Fender Champ Capacitors and other issues

[Paleo Pete]

Howdy guys
I have a 1974 Champ I've played (usually pretty hard) since about 1990 or so. Got it used, no tubes or speaker, got those and found out later I could use a 6L6 power tube, been using it that way since around 1992 I guess. No problems. Several years ago I swapped out most of the capacitors, beginning with the electrolytics, then orange drops all around. At this point resistors have been replaced also, it had started hissing a bit and that cured the hiss, so the only thing not replaced is two disk capacitors.

One is mounted on top of the power tube, marked 330K (30pf or 300?), pins 5-8 [I'm not great at deciphering the cap markings and apparently some have changed] the other is connected to the wire that comes from pin 1 of the preamp tube. The 330K is not shown on my layout diagram, the 250pf is, and is marked as such on the layout.

Couple of questions...this thing a couple of years after replacing all the caps and most of the resistors, started arcing across both power and rectifier tubes. Had to replace the power transformer, not entirely positive it was bad, but decided a new one was a good idea especially since I'm using a 6L6 in it. Still arced...replaced the rest of the resistors and a different 6L6, (and replaced tube sockets each time) still arced...

At this point the rectifier tube and those two caps are the only items not replaced, and the resistor going from ground (chassis) to the tone pot. I have a replacement but haven't dug out my old Weller...but I don't think that's it.

Just got a replacement 5Y3, left the 330K on the power tube disconnected, it works and sounds great, I only played it 10 minutes or so. When it arcs it's always after playing it dimed for over an hour so things get heated up really well, I left it alone once I knew it worked and sounded good.

OK I think that's most of the background, here's what I'm wondering.

What will happen if I leave the cap in the power tube (cathode bypass?) disconnected. Will that cause any serious problems?

What values do I need in current parts? I'm planning to try and stay within 10% if possible, that was not a big issue with the rest, a couple of minor changes in resistors, still within 10% of original values, and I used 2% tolerance resistors when I could get them.

One mod other than the 6L6 tube. I don't know the proper terms here, but the wire on pin 8 of the power tube goes straight to the board, to a 470 ohm 1 watt resistor with a 25-25 cap across it. That was changed to a 290 ohm on the advice of the amp tech who soldered in the transformer for me. The reason as I remember it was to try and bring the voltage of the 6L6 in line. Still arced after that...

Could the cap across the power tube be bad and causing it to arc? Or the rectifier tube? The rectifier was never changed until just recently because I didn't have one.

OK I hope I gave enough info, scattered though it may be, I can get voltages if needed, I've checked it before the last time it committed suicide, everything was well within tolerance everywhere. At this point only the tubes have been replaced, and the cap disconnected temporarily. I've been trying to keep the mods to a minimum, I'm not looking for more power, just the fatter sound I get with the 6L6 is excellent. Speaker is a Gorilla 4 ohm 8 inch, and has always sounded great, is still in good condition. I've gone over the wiring, everything is right according to the layout, all components match also except the one swapped resistor.

[Paleo Pete]

Found out reading another thread the 330 is 330pf, after I posted. I read everything I could find that mentioned Champ for the past 3 days looking for info before posting...5 minutes after I posted I find one of the answers I'm looking for...go figure...I wasn't sure from the info I looked up exactly how to read this style cap code. If I got it right, 33 pf would just be marked 33 not 330...

I'm still digging for my box of parts, just moved and it's packed away, haven't found it yet. So far I've only found a couple and nowhere near the values I need. 272 and 333 would NOT be in the ball park, I know that much...I think that would be 2700 and 33000pf, am I right? Both pencil eraser sized disk caps.

And that brings up another question, I have some other types of caps. If I understand what I've been reading, I can probably use another type as replacements, without any issues. Just not electrolytic, I doubt if I could find those values anyway.

[pontiacpete]

It looks like it's a 330pf cap, according to the schematic.

Maybe the tube socket is having issues, and causing it to arc, or perhaps you've already checked that.

[Paleo Pete]

pontiacpete - Yep, according to another post it's a 330pf. Not listed on the layout at all though. That's why I'm wondering if it has to be there at all. Everything else is listed, all of it exactly what was originally in the amp.

I've replaced the tube sockets each time, it still arcs. Just replaced them a couple of days ago with new ceramic ones.

Thanks a lot for taking time to reply.

