecht's JCM800 6V6

Marcus used a tube rectifier. Photos by Marcus Albrecht.

JCM800 6V6

The JCM800 6V6 comes in a Princeton Reverb size package but it can run 6V6, 5881, 6L6 and EL34 power tubes for 22 to 25 watts of output. I spec the AllenAmps.com TP25 power transformer and TO26 output transformer which support 5881, 6L6 and EL34 power tubes. Both of these transformers are Princeton Reverb upgrade transformers with a lot more oomph than stock transformers. They fit perfectly in the Princeton Reverb chassis.

The preferred choke is the Classictone 40-18032 5H 120ma. The bias circuit uses the power transformer's 50v tap and features bias balance and bias adjustment for 6V6, 6L6 and EL34 tubes. The power tube sockets are wired for EL34 compatibility.

Power output will be around 22 watts with 6V6 power tubes and somewhere around 25 watts with 6L6 and EL34 power tubes. Output is limited due to output transformer saturation. A pair of 23 watt 5881 power tubes is also a great match for this amp. We compensate for the different power tube plate load requirements by adjusting speaker load. For 6V6 power tubes we use an 8 ohm speaker with the 8 ohm speaker jack. For 6L6, 5881 and EL34 power tubes we connect the 8 ohm speaker to the 16 ohm speaker jack. The TO26 output transformer has both 8 and 16 ohm speaker outputs and I connect each to a separate speaker jack to make switching between tube types easy. Note that neither speaker jack has a shorting switch. Also note the NFB is tapped off the 8 ohm speaker jack.

If you don't need the 6L6/EL34 option then standard Princeton Reverb power and output transformers will work fine.

JCM800 6V6

Click the schematic to see the high resolution image. Download the pdf and DIYLC file.

 

JCM800 in a Princeton Reverb chassis. The preamp ground bus is grounded at the High Input jack ground terminal. The layout is an accurate depiction of a Princeton Reverb chassis. Note that neither speaker jack has a shorting switch. Click the layout to see the high resolution image. Download the pdf and DIYLC file.

The cap can is a JJ 40/20/20/20uF 500v can. I sourced it and its clamp from Mojotone.com. The can's terminals are marked with Y, X, U, O and "-" stamped in the terminals. The Y terminal is 40uF and the X, U and O have 20uF. The "-" is the negative or ground terminal.

You can use a standard Princeton Reverb size cab with either a 10" or 12" speaker. Mojotone sells a nice Princeton Reverb head cab too.

Circuit Tweaks

I reduced the V1A (first preamp stage) .68uF cathode bypass cap to .47uF to deemphasize lows and low-mids. I replaced the 470k treble peaker attenuator resistor in front of the cold clipper with a 330k resistor for a little added preamp gain and less shrillness. I reduced the bright cap on the Preamp volume pot from 1000pf to 330pF to keep from thinning out the tone too much at low volume. Of course you can use the original amp values instead of my tweaks but these circuit changes are time proven mild tweaks that really do improve the amp's character.

The pre-phase inverter master volume is dropped in favor of a Trainwreck Type-3 post phase inverter master volume (PPIMV). This allows the phase inverter gain to contribute to the pre master volume distortion and makes the control more useful.

The NFB resistor stays at 100k because the drop in output voltage from 6V6 power tubes is perfectly offset by moving from a 4 ohm secondary tap to an 8 ohm.

A JJ Princeton Reverb 40/20/20/20uF 500v cap can is used for B+1 through B+4 and a 22uF 500v cap is placed on the circuit board for B+5.

The 0.47uF V1A cathode bypass cap does not need to be an electrolytic. Any type of cap will work fine. I'm a fan of tantalum caps for .47uF and .68uF bypass caps. The bypass cap should be rated at 10 volts or higher.

The 6.3v heater center tap is elevated by connecting it to a Bleeder/Divider circuit at 65 volts. This heater elevation makes life much easier on the cathode follower and reduces heater hum (see layout above).

You can upload the JCM800 6V6 Hoffman circuit board DIYLC file to Hoffmanamps.com and Doug will make an eyelet or turret board for you. He can also populate the board for you if you'd like. The empty board with eyelets or turrets installed is $20 + shipping. I used to make my own turret boards but with HoffmanAmps.com accepting DIYLC files to make custom boards it's just too easy and inexpensive to bother with making them myself.

