Fender Champ Capacitors and other issues

[Paleo Pete]

I forgot about that...I don't know which one it is, but I replaced the power transformer after the arcing started. I do know it's for Champ and Princeton but that's it, had an amp tech in Louisiana install it since it had several more wires than the original. He told me the extra wires were for 220 if I remember correctly. As far as I know it's not a super heavy duty or anything,

Magnetic Components inc
40-18027
166-0911
Made in USA

I should have let you guys know that already, sorry I forgot about it. I replaced it because when digging for info at that time I found several references to transformers causing this, and I was getting dubious results on the original with my meter. I wasn't positive it was bad, but thought it was a good idea to put in a better one.

Thanks for jogging my memory. Output transformer is still original.

[leydenjar]

The heater tap on your PT has a current rating.

My inclination is that the PT I installed in my champ can handle the current needs of tubes that will draw more current.

I have learned through experience that an amp prone to arcing on the power sockets will continue to do so if you haven't removed the carbon "path" created by repeated instances of arcing.

Obviously, the initial cause of the arcing has to be dealt with as well.

My little silverface champ taught me a lot in the beginning of my quest to understand tube audio.

Search "power tube arcing" and you'll get all the info you need.

[Paleo Pete]

I have learned through experience that an amp prone to arcing on the power sockets will continue to do so if you haven't removed the carbon "path" created by repeated instances of arcing.

That's one of the things I found out a couple of years ago when trying to figure this out, so the tube sockets have been replaced every time it happened. I did a lot of searching and reading, never have figured out just what is going on. It's annoying...I've tried or tested everything I could find (if I knew or could find out how) and have completely rebuilt the amp except for 2 disk capacitors, rectifier tube and output transformer...

I'd like to be able to afford another output transformer, but for some reason I'm really leaning more toward a bad tube or the 330pf cap getting hot and failing. Now that I know about the cathode resistor I'm pretty sure it could be getting too hot as well. Never found that one before, the guys here are always coming up with something good to consider.

At this point the only way I can really know if I've made any progress is to run it full tilt for over an hour. Can't do that till I can replace that resistor, don't have a suitable one on hand. Should be able to find it in the next week or so, whenever I go into town.

This Champ is not what got me into this, it was my 73 Super Reverb driving me crazy. It would do great for 15 minutes or so then the volume would drop as if a power tube had suddenly been yanked. Nobody could peg it, I certainly had no idea, all I knew about amps was how to swap tubes, and found out very soon I needed to do a complete cap job. (due to limited income, 2 or 3 at a time.) The last couple of caps did it. I've been using it onstage with no trouble at all since then.

I still barely know enough to say I know what a tube amp is...but thanks to the guys here and a lot of other good websites, I've been gradually picking it up. But this has been driving me crazy...I need to do the pencil test again but don't want to crank it up until I fix the resistor issue, I do a very good soldering job but still could have a bad one somewhere, it would be far from the first time. (I've replaced caps in at least 100 computer motherboards, last one I did about 6 months ago had a bad solder joint...and I've been soldering over 20 years. You never get every single one right...)

Thanks a lot again folks, I'll keep trying to figure this out but it's driving me up the wall. If nothing else though every time I post something here I get plenty good ideas to think about...

Bob M - I Just spent the last half hour searching for that article, everything I find goes back to DIYCUSTOMAMPS dot COM and as far as I can tell it's not there. Every link I checked on that page just goes back to another link list. Any ideas? I'd like to give that a read if I can locate it...

[Paleo Pete]

OK I set it up last night and checked voltages as is. Stepped on a nail yesterday and couldn't do much barely able to walk so I had a little time on my hands. Also realized I forgot to mention another detail..when I put the tube sockets and replacement rectifier tube in a few days ago I had to replace the fuse, it was blown. I didn't think too much of it, arcing tubes, it was no big surprise. I put the voltages I got on the champ layout, I'll try to attach it. Had it on about 10 maybe 15 minutes, while double checking to be sure I hadn't missed anything it suddenly made a low pitched hum for 3 or 4 seconds, not real loud, then shut down. Blew another fuse. Speaker was connected, nothing plugged into the input jack, controls all on 10.

