Tracking down hum in the reverb circuit (Yes, the Gibson Hawk again)

Turbo7MN · 09-09-2007, 04:32 AM

Okay, so I got the bias issue worked out with this thing. It appears to have been the crappy JJ power tubes I was using, I put the old GEs in and all has been well ever since.

Except of course the nasty hum when the reverb is turned on. It does hum a little all the time but it's not really bad until the reverb is kicked on. What's weird is I first noticed it doing this after I rebuilt the bias supply and recapped it. I figured maybe one of the filter caps I used was bad so I went back through and replaced most of them with Sprague Atoms with no improvement. I started digging around and I think I have it narrowed down.

This is the schematic for the Gibson GA-35RVT which is closer to my amp than the actual GA25RVT schematic on the internet.

http://www.guitar-parts.com/images/ga35rvt.jpg

So I think it's either R49 (220K) or C29 (.001uF) that's giving me crap. I removed the wire from the reverb tank (it's hardwired) from the junction of those components and the hum remained. I put that back and removed the wire from the grid of V3A. The hum is gone. Unfortunately, this kills the reverb signal too. Is this a reasonable assumption based on the testing I've done?

What I don't understand it how either a capacitor or a resistor positioned in the circuit like these can introduce so much hum, and still seem to function otherwise normally as well. The reverb still sounds great besides the hum. It also seems like a really weird coincidence that the hum started right after I did some major work on the amp, even though I was nowhere close to those components. This amp's poorly laid out and kind of a bear to work on, and those components are in a particularly bad spot so I'd like to make sure that there's nothing else I should be checking before I try and dig in.

You guys have come through for me in the past and I really appreciate that, hopefully this can be the last time and I can sell this thing and move on to some other projects I've been meaning to get to.

Thanks in advance.

-Darren

Bruce / Mission Amps · 09-09-2007, 09:56 AM

Sorry to hear you're having trouble with that amp but I don't think it's just you and yours.
I'll probably get push back on this but for what it's worth... I refuse to work on those amps anymore... they are time and profit killers.
They are not that easy to work on, plus, none of my poor, under financed, vintage Gibson/Epiphone collector customers ever have enough labor money available to effect repairs and the hummy/noisy reverb problem ALWAYS seems to exist in those as soon as they are brought back to life.
Badly designed is being kind in my opinion and on top of that, they don't sound any good to me anyow.
I just hate them. ha ha

Turbo7MN · 09-09-2007, 04:07 PM

Yeah, I figured as much. This will certainly be my last Gibson project. It didn't help that somewhere along the line someone that had no clue what they were doing went in and made an even bigger mess of the already terribly laid out circuit. I've put more hours into this thing than I have pretty much every other amp I've worked on. So do you have any idea what usually causes it, or is it a just a weird thing and every amp's different?

Thanks for the input. I'll probably just try and sell the thing as-is then. I really love the sound of it and the reverb was one of its strong points but it just hums too much to be useable.

Euthymia · 09-11-2007, 04:52 AM

Y'know, I had one of these (or similar Gibson model) in for repair from one of my affiliated stores. He just wanted me to get it into salable condition.

Someone had hacked it up, so I restored it to stock and gave it back to him.

Too much hum with the reverb turned on. He brought it back.

I finally wound up just putting a capacitor across the input to the recovery stage to make a highpass filter and roll off all the lows. It sounded great, because reverb doesn't need to be bassy anyway. I choked it off way above 120Hz and it only improved the tone.

I guess I should have documented the mod, but it's pretty easy in principle.

Just one cap, I think from the grid of the recovery tube to ground.

09-09-2007, 04:32 AM	#1
Turbo7MN Member Join Date: Feb 2007 Location: East Central MN Posts: 36	Tracking down hum in the reverb circuit (Yes, the Gibson Hawk again) Okay, so I got the bias issue worked out with this thing. It appears to have been the crappy JJ power tubes I was using, I put the old GEs in and all has been well ever since. Except of course the nasty hum when the reverb is turned on. It does hum a little all the time but it's not really bad until the reverb is kicked on. What's weird is I first noticed it doing this after I rebuilt the bias supply and recapped it. I figured maybe one of the filter caps I used was bad so I went back through and replaced most of them with Sprague Atoms with no improvement. I started digging around and I think I have it narrowed down. This is the schematic for the Gibson GA-35RVT which is closer to my amp than the actual GA25RVT schematic on the internet. http://www.guitar-parts.com/images/ga35rvt.jpg So I think it's either R49 (220K) or C29 (.001uF) that's giving me crap. I removed the wire from the reverb tank (it's hardwired) from the junction of those components and the hum remained. I put that back and removed the wire from the grid of V3A. The hum is gone. Unfortunately, this kills the reverb signal too. Is this a reasonable assumption based on the testing I've done? What I don't understand it how either a capacitor or a resistor positioned in the circuit like these can introduce so much hum, and still seem to function otherwise normally as well. The reverb still sounds great besides the hum. It also seems like a really weird coincidence that the hum started right after I did some major work on the amp, even though I was nowhere close to those components. This amp's poorly laid out and kind of a bear to work on, and those components are in a particularly bad spot so I'd like to make sure that there's nothing else I should be checking before I try and dig in. You guys have come through for me in the past and I really appreciate that, hopefully this can be the last time and I can sell this thing and move on to some other projects I've been meaning to get to. Thanks in advance. -Darren

09-09-2007, 09:56 AM	#2
Bruce / Mission Amps Senior Hollow State Tech Join Date: May 2006 Location: Denver, Colorado Posts: 2,014	Sorry to hear you're having trouble with that amp but I don't think it's just you and yours. I'll probably get push back on this but for what it's worth... I refuse to work on those amps anymore... they are time and profit killers. They are not that easy to work on, plus, none of my poor, under financed, vintage Gibson/Epiphone collector customers ever have enough labor money available to effect repairs and the hummy/noisy reverb problem ALWAYS seems to exist in those as soon as they are brought back to life. Badly designed is being kind in my opinion and on top of that, they don't sound any good to me anyow. I just hate them. ha ha __________________ Bruce Mission Amps Denver, CO. 80022 www.missionamps.com 303-955-2412

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09-09-2007, 04:07 PM	#3
Turbo7MN Member Join Date: Feb 2007 Location: East Central MN Posts: 36	Yeah, I figured as much. This will certainly be my last Gibson project. It didn't help that somewhere along the line someone that had no clue what they were doing went in and made an even bigger mess of the already terribly laid out circuit. I've put more hours into this thing than I have pretty much every other amp I've worked on. So do you have any idea what usually causes it, or is it a just a weird thing and every amp's different? Thanks for the input. I'll probably just try and sell the thing as-is then. I really love the sound of it and the reverb was one of its strong points but it just hums too much to be useable.

09-11-2007, 04:52 AM	#4
Euthymia Senior Member Join Date: May 2006 Location: Alameda, CA, USA Posts: 185	Y'know, I had one of these (or similar Gibson model) in for repair from one of my affiliated stores. He just wanted me to get it into salable condition. Someone had hacked it up, so I restored it to stock and gave it back to him. Too much hum with the reverb turned on. He brought it back. I finally wound up just putting a capacitor across the input to the recovery stage to make a highpass filter and roll off all the lows. It sounded great, because reverb doesn't need to be bassy anyway. I choked it off way above 120Hz and it only improved the tone. I guess I should have documented the mod, but it's pretty easy in principle. Just one cap, I think from the grid of the recovery tube to ground.

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