New Express..slight problem.

[MGW]

Just finished up an express build. Sounds INCREDIBLE...except for one thing. When I do the chugga chugga thing on an open A chord, it sounds a bit like blocking distortion. I have changed out the .1 before the PI to .022 and really didn't change much. Swapped out the 1k5 gridstops on the the output grids with 10k...no change. Added a 33k gridstop to the input grid...no change.

Thoughts?

[M Fowler]

Cold solder joint?

[John_P_WI]

Add a grid stop to the third gain stage and lower the value of the 150k to ground preceding it. Just clip lead another 150k r in parallel to test.

[redshark]

I would keep it stock and look for cold solder joints first!!

Turn the amp on with a speaker hooked up of course.
Turn up volume to at least 12noon
You will hear hiss(normal)
then with a chopstick start tapping light every joint in the amp
If you want to make sure there's nothing wrong hit the chassis with a rubber mallet in diferent places and if something is wrong you will hear it.

I do that thing to find cold solder or bad joints.

[pablogt]

I had a similar problem. Went away swapping the pa tubes with another amp. Both amps sound good now. The express is very demanding regarding tube quality. Tubes that are perfectly ok for other amps are microphonic or too noisy on the express.

Pablo

[MGW]

I put some 6V6s in it...granted they were biased way too hot, but it didn't make the noise. I have another set of EL34s in my marshall that i'll swap out and see what happens.

[MGW]

Another thought...Is there any way I'd be killing the OT with low freq transients or something? It's a 30 watt Edcor and it's biased right at about 15w a tube.

[JoeCon]

MGW

I used Edcors in my two builds so far and they work great. Very conservatively rated so I doubt that the transformer is the problem.

[MGW]

Good news fellas...apparently the new pair I got for this build is a wuss pair. Cause I swapped out the tubes from my marshall and the amp sounds completely different. Both sets are JJ EL34 too. It was so nice...now I gotta go undo some stuff I did.

[ampmike]

Awesome!Great to hear you got her tweaked into place,You got a great amp now,have fun now,Mikey