JCM800 Preamp 6v6 Push Pull

[mwelch55]

A friend of mine gave me some parts he intended to use to build an 18 watt 6V6 push pull amp with a JCM-800 preamp.  He had these parts for about 2 years and finally decided that he was not going to build it. He gave me a populated turret board, a bunch of high quality resistors, 3 Can Caps, a choke, cliff jacks, fuse holders, potentiometers, power switch, standby switch, tube sockets, nuts and bolts, and a heavy duty 17 x 8 x 2.5 aluminum chassis.  He also included a nice face plate.
The only parts I needed to buy were knobs and transformers.  I bought the 18 watt Marshall style transformers from Classic Tone.
After getting everything assembled and connected, I fired it up.  The bias was perfect with the bias adjust pot right in the middle.  Plate voltage is about 390v with plate current is about 27 ma.  It worked on the first try, but had a lot of AC hum.
Here are the troubleshooting steps I used to try to get rid of the hum:
1.   Connected all grounds and heater center tap to star ground with ground bus wire soldered to back of pots.
2.   Separated the power amp and pre amp grounds with heater center tap connected to the power amp ground.
3.   Created artificial center tap and attached to Power Amp ground
4.   Removed artificial center tap, re-attached heater center tap to power amp ground and un-soldered ground bus wire from the back of pots.
5.   Re-routed the heater wires so they don’t come close to B+ wires.
6.   Attached heater center tap to elevated heaters (AX84 style).
7.   Used shielded wires on first preamp tube, removed 68K resistor from the circuit board and soldered it directly to the socket of V1.  The hum was finally gone!!!
If I had tried step number 7 first, I probably would have save a lot of time and frustration.
When unsoldering the original wire that goes from the 68K resistor to the input of V1, I noticed that the wire was almost broken in two.  There was only one strand left on the wire.  I am thinking this could have caused the AC hum.  I already had the rg-174 coax cut and ready to install, so I decided to go ahead and use the shielded wire for the two inputs to V1.  I will probably use shielded wire on the inputs of V1 on future builds.
It sounds great!!  I am very pleased with the results of this build. The layout is based on the Mojo JCM800 2204.
Mike

[mwelch55]

Here is the layout I used:

[shooter]

If I had tried step number 7 first

the fix is ALWAYS the last thing you do
I'm sure it's a law of physics or something!

[mwelch55]

Gut shot and front view

[tubenit]

Very nice looking amp!  Have you got a schematic to go with the amp that you could share?  Looking at your chassis photo, is this a 2204 that has been jumpered at the input?   I can't quite line up the photo with the schematics of Marshalls I am looking at.

With respect, Tubenit

[12AX7]

This is quite similar to my amp which started life as a 18 watt and my first build. Stuck it in a old JCM800 lead 20 solid state combo i gutted. But i love JCM800 tube amps and hated the 18 watt sound, so i converted to 6V6 and rebuilt the entire thing as a 800 style MV type amp but with a regular gain stage where the 800's cold bias stage was and made a number of changes to tweak it to my liking. I think the 6V6 platform is perfect for a small club amp.

I have one comment tho...mine uses a 2k resistor in place of a choke, and when i first converted it i tried a choke instead and it really seemed to feel and sound better with a resistor. I also had built a el34 version that was otherwise the same amp and it DID had a choke and it sounded/felt better like that than with a resistor. You may wanna try a resistor and see. All i know is the choke was better in the EL34 amp but in the 6V6 amp the resistor was. Anyone have an idea as to why i experienced this? I don't think it was just my subjective feeling about it, i thing the difference was one anyone would have felt the same about.

[mwelch55]

Here is the schematic for my build:

[mwelch55]

I only used a choke because I had it and didn't pay anything for it.  I will try the amp without the choke and see if I can tell the difference.  I am very pleased with the tone of the amp.  My friend who owns a real JCM800 said it sounds a lot like the original, except not as loud.

I was really amazed at how loud the amp is.  Also, the gain control is useable all the way up to 10.  I have tried this amp with the Gain and Master Volume all the way up and there is no oscillation, just great tone.

The bright cap on the gain control is switchable between 1n and 5n.  The 5n fattens it up a bit.

Mike

[12AX7]

Thats very close to mine except instead of the cold bias stage mine has a regular 1.5k/.68uf gain stage. I also have the bright cap on the gain switchable, tho mine is in/out. I did have it like you before tho different values so i could switch between 2 values. But i find this more useful i think. I believe in a band context i may find it useful for those times the room is bright and the amp needs a lot of attenuation in the top, which i hate doing via tone controls. But i have yet to test it. As to oscillation, none here either but i found the most important thing was using shielded wire for the input and for the gain pot output signal. Long as those are shielded no oscillation at all.

[mwelch55]

Yes, I learned the hard way to start off with shielded wire on the inputs of V1.  Instead, I tried everything else first (see post #1).

Mike

[HotBluePlates]

Great job on the build, and on fixing the hum issue!