Problem with a JCM800 2204 homebrew - out of ideas :(

[HiWaTTage]

Holy Guac:

Here is a link to my 2204 (Ceriatone):
Joe Popp's JCM800 2204
Good luck and stay on it!!!

Is that just me or your electrolytic caps on the board are wired the wrong way as you have grounded the + side (or it should be like that? - don't see the point, only if -> was standing for + ???)

Cheerio

I'm pretty sure that those are supplying the bias for the power section, which is a negative voltage, and as such, I think that the electrolytics are supposed to be reversed. Someone correct me if I am wrong, or elaborate if I'm in the ballpark.

[Bulatovic]

Oh, i've missed that
thanks HiWaTTage!

[soundmasterg]

I'm going to second the opinion that your preamp voltages seem pretty high. Higher voltage on the preamp means higher gain and higher frequency response, which also means higher suseptibility to oscillation. You may try going to a larger dropping resistor for the preamp stages and see if that does anything with the oscillations. It will reduce the voltages on the preamp and reduce gain and high frequency response. The other JCM 800 builds that I've seen seem to have lower voltages overall in the preamp. The one I'm working on in my project amp isn't finished yet, but I've been shooting for lower voltages myself.

[hoelyguacamole]

Many thanks for all the replies and tips; you guys rock

I pulled the board last night to tidy up the wiring and am going to also shield the bottom of the board while I have it out - I figure even if that doesn't solve the problem it would be good to do it.

I'll update as soon as I've fired it back up again (hopefully this weekend) and try to lower my preamp voltages at that time. Peace

[populartsl]

You need to sheild the bottom of the cab that you put the chasis in not the back of the component board. I may of misunderstood what you said if i have my apologies in advance.

Ben

[Strat]

While you have the board out (although you could meter from above) check that the power resistor (R8 ) has the feed to the V1 plate resistors (R4 ,R7) connected on the 'downstream' side - it's very easy to miswire underboard when everything is flipped.

Also, use your meter and check that the V1 plate resistors are the value they're supposed to be, same with the cathode Rk/Ck.