I have a 74-ish Fender Champ that makes popping & scratching noises when I'm warming up the tubes. Nothing plugged into it. Volume at zero. Any ideas what the problem could be?
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I have a 74-ish Fender Champ that makes popping & scratching noises when I'm warming up the tubes. Nothing plugged into it. Volume at zero. Any ideas what the problem could be?
Old tubes?
My first guess would be the power-rail resistors in the power supply. These are 1-watt carbon-comp types which are typically good for about fifteen to twenty years. At that point they become brittlle and prone to absorbing moisture from the ambient air. This often causes the noises you've described. When replacing them I recommend the metal-oxide types which are more durable and resistant to this phenomenon.
Thanks for the feedback. I'll have to research the power-rail resistors. Is that something I could replace myself? As for old tubes, I just had this thing totally gone through & rebuilt. I assume the guy replaced all the tubes, but maybe not. I'll check my bag of spare parts he sent back with it.
You can, as long as you know how to discharge the filter caps properly and check for voltage, how to solder, and how to select the correct type and ratings of resistors.Quote:
Originally Posted by musicallydeclined
Looks like I've got some learning to do. Where would a feller look to find info on that sort of stuff?
A two-year program at a junior college or military tech school would provide the essential fundamentals.
Remember......there is shit inside these amps that can fucking KILL YOU! Electricity is terribly unforgiving.
And, it's really quick. I'm thinking it would also be quicker to put it in a shop rather than waiting to graduate a 2yr program.
Technically, the high voltage/low amperage in our guitar amps "could" kill you but I have never found any documentation that supports it. Mechanics work with higher voltages, still low amperage, but also no evidence anyone ever died from a spark plug shock. From experience, I guarantee you can hurt yourself from your body's involuntary reaction!
Electronics, and tube amps is a fun hobby, but you really need to spend some time learning the how and why if you plan on working on them. Be careful to disconnect the amp before servicing, as the 15 amps or so from the wall has been known to be deadly!
This may help:
https://www.stewmac.com/luthier-tool...waAgfJEALw_wcB
I bought the book, but still have many questions.
I have determined my Champ is 1970 -1974 with AA764 circuit. The tube chart is torn at the bottom, so I can't find the date code. It is silverfaced, blue sparkle grille with no aluminum trim (1970 - 1975), Fender logo with tail (1968 - 1974). I bought it way back in '75 or '76. I've replaced the speaker numerous times. I have no clue if I have the correct speaker. Everything I've read says it should have a 4 ohm 8" speaker. It currently has an 8 ohm 8" speaker. Does anyone know which is correct, or how to determine the proper output resistance?
The original output spec for a Champ Amp (or any of the derivatives) calls for a 3.2 ohm load. Most folks read that as four ohms so that's the driver your amp needs for optimum performance. There are ample options available from a number of different sources -- Jensen, Oxford, Weber, etc. As with most silverface Fenders, you'll need to examine the EIA date codes of the cap can, pots, trannies, etc to determine the exact year that the amp was made. Tube charts used during this era were notoriously vague as far as dating goes.
Does anyone know what the value is of the capacitor that connects to pin#8 and pin#5 on the 6v6 tube socket? Mine looks a bit damaged and I can't read it.
[QUOTE=Bill Moore;662657]Technically, the high voltage/low amperage in our guitar amps "could" kill you but I have never found any documentation that supports it. Mechanics work with higher voltages, still low amperage, but also no evidence anyone ever died from a spark plug shock. From experience, I guarantee you can hurt yourself from your body's involuntary reaction!
Electronics, and tube amps is a fun hobby, but you really need to spend some time learning the how and why if you plan on working on them. Be careful to disconnect the amp before servicing, as the 15 amps or so from the wall has been known to be deadly!
This may help:
https://www.stewmac.com/luthier-tool...waAgfJEALw_wcB[/QUOTE
1.5 volts can be deadly in some conditions.