I can only guess, but when i looked up info on arcing tubes I saw the output transformer was a pretty popular culprit, but I do also have a couple of other questionable items. Wrong cathode resistor for example...I guess I can't do much till I can swap that out. I looked up a little info on how to test the transformer, not sure if I got it right and still need to look a little more. I got 287 ohms testing the blue and red wires, when testing the speaker out leads it shows the same as a dead short, as if it were testing a straight wire.

Don't know if either is right, but that's the only places I got any reading at all. Amp shut down, plugs pulled and speaker disconnected. If I understand what I read, it should show 70-140 ohms, I got 287 several times. I'm not sure if the output wires should show a short I'll have to do a little more reading.

If this works, I put the voltages I see on the Champ layout from Fender Amp Field Guide, what my meter says is in red. From what I see, voltages don't look bad. Everything but heater voltage is low. Heater wire showed a solid 6.3 every time. 318V at pins 4&6 of the 5Y3 was fluctuating + or - 2 or 3 volts, 318 was where it settled now and then. Everything else was steady, only those fluctuated. I checked each at least 3 times, always grounded at the same solder spot as the ground wire.

I'll keep doing some looking around, I was tired and in pain and didn't spend a lot of time searching for how to test the output transformer, but something is obviously creating a short, either that or the resistor getting hot. Other wise it wouldn't be blowing fuses. I didn't touch the cathode resistor to check that...I don't like high voltage...as quick as I could turn it over (it was sitting outside the case) the power transformer was not even warm, output transformer only slightly warm. I could have left my hand on either one all night, very little heat, the PT was probably body temp, not warm at all. I'd say the OT was 105°F to 110° at most. Far from what I call hot. I've checked the transformers in my Super Reverb, both get warmer but not hot.

I'm guessing, either bad output transformer or the 290 ohm resistor is getting really hot and shorting. But that's a definitely uneducated guess...One good thing, I see no indications that it arced this time, I checked both tube sockets closely. Both still look like new, no black traces...

[nickb]

Making the assumption that you haven't actually seen this arcing but just heard something, I'm thinking that that 330pF between the plate and grid cap is bad by going leaky intermittently after a little bit of heat. This would cause the hum and fuse blowing as the 6L6 will turn on hard. It's there to roll off high frequencies either to prevent oscillation and/or for tone.

Other leaky possibilities in order of suspicion are a leaky grid coupling cap to the 6L6 or the 6L6 cathode bypass cap going short cct.

Easier to replace these that the transformer and see what happens.

You could test the OPT by simply completely removing the primary: wire the plate to B+ and set volume to ZERO. Leave for a long time to see if the fuse blows.

[Paleo Pete]

No, I haven't actually see it arc, just the burnt traces on the tube sockets after it dies, usually while playing it. No odd noises, just suddenly fizzles out and it's dead. Until now the power lamp would stay on, I don't remember if it was crackling, no obvious hum that I remember. Just no sound and a burnt smell from the arcing, and no fuse blown the first 2 times. This started only the last time it arced, and last night when I checked voltage. No arcing last night that I can tell, I checked the sockets. I also didn't play it, just checked voltages.

I'll have to do some digging to find out what most of that means. Maybe you guys do this every day and have it all memorized, I don't know B+ from a frog's ass, most of this is greek to me, you're assuming I've done this 30 years like you and know what you're talking about. But I think I have a plate in the kitchen...I'm still trying to find out what screens mentioned above is, I see it mentioned all over the place, nobody has bothered to simply say oh by the way that's pin 18 or whatever. Bob realized this and let me know the resistor I was talking about was the cathode resistor, I thought that was great, I finally know what one of them is...

