This JCM800 – Sold

This amp is now sold. 

Click below images for huge versions!


Tweaks – It’s never really done…

For my other amps, I built them with mods before even testing a stock configuration. I learned about these mods from the Tone Lizard website. For this build, I wanted to start stock and then gradually make the mods in the spirit of keeping the amp as close to original as possible. But after playing the amp and comparing it to one of my previous builds, I decided to do the mods. The amp is way too bright when lowering the preamp gain knob. I was sort of expecting this since I installed the .001uF bright capacitor that many people clip out. I removed the capacitor and this solved the brightness issue.

The next step was more gain. I lowered the cathode resistor to 5.6K (stock is 10K, I had a 8.2K installed) and removed the treble peaking circuit. I then placed a 68K right on pin 2 of V1. The amp now sounds the way I want it to sound!

My next step will be experimenting with some NOS tubes.


Killing the Hum – Now it’s really done!

I have done a lot of research and asked a lot of questions over the course of the last three weeks. Unfortunately the Metroamp forum has crashed and no longer a source of information. I turned to another forum called The Amp Garage. A lot of people jumped in to help and many offered suggestions. I was curious about the “Larry” grounding scheme some people have incorporated. I also thought that phase inverter filter cap might be bad as the hum vanished when I pulled the phase inverter tube. A leaky diode could also be the problem as my research hinted. I decided to make some changes, but it would mean pulling the board and redoing a lot of wiring. Since this is my “forever” amp I decided to go for it. I found a JTM version of the Larry grounding which I adapted to my 2204 circuit. Since I was pulling the board anyway I decided to make some other changes I wanted to do. They are as follows:

– Swapped out the top hat diodes for Ultra Fast UF5408
– Swapped all of the 22AWG wire on the board for 20AWG
– Swapped out the skinny shielded input wires for Lava Mini ELC
– Added a 1000pf bright cap (mustard)
– Incorporated the “Larry” star ground scheme

These changes took a couple of days but I took my time and did them carefully. I ran the same voltage tests and fired up the amp. I thought I had mis-wired something because it didn’t seem to be making any sound at all. But low and behold, plugging in my Tele proved differently. The amp blasted to life. The hum was gone and it sounded even better than the first test! I jammed on the amp for a while. I felt proud. This thing took damn near a year but I eventually got it done.

Am I really done? No way. I want to record some sound clips. I also want to change the JJ tubes and try some N.O.S. glass. Mike from KCA tubes and I have been discussing which tubes fit into my budget. That’s a good one – budget! I have also acquired a 1990 Marshall 1960a 4×12 slant cabinet as I believe this amp deserves some serious paper to push the sound through. I have ordered a separate head shell cabinet from Mojotone so I can stack it with the 4×12. This will give me the option on putting the amp in the combo cabinet for smaller gigs, or swapping it to the head cabinet for bigger venues.

What a great day.

Installing the Circuit Board and Completion.

With the leads all wired the next step was to drop in the board and wire it to the tube sockets, potentiometers, and transformers. This was time consuming for me as I have raging O.C.D. so every wire must be just right. I took my time and I am happy with the lead dress.

*** WARNING – THE VOLTAGES INSIDE OF A GUITAR AMP CAN KILL YOU! Even adfter the amp is unplugged the capacitors can store lethal voltage. Learn how to drain caps by clicking here.***

I did some preliminary voltage testing as described in the Metroamp 50 Watt Kit Instructions. My voltages were slightly different because of the transformers and layout I was using, but I found correct voltages on the internet. Everything looked good. With a speaker attached, I biased the power tubes to 40ma as suggested by Chris Merren.

Now the final moment – I plugged in my hot rodded Tele and flipped the standby switch. The amp came to life. What a great St. Patrick’s day present! I goofed around a little with the amp and the tone is unreal with a beautiful aggressive and full crunchy sound. I let it burn in for a while, hitting a chord every now and then.

