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JayB
Senior tube assistant
Posts: 105
(5/22/05 8:58 am)
Reply Humming tube?
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What would make a tube humm? Volume doesn't change it. This guy brought in a twin-II with a humm, 60 cycle. Nailed it down to V5 tube. Switched tubes with an old good one I got and it dissappears. I can touch the tube and hear the static click with either tube. Just like touching the guitar strings, get that little click. It's driving me insane trying to figure this one out. I remember my concert-II doing that but I just switched tubes and lived with it. I'm beginning to think it's a grounding issue associated with these amps.
JayB
Senior tube assistant
Posts: 106
(5/22/05 11:25 am)
Reply Re: Humming tube?
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ok, found a loose solder joint on the heaters with that tube. Fixed that and no crackling any more. Leaking cap that's supposed to block dc off the cathoded follower was replaced. Sweetened it some. That helped a little. Just the stupid hum left. It's not bad, can't hear it while playing. Replacing the tube works but the tube isn't bad at all. Switching tubes around does nothing. Using that weak old long grey plate 12ax7 kills the hum but that weak old tube sounds like shit.
Thinking of elevating the heaters to see. The stand by switch makes noise when switching back and forth. Might take that out and by pass it for a quick test. Any help would be greatly appreciated before a rack my mind!
HotBluePlates
I only work on Fender's
Posts: 1184
(5/23/05 12:06 pm)
Reply tube hum
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V5 is a 12AX7? It wasn't obvious in your first post, since V5 means nothing if we don't know what the circuit is.
The tube itself can be humming. Switching to a weaker tube to kill the hum isn't a good option, because you're not solving the problem, but adding a crutch. I'd try a strong new tube, to see if the problem was just the original tube or the amp.
If the heaters are the problem, temporarily powering the heater circuit from a 6v lantern battery would tell you for sure. Then you could deal with that.
If not, you've already found that the problem must be after the volume control, as it doesn't affect the volume of the hum. If there's a leaky cap, then this is probably an older amp. What condition are the filter caps in? How are their grounds? What a.c. voltage (which is ripple or hum) is there on the B+?
Usually, amps hum because of the heater wiring (in a new amp or a single ended amp) or because of hum from the B+ (in an older amp where the filters are failing). Sometimes in a fixed bias amp, hum is injected straight to the output tube grids because the bias cap fails. That would be diagnosed by pulling the phase inverter tube (which doesn't stop the hum), then pulling the output tubes (which does stop the hum). And if the amp is old, and you find one bad solder joint, there are likely more to be found.
Be sure the basics are covered by checking the heaters, B+, and filter cap grounding.
JayB
Senior tube assistant
Posts: 112
(5/23/05 3:17 pm)
Reply Re: tube hum
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It's a 12AX7A. Tried a new sovtek from doug, tried the whole pack of tubes I got from doug and I know their not all bad. Has to be the amp. V5 is the reverb mix and dry tube, just after the 3meg/10pf cap combonation. I elevated the heaters from ground and that kills it but I'm wondering if that is just a crutch too. Could be a bad filter cap, they reasonably good but they been there since the 80's. What exactly is the difference between 60cycle and 120 cycle hum? I know one is a different pitch but I forgot which is which.
JayB
Senior tube assistant
Posts: 113
(5/23/05 4:42 pm)
Reply Re: tube hum
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Pulling the phase inverter kills the hum. Double check the grounding again. Think I'll just get the caps from Doug and replace them all. I noticed a funny hum when turning the amp on. Sounds like a motor boat decreasing in frequency until it stops then the hum. Flipping the standby switch while it does that stops the motor boating but then the hum remains. Looks like someone sealed the cap ends with some kind of sealer. It's brown in color. Nice looking mallorys.
HotBluePlates
I only work on Fender's
Posts: 1188
(5/24/05 7:32 am)
Reply Re: tube hum
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O.K. you just did your own troubleshooting. The motorboating sound is a low freq. oscillation caused by the filter caps failing. If I were you, I wouldn't try to nail down the one electrolytic cap that is failing, I'd just go ahead and replace them all. If one is going, the others are soon to follow.
