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Old 29th December 2005, 08:23 AM   #1
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Red face Guitar Amp HUUUM with an MP3! Reverb Problem

Gentlemen, I (unfortunately) have an Epiphone Galaxie 25 tube amp from the Poeple's Republic. Pretty straight forward el84 push pull with spring reverb. The schematic is here:

http://members.cox.net/ted007ted/epiphone/epiphone.pdf

With a shorting plug in the input, there is a modest buzz that is not volume dependent. But when I turn up the reverb control, it injects this loud overpowering hum. I can faintly hear a little reverb through the hum. What does the hum sound like? Well here is an mp3 for your listening enjoyement. I made this with one of those super crappy plastic computer mics direct into my soundcard.

http://members.cox.net/ted007ted/epiphone/gal25.mp3

The spring reverb seems to work, if I tap the pan, I can hear some spoinging sounds, but pretty hard to hear over the hum.

I swapped the reverb recovery tube with a known good tube, and the same thing. In fact, I swapped out every tube in the amp! I biased the individuel EL84 output tubes very closely. The only change that I have made to the schematic is that I put in two parallel 100 ohm resistors from the pilot light to ground because the filaments were floating.

I only have one guitar amp, so this is driving me crazy because it gives me a headache if I play for more than 10 minutes. I did notice that female bees in heat were gathering outside of my window! I am starting to wonder if I should just eliminate the reverb all together, but I figured I would see if anyone could point me straight.

I did try the chopstick method and could get nary a change in the hum if I moved the wires around. If I clip pin 7 of v3 (recovery) to ground, the amp goes silent. I really do not know very much about tubes, trying to learn as I go. I also searched on the web and the posts until my fingers were blue. Please save me from throwing this thing out the window, or worse yet, slinking around at ham fests looking for an o-scope that I have no idea how to use.

Thanks in Advance for any help....
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Old 29th December 2005, 08:50 AM   #2
Giaime is offline Giaime  Italy
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Hello Ted,

You could have open ground/shield on reverb cable, or dirty/corroded RCA jack/plug on reverb cable but also broken ground wire inside the reverb tank. Check this out.
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Old 29th December 2005, 01:17 PM   #3
Trout is offline Trout  United States
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The Constant hum sounds power supply cap related, Though the amp really isnt that old, Still possible.

The Reverb sounds like bad ground someplace. I agree possible dirty rca jacks.
Also look for loose connections at all the pots.

That was a great idea posting an MP3 of the problem!!
Gene
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Old 29th December 2005, 01:25 PM   #4
anatech is offline anatech  Canada
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Hi Ted,
Really sounds like supply issues. Don't forget the reverb is just another input. It'll make your supply hum sound much worse as you turn it up.

If you are going to do much work with this stuff, consider slinking around ham fests looking for a scope you will learn how to use.

-Chris
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Old 29th December 2005, 09:31 PM   #5
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Hi guys, thanks for the replies. Well, I checked out the rca jacks and the reverb pan. The jacks are clean and I get continuity from the rca jacks both on the center pins, and continuity to ground on the shields. The metal reverb pan shows a ground to the amp ground.

I tested both sides of the inside of the reverb pan, measuring 1 ohm and 200 ohm dc resistance on the tiny transformers. The springs are all intack in the pan.

Moving the reverb pan or the reverb tranny does nothing to affect the hum by the way, so I don't think it's being picked up in the pan. The thing that does not make sense to me, is that if I use the footswich to disable the reverb, the reverb control still produces the same hum.

Sounds like power supply hum? So try adding a filter cap?

Does it make sense to try to remove the reverb circuit? As in if I disconnect C8 and broke the connection at R15 in schem?

Thanks,
Ted
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Old 29th December 2005, 09:36 PM   #6
anatech is offline anatech  Canada
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Hi Ted,
At this point an oscilloscope would tell you what you need to know. My feeling is that the filter capacitors are not working properly. This may be due to a poor ground to the chassis also.

I don't believe removing the reverb circuit will help you in any way, you still have the constant hum.

Just thinking. Some of those signal tubes may have developed a heater - cathode short. Check for that as well.

-Chris
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Old 29th December 2005, 10:11 PM   #7
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Okay, so this probably not a revelation, but I pulled the reverb pot and grounded the middle terminal (#2) which the output of the pot going to r15a on the schem. All hum goes away! Wonderful, at least I can play my guitar, rather than look at it. Well, some reverb would be dleightlful, so I want to keep going. Could anyone point me where to look next while I have this beast open?

I started reading about scopes, so my next purchase will be a scope...and a signal generator. Man this stuff gets expensive fast!

Thanks!
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Old 29th December 2005, 11:28 PM   #8
anatech is offline anatech  Canada
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Hi Ted,
Was the reverb footswitch plugged in? If so, check the shield on that too.

-Chris
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Old 30th December 2005, 12:19 AM   #9
Trout is offline Trout  United States
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Quote:
Originally posted by ted777ted
Okay, so this probably not a revelation, but I pulled the reverb pot and grounded the middle terminal (#2) which the output of the pot going to r15a on the schem. All hum goes away! Wonderful, at least I can play my guitar, rather than look at it. Well, some reverb would be dleightlful, so I want to keep going. Could anyone point me where to look next while I have this beast open?

I started reading about scopes, so my next purchase will be a scope...and a signal generator. Man this stuff gets expensive fast!

Thanks!
Ted, Did the constant background hum disappear also?
Gene
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Old 30th December 2005, 12:23 AM   #10
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Ground loops are very common with reverb tanks. The grounding scheme should be dictated by the tank's design. Some have the in and out jacks grounded to the tank, while others have one or the other or neither grounded.

Find the tank# and use this (or just teest it with a meter):
http://amps.zugster.net/tools/accutronics.html

Use all capital letters of the script won't work.

Then when you know the grounding scheme of the tank, the you can arrange the tank to eliminate ground hum, by either isolating the jacks from the chassis and running the grounds to s inngle point, or otherwise, depending.
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