I'm wondering if that 330pf cap, being on top of the power tube, was getting hot and causing trouble. The arcing always happened after over an hour of playing at full volume so I'm really suspecting heat plays a part in this. If it really needs to be there, I'll probably try to place it off to the side, even if I have to add some short wires to get it out of the heat.

[pontiacpete]

It will work without that cap.
But I would concerned about the socket. It may be shorting between the pins.

[Paleo Pete]

But I would concerned about the socket. It may be shorting between the pins.

I've wondered about that but would the original socket and 2 replacements (new) ALL short between pins? I also wondered if they might be too close together, but they are identical to my Super Reverb sockets, with no problems at all, still all original sockets. Spacing shouldn't be it. I found something about that in another post, seems it would require way more voltage/current than the Champ produces to cause an arc form one pin to another by just being too close. If I remember correctly something like 1/64 inch.

If this were still the original socket, I would say you have a very good point. (Not to mention that after arcing once, from what I've read, it will always tend to arc across the leftover burnt area) The sockets have been changed each time though, and I really doubt if 3 would all short between pins. The original, over 30 years old, maybe...that wouldn't surprise me. I also looked them all over under magnification, no cracks I could find in any of them including the original.

Thanks again, good ideas here. As I said I'm definitely no tech, but I'm still wondering about 2 things. The cap getting hot, or bad rectifier tube. I've been using the same 5Y3 all along, didn't have a replacement. Just got one. That 5Y3 was used but good when I got it, old RCA. It worked for 20 years before starting to do this. My tester is down, also have some questions about it, but will not go there in this thread. I had the 5Y3 tested years ago by a shop in Louisiana, it was good a couple of years before this started happening.

Sorry to digress, but I'm trying to offer all the info I can, hoping I can answer some of the basic questions before they are asked. All tubes were tested good 2 or 3 years before this thing started to arc. That doesn't mean they were all still good when it started though...I've also tried 3 or 4 different power and preamp tubes, different set of tubes each time it arced, except for the 5Y3. It's the only one that stayed the same.

Like a dummy I pulled the sockets without marking them, but it looks like the power tube is arcing across pins 2-3 as best I can tell. I'm really kicking my self for not marking them, I intended to...and forgot...DUMMY! Reminds me of testing a video card in a customer's computer then finding out I didn't note WHICH video card they originally had...grrrrr...not hard to find out though, let it boot into Windows and see if it wants to install drivers...wouldn't have been a big deal if I hadn't had 4 others on the bench and didn't need to waste the time. Turned out they had a bad monitor, not the video card. I also didn't check that...[note to self] go ahead, get in a hurry again...dummy...

[pontiacpete]

Can you post a picture?

[Bob M.]

Hi,

If you're 'diming the amp' and having arcing problems, I'd recommend you read the article, 'The Remaking of a Champ'. I'm not recommending that you rebuild your Champ as Mr. Elliot has done but only to read his sage advice on screen protection and other related things. You really want to protect those screens in full on volume/power situations.

Also, I wouldn't use a cathode resistor (power tube,pin 8) that is smaller than the stock 470 ohm but larger instead. I usually use 510 or 560 ohms cathode resistor, and at least 5 watts. 7 to 10 watts is better in light of the fact you're using a 6L6 type tube. Another thing, one of Leo's Champ mistakes was to leave the cathode cap right next to the usually very hot cathode resistor on the board. Relocate that cathode cap away from that big resistor. Electrolytic capacitors hate heat; it must go from pin 8 (of the power tube) to ground, as before, but be relocated away from that very hot resistor.

Good luck,

Bob M.

Arcing is sometimes associated with high B+ before warming up of the heater circuit. As this amp doesn't employ at standby switch (which is a very easy mod) how about using a thermistor, which works very well in these situations?

[Paleo Pete]

pontiacpete - One picture coming up. Took me 5 tries, my connection out here sucks out loud...

OK maybe not...workoing on attempt #6 now, if it works I'll come back and add it later...I'm sick of starting over.

Bob - Thanks a bunch, some good stuff to look into. I'll have to look up that article. I was completely unaware that a higher wattage resistor would be a good idea in that spot, I'm still using the 1 watt specified in the layout. I'll have to go shopping for that one, and I don't even know where to get anything around here, moved a month ago and haven't even tried to find things yet.

Your last comment - Arcing is sometimes associated with high B+...