Unplugging a tube amp from the wall doesn't remove the chance of electrocution. If you don't have at least a basic amount of knowledge and experience in electricity and circuits do not attempt to work on a tube amp. There are potentially lethal voltages inside even after it is unplugged. It would really suck if you became the "R" in an RLC circuit.
Chuck
I have looked on the web, and asked on several forums if anyone has evidence of anyone killed from the low amperage "lethal voltages" we find in our amps, there seems to be no documented evidence! All the manufacturers have to parrot the "lethal voltage" trope as the 15A wall voltage can be deadly, so everyone assumes it is the LOW AMPERAGE high voltage circuits.
Sure 1.5 volts with high enough amperage applied across your heart could be fatal to some folks, but you would have to work at it to get it done.
As I said, you are more likely to get hurt from the reaction to the shock, than the shock itself.
+1Quote:
Originally Posted by Offshore Angler
But what would it do to the amp's tone?
:biglaugh:
Quote:
Originally Posted by Bill Moore
OK, believe what you want, but please don't argue with engineers and technicians based on your internet "research".
Those filter caps can pack quite a wallop. If you don't understand circuit time constants then you don't understand how much current those caps can deliver. Remember in a short-circuit the current goes to infinity.
If you want I'll be more than happy to show my math, and you can show yours.
Chuck
Some of us have experience in these matters, I have been working on and building electronic circuits for most of my life. I have been shocked from HV electronic and automotive ignition circuits, evidentally none lethal.
Others may have "theory", if there is any evidence of death from guitar amplifier filter caps, I would like to see it!
Here are the specs for typical capacitors used in guitar amps, their physical size keeps them from storing more than milliamps of current.
https://www.tubesandmore.com/sites/d...eries_spec.pdf
Bill, your answer shows you have no basic understanding of electical circuits. Capacitors do not store current. In fact, "stored current" makes zero sense. Those caps on the power supply, BTW, are pretty good honkers there with a decent potential across them.
Maybe you should start by looking up "Coulomb". Even the tiniest capacitor can produce a very large current. Look at Ohm's Law , then tell me what the current is out of a tiny little 1 picoFarad capacitor with 12 volts potential when the resistance is zero.
And your advice about don't worry about it is - dangerous. How do you know the person reading it doesn't have a medical condition or a pacemaker? In some cases the generated magnetic field of a capacitor discharging across an inductor can cause medical implants to malfunction.
Like I said, believe what you will but sometimes you should consider the input of people who are specifically educated in things like this. If you want to put yourself at risk of electrocution that's your own business, but when you start passing out opinions as fact and basically telling people to disregard the amp manufacture's safety notices and that of engineers and technicians, you put this forum at risk of liability.
Your whole argument is basically "I haven't killed myself yet." Sheesh.
Chuck
Well which is it?
"Capacitors do not store current. In fact, "stored current" makes zero sense."
"Even the tiniest capacitor can produce a very large current."
I take it you haven't any experience working on electronic equipment, and it seems if things were so dangerous the instructors might have mentioned it in my service schools. The warning were "be careful, you can get shocked, and your involuntary reaction may get you hurt!"
Once again, there is NO documented evidence of someone being killed from the low amperage, high voltage curcuits in guitar amps. (If you have any evidence, please let us know!)
This is why I never even try to muck around with my amps. My knowledge of electronics ends at the point I plug in and flip the switch.
I'll leave the discussion about amps, ohms, dip-switches, leads, capacitors and the like to others.
Bill, you really are displaying your ignorance in electronics. A capacitor stores a charge, the charge creates a potential, and when the capacitor is discharged a current is produced in the circuit. If you took training in electronics you should have that basic level of knowledge.Quote:
Originally Posted by Bill Moore
I have multiple engineering degrees and work with (extremely) high-energy systems on a daily basis. My actual field of study is aerospace, but we use electrical analogies all the time, since the movement of fluids uses a lot of the same equations and I've even used analog computers to solve flow problems. I've also done work in magnetics and worked for one the world's leaders in magnetics.
Like I said, I'll show may math and you can show yours. I've already showed you some math with Ohms law. Apparently, you think Ohm's law isnt' true. Please prove it.
