Marshall JCM800 - 2204 build preamp problems

[arwokc]

Hey guys, I decided to try my hand at a JCM 800, 50-watt, 2204 clone. As you might guess, I've run into a snag. I used the Ceriatone layout found here.

http://www.ceriatone.com/images/layoutP ... iatone.jpg

Upon power up, I get an awesome, crystal clear, clean tone. No hum or hiss but no gain either. At full volume, I get the same clean, clear tone and only about 1/4 the volume it should have. After reviewing the voltages of a stock JCM 800, It appears that the power section is about right. But the preamp voltages are out of wack. I've double checked all the resistor values three times, not to mention I ohmed them out as I installed them. I've triple checked the capacitors as well. I thought for sure, I must have gotten one of the leads underneath the back side of the circuit board wrong, but I've triple checked them also. I rang the leads out from the preamp tubes to each spot on the circuit board to verify correct connections. I've replaced the tubes with other new tubes. I've even swapped in transformers out of another 50 watter I'm preparing to build. I reviewed the input jack wiring as well. It kills me to admit defeat or to ask for help, but I'm stumped. Maybe I just need a fresh set of eyes to look at it objectively. Here is a schematic with voltages from a bone stock 2204 from Mark Huss's site that I have been referring to.

http://www.mhuss.com/MyJCM/JCM800_2204.pdf

Here are the pin voltages on my build.

http://www.el34world.com/forms/valvedata/2204volts.htm

It would appear that my problem is in the preamp section somewhere. My first impression is that the preamp tubes are not biased correctly and have the tubes nearly shut off or that I'm not dropping enough voltage. Anyone have any suggestions where my problem may lie?

Thanks in advance,

Andy

[Cameron]

Hey guys, I decided to try my hand at a JCM 800, 50-watt, 2204 clone. As you might guess, I've run into a snag. I used the Ceriatone layout found here.

http://www.ceriatone.com/images/layoutP ... iatone.jpg

Upon power up, I get an awesome, crystal clear, clean tone. No hum or hiss but no gain either. At full volume, I get the same clean, clear tone and only about 1/4 the volume it should have. After reviewing the voltages of a stock JCM 800, It appears that the power section is about right. But the preamp voltages are out of wack. I've double checked all the resistor values three times, not to mention I ohmed them out as I installed them. I've triple checked the capacitors as well. I thought for sure, I must have gotten one of the leads underneath the back side of the circuit board wrong, but I've triple checked them also. I rang the leads out from the preamp tubes to each spot on the circuit board to verify correct connections. I've replaced the tubes with other new tubes. I've even swapped in transformers out of another 50 watter I'm preparing to build. I reviewed the input jack wiring as well. It kills me to admit defeat or to ask for help, but I'm stumped. Maybe I just need a fresh set of eyes to look at it objectively. Here is a schematic with voltages from a bone stock 2204 from Mark Huss's site that I have been referring to.

http://www.mhuss.com/MyJCM/JCM800_2204.pdf

Here are the pin voltages on my build.

http://www.el34world.com/forms/valvedata/2204volts.htm

It would appear that my problem is in the preamp section somewhere. My first impression is that the preamp tubes are not biased correctly and have the tubes nearly shut off or that I'm not dropping enough voltage. Anyone have any suggestions where my problem may lie?

Thanks in advance,

Andy

Check the wireing on v2..pin 2 should not have 1.69 volts on it. Check the 820 ohm cathode resistor...Check all your grounds with a meter so you are sure they are all hooked up right.....If you didnt have the 470k grounded it would just be hooked up to the v1 cathodes.

[Andy Le Blanc]

just had an old princeton in with a pre amp tube with a crack in its envelope
did the same.... clean 1/4 power...... eyeball the tubes in the pre pretty close
as well.....