But all is not perfect in paradise. I noticed the amp has a slight hum even when no guitar is plugged in or even if the master volume is turned down. It is almost too small to complain about, but this is to be the keeper amp of my life. I must track down the source of the hum to make the amp perfect.

Yes, I know I am nuts.

I will do some research and hit the forums for some ideas.

Stay tuned.


Adding Wire Leads to the Circuit board

Soldered the components to the turret board before adding the wire leads. This is actually the incorrect order of normal wiring as soldering the leads to the turrets can introduce heat into the resistors and caps possibly damaging them. I encourage you to wire the leads first and then mount the resistors and caps. Since it was too late for me to do this, I clipped an alligator clip to each component just above the solder joint to act as a heat sink and dissipate the heat away from the component. It worked well because when I felt the components right after soldering they were hardly warm to the touch. I like to solder using very high heat. Around 750 degrees on my Weller WESD51 digital readout soldering iron. The idea to to get in and get out quickly so the heat does not have time to spread to other parts of the circuit.

I used a pair of standoffs on the mounting holes to provide a stand for the board when working on it so the component leads would not get bent.

I used 22AWG PTFE wire and left the tails slightly longer than normal.

*** NOTE – I later changed all of the wire to a thicker 20AWG. This wire holds its form better when bending and may provide increased signal flow. I also later routed the wire through the perfboard and soldered the wires on the top of the board by wrapping them around the lower portion of the turrets. This was done for two reasons: strain relief (prevents the wires from breaking when pulling on them during assembly) and the ability to see the wire actually connected to the solder point. ***

Potentiometer and Input Jack Wiring

The next step was to mount the pots. I went with sealed the P.E.C. brand that I purchased from Valvestorm. The trouble with these pots is that they are stainless steel and very difficult to solder a bus bar across the backs. Doing some research I found that a buss bar could be run just connecting the lugs that needed to be grounded.

*** NOTE – I later abandoned the buss bar and switched to the “Larry” grounding scheme due to a hum in the amp. ***

I next wired up the input jacks using shielded cable. I originally had the 68K resistor mounted on the board but after several suggestions I moved it directly to pin 7 on the V1 preamp tube. This is to help reduce noise.

*** NOTE – I later replaced this very skinny cable with Lava ELC which has a braided shield for better noise rejection. It is also a thicker gauge so breakage on the solder points is minimized***

Switch and Back Panel Wiring

I installed the power and standby switches. I am not putting the front panel on yet as this only needs to be installed when the pilot light is mounted.

The left lug of the power switch is wired to one conductor of the mains fuse. The right lug of the power switch is wired to the black 120V primary wire from the power transformer.

I installed the Neutrik powerCON connector. I wired the ground to it’s own ground lug, the neutral to the to the power transformer primary common, and the live to the other terminal on the mains fuse.

The left lug of the standby switch is wired to the left filter cap. The right lug is wired one conductor on the HT fuse.

The other lug of the HT fuse gets the output transformer center tap and one wire from the choke.

I grounded the 6.3 volt center tap and high voltage center tap to a transformer bolt.
***Note – I later moved these to a cap can clamp bolt***

I soldered the 1K 7watt Welwyn resistors to pins 4 and 6 on tube sockets V4 and V5 as well as a length of wire to pin 6 on V4.

Next I wired the bias test points. These are female banana connectors. I installed 3 across the back panel. Two red with a black in the center. Between each red and the center black connector I soldered a 1ohm 2 watt resistor. I joined pins 1 and 8 from the EL34 tube sockets V4 and V5 and then soldered each one to the nearest red test point. The black test point is soldered to a ground lug on the V4 tube socket bolt.


Switches and back panel wiring.

Switch and back panel wiring.



