Mark III Loud, High Pitched Noise

[Agustín Collia]

Hi guys, I really need your help! I pulled out my Mark III yesterday in order to add a courtesy outlet as it didn't have a factory installed one. While I was at it, I also cleaned up the chassis. I didn't touch the circuit board at all, and didn't even remove the preamp tubes, except for V1's metal protection to clean it, I then put it back in its place. I then pulled the chassis into the cabinet and flipped the power switch to on, waited a few minutes and flipped the standby switch to on. I didn't have anything plugged into the amp but the speakers (nothing plugged in the input or in the effects loop). As soon as I flipped the standby switch to on, I started hearing a loud, high pitched sound, that at times seemed to go up and down in intensity, like in cycles. The amp was in the lead channel, so I switched to Rhythm 1 and the sound changed into a loud hum. What might be causing this? The strange thing is I didn't touch a thing but the fan's ac outlet (I wired my courtesy outlet from the fan's outlet). The power tubes are brand new STR 440s (bought them in february), however the preamp tubes are the Mesa 12ax7s that came from the factory (the amp was made in 1993, I was planning to make an overdue NAD thread with gut pics and everything, but I was surprised by this issue). Thanks a lot, hope you can help me!
The settings are: Volume 1: 8, Master: 2, Lead Drive: 8, Lead Master: 4,5, Treble: 6
The amp worked flawlessly without any strange noises before I pulled it out of the cabinet to do the mod.

 

[stokes]

Check by tapping each preamp tube one at a time and see if it affects the noise,could be a microphonic tube.If that doesnt pan out,try pulling preamp tubes one at a time starting with V1,pull it,if the noise stops the problem is in the first stage,if not move to V2 and so on till you find the faulty stage.If not a tube,my first guess would be a faulty decoupling cap in the preamp section of the B+rail.

 

[Agustín Collia]

Thanks a lot! I'll try that tomorrow and report back. Quick question, the tapping and pulling out preamp tubes has to be done with the power tubes in or can I have them out of the amp while performing this?
Thanks again.

 

[stokes]

Yes,the power tubes have to be in or you'll get no sound.Be careful,they will be hot.For tappng use a chopstick or pencil,its easier to reach the tubes.

 

[Agustín Collia]

Thanks a lot for your advice! I actually had the reverb cables connected the other way round, and that was causing the noise. Still, I also tapped the preamp tubes just to be sure and fortunately none of them is microphonic, so it's back up and rockin'! NAD thread coming soon, thanks a lot!