I know what the parts are, they're marked. I know where they belong, I obviously am not totally stupid, I rebuilt this thing (and it played for 3 or 4 years without incident before starting to arc) and rebuilt a Super Reverb, it's been working perfect for 10 years. That doesn't mean I know what all these terms mean, what they do or how to find them.

Sorry, I just get irritated when people assume everyone on here knows amps inside out, when some of us are actually struggling along trying to learn. And have already stated so.

[nickb]

Pete,

I said what I did as you said in an earlier post you said

it suddenly made a low pitched hum for 3 or 4 seconds, not real loud, then shut down. Blew another fuse.

Anyway,

No, I haven't actually see it arc, just the burnt traces on the tube sockets after it dies,

So, there really was arcing. I guess I should have read the whole thread. It was late.... I was tired...sorry.

I just get irritated..

Hmm, just trying to help you know. It's pretty hard to know how to pitch things and you came across as more knowledgeable that you assert.

Arcing in caused by the voltage on the tube base pins being high enough to cause the insulation to break down. The breakdown voltage of a socket is lowered by contamination of any kind e.g. flux, dirt, humidity, or damage. You replaced them and I assume they were clean so we're looking for a source of high voltage. You measured the plate voltage as 318V - that's not excessive. The only other thing I can think of of is inductance i.e. output transformer (Enzo already mentioned this).

The primary of the transformer always has some unwanted (leakage) inductance. The problem with that is when there is a sudden reduction of current the energy stored inside it is released and gives a high voltage that could be high enough to breakdown the tube socket. The rate of reduction of current is controlled so some extent by the 330pF cap between the plate and grid of the 6L6. on the plate. I'd replace if for no other reason than you have had arcing that that will have stressed that cap. Another reason a for sudden current change could be bad connections in the plate wiring.

I wonder if there could be a parasitic oscillation, again the 330pF has a bearing on that. You might try adding a 2.2k resistor in series to the connection to the grid (pin 5) of the 6L6 too. located right on the pin.

Something else to consider is could the inductance of the transformer be higher than it should be? The usual reason for this is bad speaker wiring, blown or missing speaker load. Under this circumstance the output transformer (OPT) just becomes a big inductor and causes massive voltages. The speaker being the wrong impedance will raise the inductance too and will elevate primary voltage.

Assuming these are OK, then that only leaves the transformer itself - somehow the windings are not coupled as well as they should be - I'm having a hard time imagining how that could happen since it's a property of the geometry and core. A pretty long shot, I'd say.

BTW the removed transformer test I mentioned is only a crude check for insulation failures. I don't think it's applicable here as you have evidence of arcing.

[Bob M.]

Try emailing Steve at: Steve@diycustomamps.com

Ask him if he might email you a file of 'The Remaking of a Champ' from Glass Audio Magazine. Informative reading.

[Tom Phillips]

... 'The Remaking of a Champ' from Glass Audio Magazine. Informative reading.

I remember reading that article. It was published ~10 years ago. Just so you know, the "reconstruction" didn't concentrate on making it a better guitar amp. More like an audiophile changing all they could about the circuit to "improve" it from their point of view. IMO, other than entertainment value and sparking some interesting comments on this forum, there was nothing in the article of value with respect to a guitar player's amp. Again, this is my opinion and I'm posting this message so those who read this thread know that there aren't any guitar amp mojo secrets in the article.
Cheers,
Tom

[Paleo Pete]

nickb - Sorry to stir things up, I was hoping it wouldn't cause too much grief...I've been trying to learn these things for a long time, and for some reason electronics is the one thing I've never really understood. It really aggravates me...I'm a machinist, musician, photographer, mechanic, carpenter, PC technician, and certainly intelligent enough, my IQ is somewhere on the 140-150 range. And I'm mostly baffled by electronics...GRRR...that's not your fault, but one of the things I know is an issue is I don't do this as a regular hobby, and have a lot of trouble remembering when it's been a year since I had to do more than solder a guitar cable...I also have a lot of trouble figuring out what you guys never really think about twice...it's second nature to you. Sorry if I implied I'm more knowledgable than I actually am..my bad...