Output Transformer and Choke Wiring

The next step was to wire in the output transformer. Using a diagram provided by Chris Merren, I wired the white primary wire to pin 3 on the V5 tube socket and the red primary to pin 3 on the V4 tube socket. The back amp plate must be installed at this time. I fed the secondary wires from the output transformer through the impedance switch hole to solder them to the switch before bolting it in. I soldered Yellow (4 ohm) to pin 1, Green to pin 2, Gray to pin 3, the orange wire is left hanging for later connection to the speaker jack. I found this method easier than trying solder the switch mounted. I used #4 stainless machine screws and Nyloc nuts to bolt the switch in place.

One of the choke wires is twisted with the OT center tap (Brown) and will go to the HT fuse holder. The other is left hanging at this time .

Small cable ties keep everything neat.

***You will notice that the extra cap is missing. This step is slightly out of order as I changed the position of the cap from the chassis wall to right next to the choke as described in the previous step. The cap clamp on the wall of this picture was removed.***


Capacitor Mounting

I am using vintage reformed ARS multi-section capacitor cans that I picked up from Valvestorm. I was originally going to mount the third can on the wall of the amp as in my previous builds, but instead drilled an additional hole in the chassis using a monster step bit,  so there is more room for wiring. It is also provides a more traditional look. Since the ARS capacitors are slightly smaller than a standard 1-3/8″ can clamp, I had to slightly modify the clamps. I used #6 Stainless bolts to bolt the clamps to the chassis and #4 stainless bolts in the hole that pinches the cap. I am using 20 gauge PTFE wire for the leads.

Time to Build The Amp

I have most of the parts collected and it is time to start putting this beat together. I ordered stainless steel hardware from Bolt Depot. I go as heavy as I can on the hardware and every nut and bolt is stainless. I also use Nyloc nuts where ever possible so the amp stays together when vibrating during transport or at loud volume. To quote Spinal Tap, “Heavy Duty – Brings out the duty in my soul.”

I inserted the 1/2″ and 3/8″ rubber grommets in the chassis that prevent the output transformer and choke wires from rubbing on metal. I got these and many parts from Valvestorm. They have a very cool 50 watt checklist of parts. This list does need to be slightly modified for a 2204 circuit.

I bolted in a Merren 1202-118 power transformer with #8-32 x 2-1/2″ bolts and Nyloc Nuts. I put three grounding lugs on the bottom right bolt.
***Correction*** I later moved the ground lugs to the bottom left bolt and used only two.

I bolted in a Merren 784-139 output transformer with #12-24 x 1/2″ bolts and Nyloc nuts.

I bolted in a Marstran 1999 choke with #12-24 x 1/2″ bolts Nyloc nuts. I used a tapered reamer to widen the holes just a bit.

I bolted in Amphenol Bakelite power tube sockets. Chris Merren (transformer supplier) insisted that I used Bakelite sockets as he swears they improve the tone. Who am I to argue? He sent an eBay link and I purchased them right away. I did have to use a Dremel with a wire brush to clean up the terminals. They are pretty tough to get centered in the tube socket holes just right as they use a spring clip. I used #6-32 x 3/8″ stainless bolts and Nyloc nuts.

I bolted in Cinch Bakelite 9-pin tube sockets for the remaining tubes. I used #4-40 x 3/8″ stainless bolts and Nyloc nuts.

I used a drill to twist a black and red pair of 18 gauge PTFE wire for wiring the heaters. These went from the 6.3V heater terminals on the power transformer to pins 2 and 7 on the power tube sockets and pins 4/5 and 9 on the preamp tube sockets. This takes a long time to get neat. I cursed many times getting them perfect.

I bolted in 3/4″ stainless standoffs with #6-32 x 3/8″ machine screws to support the circuit board. I had to custom drill these for my unique circuit board. I put a ground lug on the standoff closest to the input jacks for grounding.
***Correction*** I changed these from 3/8″ to 3/4″ in height to provide more wire clearance under the board. I also moved them over by one hole (3/8″) closer to the power transformer for more accurate circuit board mounting.