Locating Noise

[EETStudent]

Ok, I finished building a Super Reverb clone a couple days ago, and although the tone is incredible, there is a little noise problem that is bugging the piss out of me. Here are my observations:

-Pulling PI tube eliminates said noise completely, leading me to believe the noise is generating in that section or one before it in the preamp.

-The volume control does NOT affect the noise, eliminating the first stage/input as a suspect.

-I installed a simple post-phase inverter master volume across the phase inverter outputs, an idea taken from a Matchless Chieftain schematic I saw. This volume control DOES majorly affect the noise.

-

After all this, I am correct in believing the noise is being generated somewhere between the phase inverter and after the first volume control, right? I bought a turret board from Watts Tube Audio, one that is identical to the original SR boards, and I made every attempt the follow the layout diagram verbatim. Any suggestions?

[tim]

can you describe the noise? hum, shot noise, crackling, arcing, what?

[Mandopicker]

Based on how you said the MV effected the noise, perhaps try using a shielded wire for the MV control. (grounded at the pot end only of course.)

Good luck.

[Enzo]

He already established that pulling the PI tube killed the noise, and that is before the post-PI MV.

Yes, the source or entry point of your noise is after the volume control and up to or at the PI.


But tim asks the magic question: WHAT noise? Hum hiss, static, random impulses all have completely different sources and cures. The doctor can;t make you feel beter unless he knows if you have a headache, a stomach ache or a backache.

I don;t know what version of the SUper Reverb you built, so I am looking at the AB763 schematic. You have pulled the PI, fine. Put it back in and pull the other tubes one at a time. Replace after each test. DO any tube pulls affect the noise?

Until we know what we are chasing, I'll refrain from more guesses.

[EETStudent]

Yea should have described the noise. Its more of a consistent hum, but not 60 Hz hum. No squels, crackle, static, or added hiss(of course the channel volume adds hiss when turned up).

As for pulling the other tubes, Im only running the PI tube and channel 1 preamp tube. I'm not even concerned with the other channel at this point. As mentioned before, pulling the PI tube eliminites 100% of the noise. Inserting the PI tube and pulling V1 still yields a dead quiet amp.

[Enzo]

Never think of reasons not to check something. Just because you are not using a channel doesn;t mean a tube in it can;t affect the performance of the amp. Or are the othwer channel tubes already removed?

Pulling the PI tells you the problem is either at or before the PI. But that means most of the amp stiil. So individually pulling the other tubes to see which ones have any effect helps localize the source of noise.

Pulling V1 leaves all the rest of the amp runnig. If that is clean, then the problem is at or before V1.

SO we are talking the Normal channel as oppesed to the Vibrato channel. Right?

You don;t care about the other channel, but does it have the same hum? Or did it?

And a long shot, does the trem intensity have any effect? Also, is the mmetal cover of the intensity pot grounded to teh chassis? Make a resistance reading.

How many different 12AX7s have you tried in V1?

I am still looking at AB763 Super Reverb. Please tell me if you built a different version.

In this version, the cathode of the second stage of channel 1 shares a cathode resistor with the other channel in the same spot. If that other tube is removed, then you won;t have proper bias on that tube.