So enough, the readers can decide who to trust.
Chuck, you should know that nothing happens until there is electron flow "current", discharging the cap allows stored energy to produce current!
You haven't shared any math, but we can use I=E/R to find current produced from the Champ's "lethal voltage", 355v at the first node on the schematic. I currently measure 2.5megohms hand to hand, which would yield .000142A if I discharged the cap through myself, not near enough to damage anything!
Quote:
Originally Posted by Bill Moore
Well Bill, I think V=IR qualifies as math but if you feel compelled I will do some simple calculus to calculate the time constant of a circuit for you. Let me know.
OK, you measured 2.5 MO hand-to-hand at an extremely low potential, which is not reality at higher potentials. At voltages around 100V human tissue becomes conductive. That means the current will rise significantly, and at higher voltages your body will actually burst into flames and nothing is left.
That's what HiPot testing is for. We need to know that the insulation is sound enough not to break down and allow current to start flowing.
Judging by your demonstrated level of knowledge of electricity I would urge you not to work on your own amp, or anybody else's.
I would also urge you to look up the term "death cap" as it relates to guitar amplifiers.
As I stated earlier, you are fully allowed to electrocute yourself but in my informed opinion you really should refrain from using the forum to pass yourself off as a subject matter expert on something you are clearly in the dark on and giving people dangerous advice.
Chuck
Taken from one who seems to have little knowledge of working on amps, I take your advice with a grain of salt. You evidentally have not spent any time with "hands on" experience, theory and "what if's" don't fix things in the real world. Maybe after you build and repair a few, you might be a little more enlightened, "death cap" was used back in the day of ungrounded wall receptacles, and no, no evidence of death with their use. (They also had a "ground" switch on amps that had this circuit). A musician in those days had to rotate the wall plug for amps without a "ground" switch.
As far a "conductive" skin, you would need a substantial amount of current flow to overcome the high resistance at even 100V. I haven't heard of anyone catching on fire, but I know a lineman who lost part of his heel after contacting 7200V through a hole in his rubber glove, he made the ground connection through his right hook, no fire though!
Sure V=IR, (it was E=IR when I learned it), is a great tool if you need to find voltage, but looking for amperage, you need the right formula.
I think I may have fixed my little Champ. After replacing a blown capacitor (twice), and greatly improving my soldering skills with a new iron, I stumbled across a post on another forum complaining of the exact same symptoms as mine. I replaced all the tube sockets. So far, no loud, scary noises during warm up. I found a 10k resistor running parallel to the blown capacitor. According to the schematic, it should have been 470 ohms. I replaced it with the correct one. I replaced the cap on my 6V6 pins with a new 330pF (still not sure that value is correct). I replaced the 8ohm speaker with a 4ohm. I also did a mod from Ampman, so I moved a couple of things around a bit. So far, I've played it a couple of times for about an hour each with good results. Fingers crossed!
That’s great to hear! Glad it wasn’t too serious!
I played my little Champ again yesterday for about 2 hrs., and I let it idle for 4-5 hrs. I never heard any strange noises. It sounds like a vintage Fender amp should. At this point, I'm going to call it a success. Thanks to everyone for all the input. I certainly would not have gotten this far without it. Even what I couldn't use this time may come in handy for another time.
So, I'm moving on to a new, external, 2-12 speaker cab. I have the Champ and a Princeton with which I hope to share the ext. cab. I'm trying to decide whether to use 4 ohm or 8 ohm speakers. I'm leaning towards Celestion since they are geared for guitar, but I'm not married to that. I didn't find a good selection of 12" x 4 ohm Celestions, so I'm leaning toward 8 ohm greenbacks.
Will the Champ push 2 12's with a 4ohm combined load?
Anybody have suggestions for other speaker brands?
