Fender Champ Capacitors and other issues

[Paleo Pete]

Well, 620 Ohm resistors finally came in, did some solder sniffing last night and it's working, and sounds great. Still a bit heavy handed on the treble, I have to turn it down to about 5 or it's too much, otherwise it sounds really good. Nice and fat, plenty raunch, so far no problems at all. I played it about an hour last night, checked voltages, everything went up slightly but not much, I don't see anything that makes me worried. Heaters still 6.7V.

620 resistor got warm but not too hot, power transformer warm but not hot by any means, so it looks like heat is not an issue. Everything else looks pretty good, and since I haven't done it in a while I grabbed a pencil, cranked it to 10 and poked around all over the eyelet board, nothing but silence. It runs almost dead quiet dimed, a very slight hum I can only hear if I put my ear a foot or less from the speaker, and that's with a single coil strat plugged in, on a stand far enough away it won't try to feedback, and playing it I can even get the strat to try and get a little feedback now and then. That never happened...I could only make it feedback before by plugging in my Ibanez SD9 distortion pedal...the Cort CL 1500 hollow body will feedback but that's to be expected, I plug it into the Super Reverb and at stage volume it will feedback if I look at it wrong...

so it looks like this thing is working fine now. I'm not sure if I like the 510 ohm or the 620 better yet, I'll have to play it a while and see. Might swap them a time or two and listen, maybe record both for comparison. Already recorded it with the 510, but my timing sucked, can't do a lead over it and have both on one track, I'll have to redo it.. The main thing is I've played it quite a bit lately and no arcing across the tube sockets, that was the main problem I've been dealing with. I'll be setting it up and recording it in the next few days, gotta get a little practice first though...

Thanks a lot guys, I appreciate the help greatly. I'm going to call it fixed.

[Leo_Gnardo]

I grabbed a pencil, cranked it to 10 and poked around all over the eyelet board, nothing but silence.

The "lead" in a pencil is carbon & clay, much like you find inside a carbon comp resistor. And it IS conductive. Please for your safety use a chopstick*, dowel or plastic probe. One day I made the mistake of pencil-probing and wound up getting thrown across the room by shock. Shocks, who needs 'em. Not you & not me.

Everything else sounds terrific, glad you got your Champ revived!

* Which is how 'chopstick' got turned into a verb, at least amongst us electronics builders/experimenters/repairmen.

[Paleo Pete]

I always suspected the lead in a pencil may be conductive, I normally use an all plastic TV adjusting screwdriver. The pencil I used was unsharpened and had no eraser, (so no metal cap) and I made VERY sure I didn't touch the end...didn't know where my TV tool was...and of course, found it 30 minutes after I shut down...I do have some chopsticks, but they're packed away in a box and I have no idea where...Also have some wooden dowels, probably need to cut one to shorter than 3 feet and put it in the bag...I keep the soldering rig in a canvas bag, the TV tool was down in the bottom against the seam, almost invisible, found it putting everything else away...

I'm glad to have it revived too, best practice amp ever made and it sounds great dimed. I still think the treble is too heavy, but I can turn it down easy enough. So far around 4 works well, while before I usually set it about 7. Quite a difference...I'll have to play it for a while to get used to it again, and see how I really like the sound before I think about changing anything. But I do still plan to swap between the two bypass resistors and see what I like best. I know both work well, and I think I'm hearing a little more "spongy" response than I might like, compared to the 510 ohm, have to wait and see...

[Paleo Pete]

Thought I would pop in and let you guys know this amp is still working great. I took it to practice yesterday (and a jam session last week) and let it sit there running with volume knob at about 7 1/2 for at least 3 hours each time, no problems at all. Still haven't gotten a decent recording but I'll be trying again before long. Too much going on the past week or so to take time to try it. Guys I played guitar in a band with over 10 years ago just decided to hook up and try it again, 1st practice was yesterday, mostly acoustic so far and some electric here and there and the Champ is perfect. Low volume, great sound, and I don't have to carry a 80lb amp around.

