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RR2104 Master Volume 6V6 & Micro Guitar Tube Amps

By Rob Robinette

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


Design Philosophy

This amp is a work in progress. Check back here for updates on these Marshall Master Volume/JCM800 based amps.

The Marshall 2104/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 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 layout is designed for use in a standard Princeton Reverb chassis and cab which allows the use of a 10 or 12 inch speaker.

Master Volume 6V6

For the 6V6 amp I spec the AllenAmps.com TP25 power transformer and TO26 output transformer which also support 6L6 and EL34 power tubes. Both of these are Princeton Reverb upgrade transformers with more omph 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 of 6V6, 6L6 and EL34 tubes. The power tube sockets are wired for EL34 compatibility.

JCM800 in a Princeton Reverb chassis. 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 150k resistor for 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 NFB resistor stays at 75k 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 4 and a 20uF 500v cap is placed on the circuit board for B+5.

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.

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 Master Volume 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. The Celestion Classic Lead is also a good choice. I recommend the Celestion Vintage 30 if you like a slightly darker tone.

You can upload the Master Volume 6V6 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.

To summarize the parts sources I used: I purchased the Princeton Reverb chassis 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 made the turret board.


Master Volume 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 triodes work as our power tubes without being overwhelmed.

The NFB resistor is reduced to 22k due to the very low output voltage of the Micro.

The 2104 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 now smoothes the entire amp's power supply.

The 6.3v heater center tap is elevated by connecting it to the power tube cathode resistor (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. Click the layout to see the high resolution version. Download the layout pdf here and the DIYLC file here.

I am in the process of building the first Master Volume Micro but I'm not expecting any issues since it uses the power amp from the very successful Bassman Micro LTP amplifier.

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 Master Volume 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. We connect the 6.3v center tap to the hot side of the power tube cathode resistor to elevate its ground reference to reduce heater hum. 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 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 150k resistor for 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+1 through 4 and a 20uF 500v cap is placed on the circuit board for B+5.

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.

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 Master Volume 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. The Celestion Classic Lead is also a good choice. I recommend the Celestion Vintage 30 if you like a slightly darker tone.

You can upload the Master Volume 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.

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 made my turret 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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