SOLVED Microphonics ONLY on left channel - Crack 1.0 Speedball

[MOAR_CORES]

Hello all!

I am experiencing some intense microphonics on the left channel only.

Tapping hard on my keyboard, or on the crack creates a harsh thumping sound in the left channel.
I have put up with it till now. Otherwise the amp sounds great.
It happened before the speedball upgrade iirc.
I have double checked voltages, grounding, and all resistances. Everything seems right. I have also resoldered everything. No dice.
I have tried replacing the 12au7 too. I put in a russian reissue "mullard" tube (not really mullards but should be good enough), and no change was observed in microphonics.
Could it be the pot? RCA jacks? or something wired wrong? or ground not wired correctly?

Let me know what you guys think.

[Paul Birkeland]

A harsh thumping sound is likely a cold solder joint that's moving around a little bit from the vibration of the adjacent keyboard. 

[MOAR_CORES]

A harsh thumping sound is likely a cold solder joint that's moving around a little bit from the vibration of the adjacent keyboard.

Actually, it was the bigger 6080 tube! I tried the original RCA that it came with, some Sylvania tube, and some GE tube. The GE tube gave decent results. I kept looking to see if I had another tube and I found some philips ECG tube, and now it's dead silent no matter what. My left ear is relieved now.

The reason I thought it might be the tube is that even though tapping the amp anywhere with a pencil eraser caused the microphonic effect, tapping it at the tube caused it to be ever so slightly louder.

Or maybe it was a bad solder joint and swapping the tubes made the joint be better connected and thus be working again, idk.

thanks though!

Now time to get a better pot lol

Edit: Plugging back the tube brought the issue back. huh

[Paul Birkeland]

Or maybe it was a bad solder joint and swapping the tubes made the joint be better connected and thus be working again, idk.

This is quite likely.

[adydula]

I had this same experience with a larger coke bottle 6AS7G tube. Typing on a nearby keyboard caused as thumping sound. I removed it carefully and used a smaller mil spec version and no thumps...put the 6AS&G back in and thumps came back...did this a few more times....turns out the glass bottle was slightly loose in the base!! I carefully re-glued the glass tube in the base...let dry 24 hrs and now the thumps have gone away!

Its seems these larger tubes tend to be more prone to microphonics and are tempermental! I have 6 of these tubes and most are more sensitive to mechanical "knockings' etc..than the smaller "better" constucted tubes.

Its funny, I have built a "HAM" tranceiver with 21 tubes, all small 12au7 size...and beating on a CW key on a desk right next to it yo dont have these issues!!.

Alex