How to troubleshoot ground hum?

[JustMike]

 I've got a home built Ceriatone JCM-800 2204. I used the Hoffman grounding scheme but it still hums. I put an effects loop in it (between treble & MV) and when I insert an external pre into the return, there is no hum. But I can't figure out if it's because of multiple ground paths or something else. Any suggestions?

[firemedic]

You sure it's not your heater wires?

[stingray_65]

One trick I learned is that I had to identify between 60 and 120hz.

120Hz hum is post rectified HT. 60Hz is heaters or an errant signal.

If you take another amp, plug in a guitar cable and touch the tip of the cable, thats 60Hz.

JCM 800 is a high gain amp, Lead dress COULD be at fault, chopstick your leads, especially near the tube sockets and your FX loop mod. Making it worse is almost as good of a sign as making it better,gives you a place to start.

You sure it's not your heater wires?

Did you phase the heater wires? with the tubes pulled you can check continuity and make sure one wire supplies pins 4-5 and the other pin 9. It is my understanding that this provides a sort of humbucking effect for heater hum.

Parallel wires?

OH! does the hum increase with the volume control? what controls affect it?

And double check the ground scheme. Doug's method has proven itself time and again.

How is Doug's grounding scheme different than what Cieratone suggested? PRR has suggested many times to build a proven design. I've never read a bad review of Cieratone, so why would we want to change their grounding scheme?

Just a few thoughts, Good Luck! and as Tubenit has advised me, "Go SLOW, be patient, DON'T GET DISCOURAGED!"

Ray

[Leevi]

does the hum increase with the volume control?

This is a valid question. If not, you can try the following (do one change at a time):

- Insert temporarily another filter cap parallel to the first filter cap
  and compare the hum
- Insert a new filtering stage (cap + resistor) before the first filter cap
- If there is no choke used replace the resistor with a choke
- change the grounding of the PI sector to somewhere else
- change the grounding of the power amp to somewhere else


/Leevi

[JustMike]

 Thanks guys. Lots of good points to check out. I'll keep you updated.

[Willabe]

Hey JustMike,

Read up on it in Dougs library.              

Try this also, look in tech info,

http://www.aikenamps.com/Home.htm

One more, Merlins site,

http://www.freewebs.com/valvewizard2/grounding.html

You'll find it.      


                  
                   Brad      


          

[archaos]

Hey JustMike,

Read up on it in Dougs library.              

Try this also, look in tech info,

http://www.aikenamps.com/Home.htm

One more, Merlins site,

http://www.freewebs.com/valvewizard2/grounding.html

You'll find it.      


                  
                   Brad      


          

+ 1

Merlin's grounding scheme is one of the best ever anyway : success every time !

[JustMike]

 Thanks for all of the reading...But it turned out to be a bad preamp tube! WTF?!?

[John]

  But think of all you've learned!

[tubeswell]

Thanks for all of the reading...But it turned out to be a bad preamp tube! WTF?!?

Tube swap is the 1st thing to check when you get hum.

[stingray_65]

Thanks for all of the reading...But it turned out to be a bad preamp tube! WTF?!?

I will ASSume that it was in position V1?

Does the tube hum in other positions? V2? PI?

[jjasilli]

Thanks for all of the reading...But it turned out to be a bad preamp tube! WTF?!?

Tube swap is the 1st thing to check when you get hum.

Ditto. [Rule 1:  Always suspect tubes first; Rule 1.a. This inlcudes new tubes.    ]