If your amp has stability and oscillation issues try chopsticking the amp's leads around. Try to separate the preamp plate and grid wires as much as you can. If you still have stability and oscillation issues, placing a 1000pf 600v+ ceramic disc cap across V1 (first preamp stage) pins 1 & 3 will help. This stability disc cap is present in many factory Master Volume/JCM800 amps.

Speaker Suggestions

An 8 ohm speaker is preferred because it makes it easy to switch between 6V6 and 6L6/EL34 tubes. For 6V6 tubes connect the 8 ohm speaker to the 8 ohm speaker jack. For 6L6, 5881 or EL34 tubes connect the 8 ohm speaker to the 16 ohm speaker jack. Doing this adjusts the power tube plate load for optimum load (7k ohms for 6V6, 3.5K for 6L6, 5881 and EL34.

I'm not a fan of the Celestion G12T-75. I prefer the standard Celestion G12M Greenback for Marshall closed back cabs but for a JCM800 6V6 combo cab my recommendation is the Warehouse Guitar Speakers Green Beret in 10 or 12 inch. I prefer the 10 inch speaker for Princeton Reverb cabs. If you want a 12" speaker in a Princeton Reverb cab consider enlarging the cab. Adding two inches to the cab's height will allow a 12 inch speaker to breathe and not sound so boxy. Most cab makers will customize their cabs if you ask. The Celestion Classic Lead is also a good speaker choice. I recommend the Celestion Vintage 30 if you like a slightly darker tone.

JCM800 6V6 Bill of Materials

You'll also need a chassis, cab, speaker, power cord, wire, power and output transformers and choke. The 40/20/20/20uF 500v JJ cap can and mounting clamp came from Mojotone.com.

To summarize the parts sources I used: I purchased the Princeton Reverb chassis, cab and most parts from Mojotone.com. The power and output transformers came direct from AllenAmps.com. I purchased the Classictone choke from AmplifiedPartsDirect.com. Doug Hoffman at HoffmanAmps.com can supply the turret or eyelet board.


JCM800 Micro

The first JCM800 Micro has been built by Paul Rico and it sounds fantastic.

Paul Rico's JCM800 Micro

For this 1 watt Micro amp the following changes to the JCM800 circuit were made:

Large value power tube grid stop resistors are used and the grid leaks are reduced. The grid stopper and grid leak resistors have been rearranged to form a voltage divider to attenuate the phase inverter output to help the little 12AU7 triodes work as our power tubes without being overwhelmed.

The JCM800 power supply is modified to keep the little power tube happy. Since our triode power tube has no screens the screen power node is deleted. The choke is moved to smooth the entire amp's power supply.

The 6.3v heater center tap is elevated by connecting it to a Bleeder/Divider circuit at 65 volts. This heater elevation makes life much easier on the cathode follower and reduces heater hum (see layout below).

Most of the Micro changes are after the phase inverter. Click the schematic to see the high definition version. Download the schematic pdf here and the DIYLC file here.

Princeton Reverb chassis layout. The preamp ground bus is grounded at the High Input jack ground terminal. Power Transformer wires match the Classictone 40-18027 for 120v USA mains power. I used a few metal film resistors (light blue) for less hiss but left most carbon comp for mojo. The layout is an accurate depiction of a Princeton Reverb chassis. Click the layout to see the high resolution version. Download the layout pdf here and the DIYLC file here.

I recommend the use of a Princeton Reverb chassis and Classictone 40-18027 Princeton Reverb power transformer. The 40-18027 supports 120v and 240v mains and has two high voltage windings. We'll use the lower voltage 275-0-275v 100ma red-white high voltage leads which are perfect for the JCM800 Micro. The entire amp uses only about 30ma at idle. The power transformer has a 6.3v center tap so an artificial center tap is not needed. If you need a stand-up transformer the Classictone 40-18085 is perfect.

The output transformer is a 5 watt rated Hammond 125B wired for 22.5K:8 ohms (use the orange and yellow secondary wires--do not connect the black wire). This little output transformer will give us some big amp transformer saturation for Vibroverb-like compression and sustain. I purchased mine from Mouser.com.

A Classictone 40-18040 4H 50ma choke easily supplies the entire amp with ripple-free power.