Decades ago my Dual Showman took a dump about twenty minutes before a bunch of my buddies were coming over for a garage jam. My only alternative was wiring my Vibro Champ to the Showman's 2 x 15 cab containing a pair of JBL D130Fs. With the little amp dimed it sounded positively monstrous and had no problem holding its own against the gear my pals brought along (a Sunn 200S bass amp, a Kustom K200 for the keyboard, and a Fender Pro Reverb for another guitar). Your Champ shouldn't have any issue pushing a pair of twelves.
Cool! It's on, now.
A mod which you may find useful is to replace your Champ's RCA phono-plug output receptacle with a ¼-inch phone jack. The change is reversible if necessary and it facilitates an easier connection when plugging into alternative stand-alone speaker arrays.
Actually, I did that. just forgot to mention it. I kept the original for the old heck of it. Thanks though.
The champ is still working properly. I'm calling it fixed. Now, I want to build an ext. speaker cab for it. I got a 12" 8 0hm Greenback. Gonna get another when I get more funds. Meanwhile, I'm using it with my Princeton. However, the Princeton sometimes sounds better through its own 10" speaker. Depends on the volume and/or guitar. If I put a SPDT switch between the 12" & the 10" speakers, will I damage my amp or speakers by switching (not while playing) from one to the other?
If you're expecting prototypical Fender blackface/silverface tones the Celestion Greenback is the last speaker to choose. You're better off with Jensen, Eminence, CTS, or -- if your wallet permit it -- JBL.
My two cents.
I think of greenbacks more for closed-back cabs.
I would get a double pole single throw switch and use it to take the second speaker out of the circuit when using it with the Princeton. I did that with a 4 x 10" cabinet I built. It has 4 16 ohm speakers wired in parallel. With all 4 speakers, the load is 4 ohms. With just 2 of them, 8 ohms.Quote:
Originally Posted by musicallydeclined
Eventually, I will use a switch to take 1 speaker out. I only have 1 speaker right now and it's 8 ohm, so it matches the output of the Princeton. I want to be able to switch between the 12" 8ohm (ext. cab) and the 10" 8ohm (original) speaker. Will repeated switching from one to the other cause any damage to the amp due to no load for the 1/2 second between switches? Or, do I need to just turn the amp off to switch speakers? I guess I could just make up my F-ing mind before I turn it on, couldn't I?
This is just my opinion, but I think if you turn the volume down to 0 when making the switch between speakers, you shouldn't cause any damage to the output transformer, because there won't be any significant load during the switchover.Quote:
Originally Posted by musicallydeclined
I have accidentally had my Vibro Champ turned on with no speaker attached for 10 or 15 seconds, and as far as I know it didn't cause any damage. Prior to changing it out for a 1/4 jack, the VC had a non-shorting rca jack.
I would use a rotary switch, with "make before break", so there is no possibility of an open circuit.
https://www.tubesandmore.com/product...les-3-position
So, I thought I had my warm-up noise problem fixed, but it started giving me 1 scratchy popping sort of sound again early during warm up. Not as loud or as lengthy as before. After that, it played without a hitch. Sometimes it would repeat, but mostly just once. So, I scratched my head and tried a bunch of stuff, and I think I stumbled upon the solution. A while back, I read an article about care for vintage amps. It said to be sure to have both a speaker and a guitar connected when you flip the switch. It didn't say why. I've only seen that in the 1 article. Every post I've read since says to be sure to have a speaker connected, but nothing about a guitar. Having done some research on repair & general function of tube amps, I do understand the reason for the speaker connection. Not knowing why, or if the previous article was even true, I eventually disregarded the advice. However, I noticed that if I happened to have my guitar plugged in at startup, I didn't get those ugly noises. It's been consistent for about 20+ startups now. Must be something to it. Maybe one of you gurus could shed some light?
Did you try cleaning the contacts of the input jacks?
I did not, but I sure will. I guess that would never have occurred to me, but it makes total sense. Thank you, sir.
I replied to this post, but it wound up posting somewhere else. Still don't have all this fancy-dancy forum stuff figured out. I have not cleaned the input contacts, but I certainly will try that. After 50 yrs., it could probably use it. Thanks for the suggestion. That probably would not have occurred to me.