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....
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....
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
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
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
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
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!
I started reading about scopes, so my next purchase will be a scope...and a signal generator. Man this stuff gets expensive fast!
Thanks!
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.
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.
Gene, about half of the background hum went away when I grounded the reverb pot at the middle pin. It's certainly playable when I ground it, and I have to say the amp sounds very good! Nice clean tones and the distortion is pretty smooth. Much better than I expected. It will also rattle the windows in my living room with ease!
By the way, in the mp3, the idle buzz sounds like it does in person, but hum created by turning the reverb control makes a deeper bassier hum than it sounds in the clip.
I took a look at the reverb spec calculator, looks pretty cool, but my tank is a 2 spring Belton unit. The Belton is physically longer than the Accutronics 2 spring units. Luckily there is a phone number to Belton in Korea on the unit. I played around flip flopping the grounding and such on the reverb pan with no change.
I would really love to get the reverb hum down and play some guitar. Any more thoughts or suggestions?
Thanks,
Ted
By the way, in the mp3, the idle buzz sounds like it does in person, but hum created by turning the reverb control makes a deeper bassier hum than it sounds in the clip.
I took a look at the reverb spec calculator, looks pretty cool, but my tank is a 2 spring Belton unit. The Belton is physically longer than the Accutronics 2 spring units. Luckily there is a phone number to Belton in Korea on the unit. I played around flip flopping the grounding and such on the reverb pan with no change.
I would really love to get the reverb hum down and play some guitar. Any more thoughts or suggestions?
Thanks,
Ted
Ted,
Can you post a picture of the inside of the chassis?
My thought is looking at the schematic, Bypass the reverb section. But of course that pretty much depends on the chassis wiring. Is this printed circuit? Tag Board? Point to Point?
If the reverb stage is fully bypassed, Then maybe you can locate the other hum source " IF " there is one.
( Just my opinion )
I never use spring reverbs, They are even in the best amps I have played less than ideal, Noisey,Common source of feedback, And lord knows terrible if the amp is positioned near the drummer on a Knock Down bouncy stage.
I am far from an expert on this, But for recording I use a no effects setup at the amp, And add things like reverb or chorus with Cubase SX recording software. If I am playing out I use effects pedal for what little I ever need.
Rarely do I ever use much reverb, Generally I just breath on the knob at the pedal to take out a bit of the dryness.
Of Course opinions vary on this.
If the reverb stage is fully bypassed, Then maybe you can locate the other hum source " IF " there is one.
Looking at that schematic, I bet that baby has a great clean sound!
Gene
Can you post a picture of the inside of the chassis?
My thought is looking at the schematic, Bypass the reverb section. But of course that pretty much depends on the chassis wiring. Is this printed circuit? Tag Board? Point to Point?
If the reverb stage is fully bypassed, Then maybe you can locate the other hum source " IF " there is one.
( Just my opinion )
I never use spring reverbs, They are even in the best amps I have played less than ideal, Noisey,Common source of feedback, And lord knows terrible if the amp is positioned near the drummer on a Knock Down bouncy stage.
I am far from an expert on this, But for recording I use a no effects setup at the amp, And add things like reverb or chorus with Cubase SX recording software. If I am playing out I use effects pedal for what little I ever need.
Rarely do I ever use much reverb, Generally I just breath on the knob at the pedal to take out a bit of the dryness.
Of Course opinions vary on this.
If the reverb stage is fully bypassed, Then maybe you can locate the other hum source " IF " there is one.
Looking at that schematic, I bet that baby has a great clean sound!
Gene
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.