The cap can is a JJ 40/20/20/20uF 500v can. I sourced it and its clamp from Mojotone.com. The can's terminals are marked with Y, X, U, O and "-" stamped in the terminals. The Y terminal is 40uF and the X, U and O have 20uF. The "-" is the negative or ground terminal.

You can upload the JCM800 Micro circuit board DIYLC file to Hoffmanamps.com and Doug will make an eyelet or turret board for you. He can also populate the board for you if you'd like. The empty board with eyelets or turrets installed is $18 + shipping. I used to make my own turret boards but with HoffmanAmps.com accepting DIYLC files to make custom boards it's just too easy and inexpensive to bother with making them myself.

You can use a standard Princeton Reverb size cab with either a 10" or 12" speaker. Mojotone sells a nice Princeton Reverb head cab too.

Circuit Tweaks

I reduced the V1A (first preamp stage) .68uF cathode bypass cap to .47uF to deemphasize lows and low-mids. I replaced the 470k treble peaker attenuator resistor in front of the cold clipper with a 330k resistor for a little added preamp gain and less shrillness. I reduced the bright cap on the Preamp volume pot from 1000pf to 330pF to keep from thinning out the tone too much at low volume. Of course you can use the original amp values instead of my tweaks but these circuit changes are time proven mild tweaks that really do improve the amp's character.

The pre-phase inverter master volume is dropped in favor of a Trainwreck Type-3 post phase inverter master volume (PPIMV). This allows the phase inverter to contribute to the pre master volume distortion and makes the control more useful. The choke is moved upstream to supply the entire amp with ripple free power. A 12AU7 based power amp is used for 1 watt of clean output power. Smaller value filter caps are used due to the very low power requirements of the amplifier.

A JJ Princeton Reverb 40/20/20/20uF 500v cap can is used for B+ through B+3 and a 20uF 500v cap is placed on the circuit board for B+4.

The 0.47uF V1A cathode bypass cap does not need to be an electrolytic. Any type of cap will work fine. I'm a fan of tantulum caps for .47 and .68uF bypass caps. The bypass cap should be rated at 25 volts or higher.

If after chopsticking the amp's leads around you still have stability and oscillation issues, placing a 1000pf 600v+ ceramic disc cap across V1 (first preamp stage) pins 1 & 3 will help. This snubber stability disc cap is present in many factory Master Volume/JCM800 amps.

JCM800 Micro signal flow begins at upper right at the High Input jack and ends at the speaker jacks and NFB circuit.

Speaker Suggestions

I'm not a fan of the Celestion G12T-75. I prefer the standard Celestion G12M Greenback for Marshall closed back cabs but for a JCM800 Micro combo cab my recommendation is the Warehouse Guitar Speakers Green Beret in 10 or 12 inch. I prefer the 10 inch speaker for Princeton Reverb cabs. If you want a 12" speaker in a Princeton Reverb cab consider enlarging the cab. Adding two inches to the cab's height will allow a 12 inch speaker to breathe and not sound so boxy. Most cab makers will customize their cabs if you ask. The Celestion Classic Lead is also a good choice. I recommend the Celestion Vintage 30 if you like a slightly darker tone.

JCM800 Micro Bill of Materials

You'll also need a chassis, cab, speaker, power cord, wire, power and output transformers and choke. The 40/20/20/20uF 500v JJ cap can and mounting clamp came from Mojotone.com.

Classictone 40-18027 Power Transformer Wiring & Specs

Note the 120v and 240v primary options. For 120v mains voltage we join the Black & Brown wires and we join the Black/White & Brown/White wires. For 240v mains we join just the Black/White and Brown wires. We'll connect the Red/White 550v high voltage wires to the rectifier tube. Shrink-tube the two unused red wires so they cannot touch each other or anything else.

Hammond 125B Output Transformer Wiring & Specs

We do not use the Black secondary wire.

To summarize the parts sources I used: I purchased the Princeton Reverb chassis and most parts from Mojotone.com. I purchased the Classictone power transformer and choke from AmplifiedPartsDirect.com. I had to go to Mouser.com to get the Hammond 125B output transformer. Doug Hoffman at HoffmanAmps.com can supply the turret or eyelet board.


The Full Size Marshall 2204/JCM800

Click the schematic to see the high resolution version. Download the schematic pdf and DIYLC file.

 


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