Anyway thanks for the new info, it might take me a little time to digest everything.

You are correct in assuming the sockets are clean, they're brand new. I'll have to read through and try to get a grip on the explanation you posted, but it seems to make a little sense on the surface.

I set it up and tried again with a different cathode resistor just to try things out, not the wattage I need but it reads 510 ohm, and I'm guessing it's 2 watts. Not what I need but should be good enough for a temporary to check and see how it acts briefly.

Briefly is an understatement, it started the humming instantly, lasted 3 or 4 seconds and blew the fuse again. You and enzo have both suggested the output transformer as the potential culprit, I was already considering it, now I think that's the most likely problem. I was originally thinking heat, especially the 330pf cap, but didn't know heat might also be an issue with the cathode resistor. (I'm still planning to get a 5 to 7 watt at around 500-600 ohm, have not had the chance yet)

Now with the humming going on it's looking like I was way off base to begin with. I've been pushing the output transformer way too hard for over 20 years from what I'm reading, and now it's starting to look a whole lot like I should have been looking there all along. Speaker impedance should be ok, it's a 4 ohm that measures exactly 4 ohm. Connection is good, I checked that when I checked voltage before my last post. I knew it had been connected and disconnected a lot lately, so I checked all the solder joints on my cable and so forth. It's not easy to do but wiggling around while holding tester probes to the (unplugged from the amp) rca jack it stays steady. (By the way it's a 8 inch out of a Gorilla amp and sounds great.) Plugged into the speaker only it reads a steady 4 ohms.

I wonder if there could be a parasitic oscillation, again the 330pF has a bearing on that. You might try adding a 2.2k resistor in series to the connection to the grid (pin 5) of the 6L6 too. located right on the pin.

I'll have to get a replacement for the 330pf cap, I still suspect heat may have damaged it over the past 40 years. I had considered running without it, but if it will make a difference I'll go ahead and get a replacement. I thought it might be there for something to do with oscillation, since they did something similar with the Super Reverb, but had no idea if that were actually the case. Thanks for confirming that, I was only guessing.

About the 2.2k resistor - do you mean in series with the 330pf cap, between pin 5 and 8, or from pin 5 to the circuit board?

I'm almost convinced at this point I have a bad output transformer. It seems to make sense, it's been rode hard and put away wet for 20 years, and from what I've been reading here and elsewhere it's not really up to the 6L6 anyway, so I guess I'll be without this amp till I can scrape up a few bucks for a replacement. Guess I'll get to check out my old Harmony for a while. It's a 60's or 70's solid state, but sounds pretty good so I might be running it a bit more for a while.

I don't have a regular internet connection at the moment, so it might be a few days before I can get back here. I have to go into town 15 miles away to find a wireless access anywhere...

[nickb]

Now with the humming going on it's looking like I was way off base to begin with.

...I'm almost convinced at this point I have a bad output transformer.

Let's recap. I understand there are two problems, arcing and hum/fuse blowing after a couple of seconds. I take it the hum/fuse is the major issue so let's deal with that first and stop randomly changing things. We have to isolate where the current is going.

(1) First pull the rectifier and leave on for a while - did the fuse blow? If no, the the power transformer and heater wiring are likely OK. Proceed..
(2) Replace the rectifier and pull the 6L6. Switch on wait. Did the fuse blow? If yes, there is a problem with either the output transformer, bad cap on the power rail, or the rectifier.
(3a) if the fuse blew, remove the connection to the power side of the output transformer - DON'T forget the discharge the caps before touching. Try again, if it blows it's bad cap or rectifier. Replace one of them and try again. If it still blows it's the one you didn't replace.
(3b) If the fuse did not blow after (2) then we know that the 6L6 is drawing too much current. We discussed this already: bad cathode bypass cap, bad connection to grid, bad 330pF cap.