When it arcs it has always been after running for well over an hour. Would this be linked to the same thing? If it's an issue and a standby switch isn't difficult, I might try adding one. I always wondered why they left the standby switch out...only thing I've ever seen without one is a radio...I don't want to poke holes in this thing, but a standby switch might be worth it. I seriously doubt if I'll ever decide to part with it, but I'd like to keep it mostly original if possible and not make any cosmetic changes if I can avoid it. But that may be worthwhile.

Thanks a lot for the replies so far, I think I may be getting somewhere...it may take a few days to round up parts, I don't know where the local supplier is yet. Also won't get a chance to go into town for a few days, that's a 30 minute drive...and I don't go often. I also have a couple other things to do, heat shrink on the wires to the filter caps, think on moving the resistor or cap, they are already separated by 1/2 inch or so, but that may not be enough judging by your comments. Might have a chance to get a better picture too.

[Paleo Pete]

OK it looks like it worked the first time and I couldn't tell it...sorry about the ugly tape on the filter cap wires, that will be fixed before I crank it up again, soon as I find my heat shrink tubing. And blow dryer...Everything is packed away in a shed and I can't find anything...living in a smallish RV for now...I had the 330pf cap well away from the contacts when testing it the other night, I moved it back close to where it goes, pin 5, for the picture. I might remove it entirely.

[Enzo]

When you said arcing, did you mean INSIDE the tubes? That is a failed tube, not a socket issue.


They didn't put standby switches on Champs and most other small amps because they just don't need them. If it were a problem ALL Champs would be arcing and sparking. The voltages in this amp are not going to arc inside the tube unless the tube is defective already.

[Paleo Pete]

Enzo - The arcing is NOT inside the tubes, but across the terminals of the tube sockets, on the inside of the amp chassis, not between tube and socket. Yes, I know the difference, but not the reasons. I do suspect a failed rectifier tube, but only because that is one of the few things not already replaced with new parts, and it seems to make sense that if it were to fail while in use, it could cause this. I've also seen bad tubes suggested as possible cause of other people with arcing across tube sockets on this and other forums.

The only things not already replaced are the 5Y3, 250pf cap (connected to preamp tube), 330pf cap (on top of the power tube) and resistor going from ground to bass pot. Everything else has been replaced. All caps replaced before this started, and it worked perfect for 3 or 4 years after replacing them, most of the resistors were replaced at the same time. After the arcing started the rest of the resistors were replaced, (only 3 or 4) no changes have been made except the 290 ohm resistor Bob informed me is the cathode resistor. That's the only modification until I replaced the tube sockets again a few days ago and left the 330pf cap disconnected. I plan to follow his advice and add a higher cathode resistor, soon as I get the chance. I probably won't crank it up again either, but I might once to check voltages.

To repeat though, so it's clear, this amp worked great for at least 3 years after replacing every cap in it and most of the resistors. The cap job was done because they were all original and near 40 years old. And it still worked, I didn't start replacing stuff because of problems. I also powered it up and checked it after replacing every item. Takes longer, but if I make a mistake I want to know it now rather than driving myself crazy trying to figure out which of 8 parts I just replaced was soldered in backwards or whatever...

Thanks for the explanation on the standby switch. I had seen that explained before but couldn't remember, it's been awhile.

[Enzo]

I don't know what you have there, but not all problems are caused by bad parts. I could for example conceive of a failed connection allowing the inductance of a choke or an OT to create an arc whenever the connection opened.

[Paleo Pete]

I agree, bad parts is not the only possibility, which is one of the reasons I thought it might be a good idea to ask some questions here. One of the things I did long ago was what I've seen called the pencil test. That's how I found out a cap was loose and that was the reason the bass control did nothing. I later went through the amp and resoldered every connection in it except the ones on the chassis. I'm thinking about going through them again and dragging out my old Weller to tackle the ones on the chassis too.

I still wouldn't be surprised by a bad connection, even though I'm sure my soldering is good.

I won't be able to get any parts for at least a few days, I'm re roofing a house and burning a lot of brush right now, and still have to find out where the local supplier is. The closest resistor I have to what I need is a 820 ohm 2 watt. I'm not sure that would work, even as a temporary measure...

[leydenjar]

I had that exact arcing problem with my silverface 73 champ.

I installed the beefiest PT merc mag has for champs.

Now it handles el84, 6v6, 6550, el34 (switch installed), 6l6 e.t.c. with no arcing.

I also put a merc mag OT in as well.