Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on January 26, 2011, 07:51:47 pm

Title: 48 Gibson GA30 / Unacceptable Hum
Post by: plexi50 on January 26, 2011, 07:51:47 pm

I have a very anoying hum. It increases as the volume pots on either channel are turned CW

There is no hum when EQ volume pots are CCW

The input jack grounding is good and working properly

I have recapped this amp including all but 4 resistors. (2) 470K's on each 6V6 tube and (2) 100K resistors in the EQ with a small square treble peak cap

I suspected the preamp tubes but have tried several different 6SC7 tubes all with the same results

This is in the preamp signal path and not PT related i believe / Could be wrong

Pots and tubes are all dated 1948

(http://i356.photobucket.com/albums/oo5/plexijtm45/th_MVC-001W-3.jpg) (http://s356.photobucket.com/albums/oo5/plexijtm45/?action=view&current=MVC-001W-3.mp4)

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: plexi50 on January 27, 2011, 07:00:51 pm

I found a portion of the hum problem. I had a 470K resistor on the 6SC7 PI tube that was not soldered

Still have a little hum though and if i put a wooden stick to the PT and to my ear i can hear it huming inside the PT

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: DummyLoad on January 27, 2011, 07:25:07 pm

does it have a tube recto? maybe one section is bad.

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: plexi50 on January 27, 2011, 08:43:10 pm

I thought about that and have tried 2 different 5Y3 tubes so far. One NOS Sylvania. Maybe NOS not so NOS. But it is NOS
(What did i just say)? :huh:

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: Iannone on January 27, 2011, 09:00:40 pm

Quote from: plexi50 on January 27, 2011, 08:43:10 pm

I thought about that and have tried 2 different 5Y3 tubes so far. One NOS Sylvania. Maybe NOS not so NOS. But it is NOS
(What did i just say)? :huh:

I find it frightening that I understand this.

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: plexi50 on January 28, 2011, 07:20:20 am

Me  too!  :huh:

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: JayB on January 28, 2011, 04:32:59 pm

There is a center tap for the heater winding and that center tap is floating using the cathode biasing for the power tubes. Remove the artificial center tap or disconnect the heater center tap. If both are used, it'll hum like crazy.

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: plexi50 on January 28, 2011, 08:15:20 pm

Yes i found that out the hard way. I removed the (2) 100 ohm artificial CT resistors

Still has this d** hum when the microphone channel volume is increased. Not so bad on the Normal channels

The hum is less since i resoldered that 470K resistor on the 6SC7 that was loose

One more thing: I have found no schematic any where that is a carbon copy of this amp

What i have is a 1948 LP Deluxe.  The GA-30 schematic is the same layout but most all of the resistor values are different

1.5k-33K- and many others. The cab Tolex was recovered half a century ago. It should be the same color as the handle

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: JayB on January 29, 2011, 10:00:05 am

I wonder if it's anything to do with the global negative feed back?

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: plexi50 on January 29, 2011, 05:34:29 pm

I thought about that as well but it all worked out.  I think the PT is just tired. The PT hums with no load on it. Sounds nice though

Title: Re: 48 Gibson GA30 / Unacceptable Hum
Post by: DummyLoad on January 29, 2011, 08:27:32 pm

I thought about that and have tried 2 different 5Y3 tubes so far. One NOS Sylvania. Maybe NOS not so NOS. But it is NOS
(What did i just say)? huh


recto socket possibly bad?