I don't want to diverge but one other thing is niggling me. With the change to a 6L6 and the new cathode resistor (290 ohms doesn't sound right, BTW, maybe it's 390?) the zero signal current is a bit higher (48mA) than it was before (40mA) - this is a problem as the output transformer can only handle so much before is saturates. Now maybe with the new transformer that is OK but it still puts an extra load on the power supply. Personally, I'd make the resistor higher until I got back to 40mA - just divide the voltage across it by the resistance to get the current. Edit: I just pulled up the datasheet for a 6L6 and I would expect more like 680-750 ohms with 30V across to get 40mA so something is not right here.

About the 2.2k resistor - do you mean in series with the 330pf cap, between pin 5 and 8, or from pin 5 to the circuit board?

No, in series with the lead to the grid pin 5. The 330pf cap (not shown) goes directly to the grid.

[Paleo Pete]

nickb - Thanks a bunch for the info, sorry I haven't been back in a while, haven't been able to get online at all. Nothing at home, I have to go 20 miles into town...it's a drag.

Just copied your info into a notepad file, no way I can remember it all.

I'll go through your troubleshooting steps, see what happens and let you know. Thanks again.

[Leo_Gnardo]

Have you tried changing the tube sockets where arcing has occurred? Often the arc leaves a carbon track the plastic socket, and from then on it's easy for hi voltage to follow that track. Sometimes it's not on the surface but within the bulk plastic of the socket. On 2/12 you mentioned seeing burnt traces on the sockets. That would be a dead giveaway - replace the sockets. After skimming through this thread, that's the only solution I can suggest that hasn't been tried. Also keep in mind that carbon tracks can develop on the tube base as well, often between pins 3 and 2.

I think we can assume that the fuse blowing is because of excess current being drawn from the hi voltage supply due to arcing. I'm less suspicious of the PT and OT than I am of the fried octal sockets.

[Paleo Pete]

Yes, as I've stated at least twice, tube sockets have been replaced every time, due to obvious signs of arcing. (re read 1st post. It's there, again on 02-07-2013, 07:45 AM.) New ones just installed recently have not shown any sign of arcing since replacing the 5y3 rectifier tube, it played great once and next time I turned it on it lasted 10 minutes and blew the fuse. When checking voltages for the post above it blew the fuse again in 10 minutes or so, still no sign of arcing on tubes or [new] sockets. Last time I turned it on it lasted 5 seconds tops and blew the fuse, low pitched hum. No time to heat up.

nickb - Sorry, I haven't taken time to open it up and check things out yet, don't have any great excuses, just been a bit lazy the past couple of days. I'll try to get it out soon and go through the list you posted.

[Paleo Pete]

Hi again guys, sorry to take so long. I live out in the sticks, about 20 miles to an internet connection, and the local library decided to block forums...I finally managed a way to get online now and then just recently.

Did the testing outlined above, and as I stated earlier I copied his instructions into a notepad file because I knew I'd never remember everything. Here's what I got, including original instructions, copied from that notepad file:

*********

Let's recap. I understand there are two problems, arcing and hum/fuse blowing after a couple of seconds. I take it the hum/fuse is the major issue so let's deal with that first and stop randomly changing things. We have to isolate where the current is going.

(1) First pull the rectifier and leave on for a while - did the fuse blow? If no, the the power transformer and heater wiring are likely OK.

My response - All controls on 0, speaker connected, no hum, fuse didn't blow. On for 3-4 minutes.

Proceed..
(2) Replace the rectifier and pull the 6L6. Switch on wait. Did the fuse blow? If yes, there is a problem with either the output transformer, bad cap on the power rail, or the rectifier.

My response - Ran it for 3-5 minutes, no hum fuse didn't blow.