SO I've been using it for practice, so far no problems at all. I'm going to start swapping out some tubes the next few days and see if I can rein in the treble a little, it's just too much. Treble knob on 3 and it's almost too much. So I'm going to start with tubes, I'm thinking preamp tube first. Other than that, it's working perfect, no noise or hum, leave it running 3-4 hours and no trouble at all.

I'm liking it...oh yeah these are the guys I'm working with

Southern Cross.mpg - YouTube

I can't wait to see what we can do with a month of practice...10 years ago we had been playing together about 4 years, it sounded so good it was almost scary. As is, about 20 minutes and we were already back to the point we knew what everybody would do before they did it...

Thanks a bunch for all the help guys, and for putting up with me while I tried to figure this out having no idea what I was doing...great folks around here.

[Steve A.]

I'm going to start swapping out some tubes the next few days and see if I can rein in the treble a little, it's just too much. Treble knob on 3 and it's almost too much.

Thanks for checking back in and telling us about the results! A lot of people don't do that so we don't know if our suggestions helped or not. It's not like we're waiting for someone to come back to say "Thank You"- we really want to know how it turned out for you.

As for the treble pot did you use a 250k audio taper pot? If you used a linear pot that would account for your observations. It is also possible that Fender used a special audio taper for that pot (as they have done with other models.) It's ideal if we can get our controls to be usable throughout their entire range but if you can dial in the sound you want I think that's good enough. (I might do wiring tricks to spread the 1 to 3 range across the full 1 to 10 dial but that is really optional.)

Another approach would be to replace the treble cap with 270pF, 300pF or 330pF caps, trying both ceramic and mica (I like to use mica treble caps in BF amps- quite often a 330pF.)

Good luck with your band!

Steve Ahola

P.S. Here is a trick to create a "super audio taper" pot using a tapering resistor. Start off with a 1M audio taper pot and add a 330k resistor between the wiper and the outer terminal NOT connected to the treble cap (the one that would be grounded in a volume control.)
That tapering trick would normally be used to convert a 1M linear pot to a 250k audio pot. I can't guarantee how it will work for you since it does change the impedance seen by the output of the first gain stage. FWIW I have never had a problem doing that and in one case it seemed to improve the sound considerably.

[Paleo Pete]

Steve - Thanks for the observations. So far I think it's doing well, I swapped out the preamp tube for another one and it took care of the treble issue. Not sure why, but now I'm playing it with the treble knob at 7 and it's acting like it always has. That's about where I always kept the treble, 6 to 7. The preamp tube I was using was a really old Westinghouse 12AX7A, probably pulled from a CB radio years ago, must be a hot one because it seems to have had just a bit more gain too. I went to a Fender branded GT 12AX7A, it's excellent now. That Westinghouse tube for some reason was punching out loads of treble. It's back to normal now with the GT in there. I tried 3 or 4, all but the Westinghouse had normal treble.

I don't know what treble pot is in it, whatever the factory put there, it's still the original. It still works perfect, a little contact cleaner every few years and that's all I've ever had to do. I've never even looked at the value I don't think. I was fairly sure that was not the problem since it always worked perfect for the past 20 years, and seemed to work right, no noise. One thing I would like to know, WHICH cap is the treble cap? I'm not sure how to figure that out...I wanted to swap the bass one at one time, but now the bass is loads better too, so I think I'll leave it alone.

I've seen the same thing as you while working with a computer tech support site, we do get someone now and then who will come back and let us know it's fixed, but most just move on and we have no idea. That's the main reason I popped in to let you guys know, been there done that...I'm also trying to get a decent recording, I'd like to let you hear the thing but so far nothing I want anyone else to hear. I'm badly out of practice, rusty, my playing is still rough and choppy but that will change soon, I just started working with some friends here and will be getting lots of practice very soon. Just need to get a decent recording...This little amp is the greatest practice amp ever made to begin with, drop in a 6L6 and it gets even better...and now it sounds like a new one. Killer tone, just enough raunch to handle the kind of raunch and roll I like to play, and not too loud for practicing in a decent sized living room. I also made up a PVC tilt back stand, that's helping a lot.