Preamp Tube

[ampbldr2]

I am stuck on a problem I can not figure out. The V2 preamp tube in my trex18 is picking up any vibrations in the chassis and when the amp is idle I can get a slight hissing and parasitic hum (Microphonic I guess). When I tap v2 with a stick it comes out (tap tap) as audio in the speaker. V1 and V3 do not.

In the trex18 schematic v2 is the 2nd and 3rd gain stage before the PI. I have tried different grounding and moving components around but no luck at changing the behavior. I do not measure any DC voltage at the tone stack caps and do not believe this is due to leaky caps. If I pull v1 tube I still get v2 to hiss. If I crank the volume at idle the hiss is still there.

Again I have tried alot to rid this problem with new tubes changing the plate resistors and different grounding by separating the trex input ground from the rest of the bussed ground. Nothing makes this hissy v2 tube change its behavior. Anything ideas?

[David Root]

Not clear here. Are you saying the V2 POSITION is microphonic/oscillating, regardless of how you change the actual tube in V2? Or have you just not changed the tube in V2?

[ampbldr2]

Not clear here. Are you saying the V2 POSITION is microphonic/oscillating, regardless of how you change the actual tube in V2? Or have you just not changed the tube in V2?

V2 maybe at a threshold just before being microphone/ocillating. I am no expert. I have tried new tubes and they all exibit the same behavior. I have swapped a few components and no change. I am not sure what is the problem. The amp works and sounds great all tone/vol controls work without and scratching etc. Just the v2 tube is hissy and picking up audio from vibration. If i flip the standby off the pop come out the speaker. If i tap the chassis or the tube it is audible in the speaker. I need to figure this out but run out if ideas.

[Structo]

You can try the high temp silicone dampner O rings or you can also rubber mount the socket by using small rubber grommets between the socket and chassis, providing a shock mount.

[dcribbs1412]

Maybe replace the V2 socket altogether...
also maybe a pic if possible

Darin

[Fischerman]

I've had that exact problem and never could figure it out. One piece of advice I can give is to make a list of everything you try, every part you swap, every ground you change, etc. After a while it can get to point where you wonder, "Did I try that?"

[markr14850]

Solid core wiring or stranded?

[ampbldr2]

All solid core 20 wire in the amp with pvc jacket. I think it was beldon wire from mbelectronics.

I will try replacing the ceramic socket tonight with a new ceramic (Same Type) and see what happens. It is good to know that others have experienced same issue. To bad no root cause though.

[ampbldr2]

Here are some pics

[dcribbs1412]

ampbldr2
I'm sure you will get this worked out
might think about heat shrinking the ground buss from the pots to ground
also your nfb switch is between V2 V3...any pics of that?

Darin

[markr14850]

All solid core 20 wire in the amp with pvc jacket.

This is pretty unlikely, but, as you've tried most everything else...

Try tapping on all the wires connected to the socket, and see if there is noise, and if it is louder than when tapping on the tube. If so, identify the noisiest wire(s) and try replacing them with stranded.

[ampbldr2]

I have tried moving the nfb switch away from v2 and no change. I tried replacing the v2 socket and no change. Tapping the wires did not make any noticeable noise like it does when you are tapping the tube.

I am starting to run out of options. From an engineering perspective, what is happening to the tube? Is it at the threshold of oscillation? The tube should not be picking up excessive noise like it does.

I will try removing the grid wire from the treble feeding gain stage 2 and ground that pin just to see if the noise is still noticeable. I suppose I could change the wires to stranded but not sure why that would affect something like this.

[Structo]

I noticed that you used ceramic tube sockets.
I have read that they can attribute to microphonics, especially on a stage that is prone to it.

You might want to try a different type of socket there (like a micalex type).

[David Root]

I would agree with Tom. At this point, a Belden Micalex is worth a try.