[arwokc]

Cameron, you are THE MAN. I've looked at this thing until I was ready to take a hammer to it. I forgot the ground wire at the 470K resistor just as you mentioned. It is VERY loud now and has more gain than I ever imagined. It's pretty darn quiet too as far as noise goes. I'm going to play through it a couple of days to get used to it, then I thing I'm going to try some of the mods from "The Ultimate JCM800" from over at the Tone-Lizard site. Any further recommendations would be appreciated. I really appreciate you taking the time to review my problem. Sometimes it really helps to just back away from the problem and let someone who hasn't been rehashing the same steps, take a look at it.

Thanks again,

Andy
Del City, OK

[Wayne]

Glad you found your problem.

Did all the Ultimate JCM 800 mods from Tone Lizard on a friends '800, except tying the bias supply to pin 1. Highly recommended. Thing sounded great when we were done. My buddy later traded the amp for something else (don't remember what now), and is currently trying to buy it back. Guess he must have liked the sound, too!

W

[arwokc]

Wayne,
From all your mods to the JCM800, what would you say was the total effect on the amp? I'm getting a bit of oscillation when I turn the master volume pot. No specific pattern though. I've checked the pot with an ohmeter after taking it out of the circuit, and it appears to go open irrradically. I'm going to replace it in an effort to correct the problem before attempting any mods of any kind. Oddly enough, I had played the amp for nearly an hour through humbuckers with no issues. Then when I went to the stratocaster is when I noticed the oscillation. I guess its an oscillation, it just goes into a loud hum until I put the amp back into standby mode. I can turn it back on and it plays normally again until I move the master volume pot. Some more investigation is in order.

Andy

[Wayne]

It's been a while, so I just headed back to the "Lizard" site to refresh my memory.

The mods all fall into one of three catagories: All the "bright" caps are removed, a *bit* of gain is added to each stage, and the tone stack is slightly re-voiced. Nothing drastic, just a few tweaks. Certainly nothing that should cause you trouble like you're having.

Is your amp a combo, or a head that lives on/close to the cab? I suspect that your trouble might go away once you replace that wonky pot. I had a tube socket do the same thing to me once. One of the pin grabbers was loose, and would react to a particular note played through my combo. Once that note was played, the amp would continue producing a really rough sound at that pitch until I shut it down. Your intermittent pot may be doing the same thing. As far as that goes, while you're under the hood, check the tube sockets too. I don't think the sockets they used in those amps are top quality gear. They may need tightening.

Good luck, and let us know how it all turns out.

W

[arwokc]

Wayne,
Once again, you may have nailed it right on the head. I had the chassis sitting directly on top of the speaker cabinet and it wasn't inside of its cabinet, so it didn't have the head cabinet or its rubber feet to help absorb some of the vibrations. It actually did seem to occur only when a certain note or certain area of the guitar's neck was played. I'm going to change the pot anyway, but I think maybe the tubes were getting microphonic from the bare chassis being sat directly on top of the speaker cabinet. I don't suspect the tube sockets since this is a new build from scratch and I've used the gold plated ceramic sockets. The tubes really fit nice and tight in their sockets. I had a similar problem with my Constellation project. The 6SN7 tube was very touchy. I put softer rubber feet on the head cabinet to absorb some of the vibrations. I also installed a high temp aviation grade silicone o-ring over the glass envelope. It solved all the microphonic issues with that tube, but I never would have thought that a 12AX7 would have gotten microphonic. I really need to build another Constellation. It was very challenging and it sounded great. A buddy liked it so much, he made me an offer I couldn't refuse on my first one.

[arwokc]

By the way, thanks again for the help.

[Wayne]

double/double!

[Wayne]

By the way, thanks again for the help.

Not at all! This place has already contributed much more to my education than I'll ever be able to give back, and I haven't even been here that long.

W

[arwokc]

Wayne,
Did you also perform the bias-feed resistors change from 220k to 100k that is recommended? Were you using the 6550 tubes or the EL34's?

Andy