Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: FranciscoPerez on September 14, 2010, 12:43:48 pm

Title: Noise in a 5e8a twin.
Post by: FranciscoPerez on September 14, 2010, 12:43:48 pm

Hello everyone,

I built a 5e8a twin several months ago and lately had some noise problems with it.
It produced a crackling noise through speakers followed by something like a continuous "shhhhhhhhhhhh" when gently hitting the input jacks or the area around them.
I opened the amp and chopsticked the input jack solder joints, and fortunately some cold solders produced the noise, so I resoldered them and the probem was fixed.

However, started chopsticking the whole amp just in case there was a cold solder joint somewhere else and found out that the wire that goes from the mixer resistors to the grid of the following preamp tube (highlighted in red in the picture) produces a noise (not sure about frequency but it's quite low) just when the chopstick is near that wire. When the chopstick gets closer the problem gets worse, and chopsticking the wire or the solder joint that connects the mixer resistors with the wire and the 2M resistor it gets even worse.

I re-soldered that joint , got rid of that wire and put another one, and changed the following preamp tube but nothing improved.
The amp seems to sound good, no crackling noises or excesive hum or hiss, but can't figure out why is that noise being produced.

Any suggestions??
Thank you very much! My apologies for my terrible english.

All the best,
Francisco Pérez


(http://img340.imageshack.us/img340/7271/twin5e8alayout.jpg)

Title: Re: Noise in a 5e8a twin.
Post by: phsyconoodler on September 14, 2010, 02:38:55 pm

The tube can produce that 'ocean' sound.Try another tube in V3.Also a shielded wire may help too if it's not the tube.
  My money is on the tube.

Title: Re: Noise in a 5e8a twin.
Post by: FranciscoPerez on September 14, 2010, 03:28:02 pm

Thanks for answering phsyconoodler ;)
 
Already changed the tube (I chose a nos tesla ecc83 and changed to a new production 12ax7) but that noise when chopsticking is still there. Anyway I'm not getting the 'ocean' effect anymore, after resoldering the input jacks.

I'll try using a shielded wire and see if it helps!
However, the behaviour I described (hum when touching the wire or approaching to it with the stick) is not normal, is it??

Regards,
Francisco Pérez

Title: Re: Noise in a 5e8a twin.
Post by: PRR on September 14, 2010, 06:38:36 pm

> hum when touching the wire or approaching to it with the stick) is not normal,

Yes, it is. It is a high-gain point, and a very high impedance. The "low frequency" is probably just hum from all the AC wiring in the room.

The fix is: don't put chopsticks in your amp when playing.

The crackles and hisses are bad joints, dirty tube pins, bad tubes. These you have fixed.

Title: Re: Noise in a 5e8a twin.
Post by: FranciscoPerez on September 15, 2010, 02:28:00 am

Thank you!!

Glad to hear that hum is not an issue ;)

Title: Re: Noise in a 5e8a twin.
Post by: phsyconoodler on September 15, 2010, 10:01:58 am

Those must be special signal carrying chopsticks from a Chinese restaurant next to a radio station.

   :laugh:

Title: Re: Noise in a 5e8a twin.
Post by: FranciscoPerez on September 15, 2010, 11:14:19 am

:laugh:
Haunted chopsticks... Should have expected something like that!!!