(3a) if the fuse blew, remove the connection to the power side of the output transformer - DON'T forget the discharge the caps before touching. Try again, if it blows it's bad cap or rectifier. Replace one of them and try again. If it still blows it's the one you didn't replace.
(3b) If the fuse did not blow after (2) then we know that the 6L6 is drawing too much current. We discussed this already: bad cathode bypass cap, bad connection to grid, bad 330pF cap.

I don't want to diverge but one other thing is niggling me. With the change to a 6L6 and the new cathode resistor (290 ohms doesn't sound right, BTW, maybe it's 390?) the zero signal current is a bit higher (48mA) than it was before (40mA) - this is a problem as the output transformer can only handle so much before is saturates. Now maybe with the new transformer that is OK but it still puts an extra load on the power supply. Personally, I'd make the resistor higher until I got back to 40mA - just divide the voltage across it by the resistance to get the current. Edit: I just pulled up the datasheet for a 6L6 and I would expect more like 680-750 ohms with 30V across to get 40mA so something is not right here.

My response - I'm pretty sure you're right. I asked, he recommended a lower value, I soldered one in...(local amp guy, usually very good, maybe he was thinking backwards...he's built some excellent Princeton clones, I've played several and they are great) But after checking it out, what you posted makes more sense. According to the math, I'll be looking for something in the 650-700 range.


The primary of the transformer always has some unwanted (leakage) inductance. The problem with that is when there is a sudden reduction of current the energy stored inside it is released and gives a high voltage that could be high enough to breakdown the tube socket. The rate of reduction of current is controlled so some extent by the 330pF cap between the plate and grid of the 6L6. on the plate. I'd replace if for no other reason than you have had arcing that that will have stressed that cap. Another reason a for sudden current change could be bad connections in the plate wiring.

My response - planning to replace the 330pf with a new one. Not having it didn't seem to affect sound, and it still had problems, but didn't arc. It'll be put back in place.

I wonder if there could be a parasitic oscillation, again the 330pF has a bearing on that. You might try adding a 2.2k resistor in series to the connection to the grid (pin 5) of the 6L6 too. located right on the pin.

********

OK I went ahead and left the entire post, including the last comments. According to what I see here, I pulled each tube and turned it on, no hum, no blown fuse, so that means - bad cathode bypass cap, bad connection to grid, bad 330pF cap...according to the above info. I'm not sure which. I still haven't tried plugging it in and playing it with the 510 ohm resistor, I don't want it to arc again and make me replace yet another set of tube sockets...(this is the 3rd set) I posted here to get a good idea what is going on before I go much further, and see if I can get what should be known good parts in there and in values that should work properly.

I'm not sure which, and haven't had the funds to invest in parts yet, but might before long. I know the resistor needs to be replaced, I have a temporary in there now, it reads 510 ohms, and I'm guessing at 2 watts. I know that's not sufficient, but should be ok for a few tests until I can get what I actually need. After what I found here, I can only guess, but I'm thinking it's most likely the cap that's bad. The original resistor may have also been bad but it tested ok. That doesn't mean it was working correctly once it got heated up and I didn't want to poke around with a meter while it was running. I also have to disconnect it from the circuit to get a decent test...I'm uncomfortable enough just testing voltages...

so I'm planning to replace both bypass cap and resistor, and go ahead and put the 330pf cap back in, and a new one for that too. If necessary after that I might add the 2.2 resistor to see if that makes a difference, if it still has problems.

The output transformer still nags me, it's been seriously overworked for over 20 years, but so far I haven't proven it's bad so it can stay...for now...I'll have to check sources and prices, I'm not sure when I'll be able to afford any parts, no income at the moment but I did sell a picture recently, might be able to get a couple of things. But I wanted to drop by and post a follow up, and post the results of the testing that was advised long ago. Again sorry it took so long, I haven't been able to get online at all since my last post. I should be able to now, but it may take a day or two.

Thanks again for all the help, I think I might be closing in on the problem...