If the Mallory's are white, then they're prone to quick failure.
JayB
Senior tube assistant
Posts: 114
(5/24/05 2:25 pm)
Reply Re: tube hum
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Yeah, I tried to nail which one. I stopped before I got the main B+ pair before the choke and their all leaking DC. A couple in the 30-40VDC. I wasn't comfortable messing with the 470VDC big guys, so I'm assumming those two are gone as well. How much of a dc voltage should I see on the tone stack caps? My meter gets to .000 and I see between .003 to -.005. Would that be normal?
HotBluePlates
I only work on Fender's
Posts: 1190
(5/24/05 9:40 pm)
Reply cap leakage
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If you're reading 0.003 d.c. leaking through the tone caps, you're fine. That's not enough leakage to throw off the bias of the next stage.
Filter caps leaking d.c. will cause hum. Coupling caps leaking will cause lack of headroom. In fact, if the leak is bad enough on the coupling caps, it can keep the grid positive enough so that the tube is clipping all the time, and you can't get a clean sound. If the cathode bypass caps go bad and start leaking, at a minimum, gain and volume will drop off. It doesn't sound like any of these were your problem.
Edited by: HotBluePlates at: 5/25/05 8:48 am
mrr3000gt
I only work on Fender's
Posts: 584
(5/25/05 12:38 am)
Reply Re: cap leakage
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I have had tubes do strange things like your describing.
I have started to recommend (and use myself) ElectroHarmonix 12AX67EHs, as they are lo/no microphonics, and have the smooth distortion and great highs/lows for preamp/tonestack applications.
I have been using Mullard 12AT7s for all phase inverters in my own amps.
The EHs seem to be real impervious to noise, and dont poop the sound up. They have been the only 'cheap' tube I can say has not made me look silly to a customer after a re-tube.
My HUGE stock of JJ tesla 12AX7As is like microphonic Russian roulette, as I never know if they will act up in someones amp. Some amps love them, and most hate them. All of my amps deplore JJ Tesla 12AX7As. I have a friend with a Carvin Valvemaster 100 that they sound great in.
JayB
Senior tube assistant
Posts: 118
(5/27/05 5:36 pm)
Reply Re: cap leakage
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Well new caps didn't help. Hum still there. Caps tightened up the sound. Sounds much better now. Only thing else I can think of is the choke?
JayB
Senior tube assistant
Posts: 122
(5/28/05 9:24 am)
Reply Re: cap leakage
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I pulled the reverb and effects loop completely out of the circuit and it's still there. Then I took out the last remaining triode, reverb/dry mixing tube, out of circuit and jumped the preamp straight to the PI and no hum. Can't even hear the darn thing until you strum the guitar. I'm beginning to think the socket is to blame. Got to be.
HotBluePlates
I only work on Fender's
Posts: 1196
(5/28/05 10:08 am)
Reply Re: cap leakage
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Well, maybe the socket, but then again, maybe it's the reverb return circuit. Does having the circuit in place as normal work when the reverb return cable is pulled? What I mean is yank the cable coming back from the reverb pan and play through the amp. You'll have no reverb, but you'll find out if the hum is coming from the cable.
If that doesn't get it, then you know that the hum is in between the reverb pan and the ohase inverter. You can then take whatever steps you like to deal with that part of the amp.
JayB
Senior tube assistant
Posts: 124
(5/28/05 11:00 am)
Reply Re: cap leakage
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Yup, done that. Still no go. I'v took out the the reverb all together and the effects loop so only that one triode which mixes the reverb and dry signal together is in the circuit. Still hums. Take that one last triode out, wire the preamp straight to the phase inverter and it's quite as a mouse. Has me stumped.
JayB
Senior tube assistant
Posts: 126
(5/30/05 11:53 am)
Reply Re: cap leakage
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Would you guys believe it was making contact from the cathode of that V5 tube to chassis ground? Then the heaters were picking it up? Right through the fiber board? Looks like a recent repair at one time when the hot solder flux seeped through fiber board melting the
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