Fender Champ Capacitors and other issues

[Paleo Pete]

Put the 510 ohm back in and tried it, lots better. Treble acts right now, works perfect all the way to 10 but really bright, sounds best about 6-7. Sounds very good overall. I'm not hearing too much gain, or anything odd. Checked voltage again, everything went up. I think the 510 ohm will be ok for a temporary measure until I can work something out with the seller, the resistors were definitely listed as 680 ohm, not 68. I screwed up, I didn't look close enough, and didn't check before sniffing solder...

So far it's looking like this amp is no longer having any serious problems, I played it 45 minutes to an hour again last night, no blown fuses, no arcing across the tube sockets, no heat problems. Sitting there at full volume with guitar plugged in and turned completely down it has a very slight hum, I have to put my ear a foot from the amp to hear it at all. Normal...it's always done that, very slight. The only hum otherwise is if I leave the guitar full volume and stand a foot away. Also normal.

As far as I can tell, the arcing seems to have stopped, I think that was most likely the rectifier tube, or very possibly the bypass resistor/cap. It hasn't happened since I replaced the rectifier tube. The fuse blowing had to be because of the wobbly power tube, which I missed. I think I caused that when I took the chassis out with tubes still in and set it down, bumped it on something and broke the tube glass loose from the base. Careless mistake...but it stopped blowing fuses when I found the power tube glass was loose and put another one in.

I'm going to still get another resistor. The voltage is high everywhere. I checked again last night, here's what I got. I used the existing picture so you're still seeing the 68 ohm resistor, the voltage in blue is what I got with the 510 ohm resistor in place. Amp idling with volume at 0, input voltage 123.0. Voltage seems a bit high around here...I usually get around 117 in most places.

Voltage readings -
Black - What's listed on the Champ layout
Red - What I got with the 68 ohm resistor
Blue - what I got with the 510 ohm resistor



At the 2 red wires on the rectifier tube, I got slightly different readings, I put those on the pins where they were checked. The only way I'll know if this thing is really fixed is to run it for a while, and of course I still have to get a good resistor. The 510 seems to be working fine so far, and from what I heard last night I don't think it should give me any trouble. I know I'm taking a chance, but I'll be playing it a bit more to see how it acts, so far it's sounding very good, the treble acts like it should, I'm not too worried about it, so I plan to plug in and play it a few times and see what happens. If it arcs again I'll kick myself and replace the tube sockets again but from what I see so far I don't expect it. I think it would have already done it if it were going to. The last couple of times it took about 45 minutes or so playing at full volume to cause it to arc, I've already played it that long twice with no problems, and let it sit there and idle for over an hour. We'll see...

Thanks guys, I'm not sure this is over with, but it's looking pretty good so far...I just checked and no reply from the seller yet, so I don't know what's going to happen with the resistors.

[Paleo Pete]

Seller just sent me a full refund, so I'm looking again for a resistor...

[Mick Bailey]

If you're getting arcing between two points it's useful to approximate the breakdown voltage of air at standard temperature and pressure as approximately 33Kv/cm. The voltages produced in an amp under normal conditions are simply not capable of jumping a free-air gap between the terminal spacing of an octal base, unless there is surface contamination or a conductive defect between the pins, or there is a fault creating flyback voltages across an inductor.

I had a problem a while back on a Twin Reverb where the OPT was faulty and producing insane voltages which shorted across one particular socket. It measured into KV and took a long time to find.

It's always worthwhile installing ceramic sockets of you have persistent problems, then at least you can eliminate the sockets even if you do get flyback-level voltages occurring again whilst troubleshooting. Once a socket has arced it's generally permanently compromised.

My comments are now probably irrelevant - didn't see the last few posts

[nickb]

If I were you I'd leave it with the 510 ohm - it looks close enough. The voltages will go higher still if you increase it further.

[Steve A.]

The 1964 Champ schematic shows a 470 ohm/1 watt cathode resistor. If you add a 6k2 resistor in parallel with your 510R resistor it should bring the net value down to around 471R. Since the 510R resistor will be handling most of the current a 6k2 1/2W resistor should work just fine.

Steve Ahola

[Paleo Pete]

Mick Bailey - Thanks for joining in, and no, I don't think totally irrelevant. The first thing I suspected was output transformer, it's the original from 1974, and has been way overworked for over 20 years. But it seems maybe not...and ceramic sockets are what I installed this time. With it working fine right now, I'm thinking the problem was more likely in the bypass resistor/cap area. Still not sure though...but it seems to be working right so far after replacing those.

nickb - The only problem I have with the 510 ohm is it's not 5 watt, I think it's 2 watt but not positive. Blue and larger than the 1watt ones I replaced a while back. Definitely not a 1/4 or 1/2 watt...Otherwise I'd leave it if I was sure it was 2 watt or higher. Treble gets a hair brittle past 6 or 7, otherwise it sounds great. I don't normally run full treble anyway, and below that it's excellent. Plenty fat, nice and raunchy, just what I'm after...I have a 620 ohm 5 watt ordered, You're right, that will raise voltage some more but hopefully not too much. The main place it worries me is the heater voltage. I'm not crazy about that getting too high. If you had posted before I placed the order, I would have looked around a bit more. Not your fault, nobody had posted when I placed the order and I know you're not sitting there just waiting for someone to post, this is not a chatroom...if I knew for sure what wattage the 510 is, I'd be ok with it. I'm afraid it's not enough for long term use though.

Steve - I don't understand 6k2. 6k, I got that, 6000 ohm. The 2 part I haven't seen...2 watt? If you would explain in a little more detail I'd appreciate it. I had the original 470 ohm 1 watt resistor in place when it first started arcing, it had worked great for a very long time, I swapped it on the recommendation of a local tech who told me to go the wrong way...it should have gone higher, not lower impedance from what these guys are telling me. Thing is, I'm using a 6L6 power tube, not the original 6V6, so that changes things a bit. I thought about going back to 470, but the recommendation here is for higher, 600-700 range, so that's what I've been looking for.

Thing is I'm far from a tech, I barely know what I'm doing, but didn't want to "shotgun" this, and am trying to get it to work somewhat right with the 6L6. So you'll know without having to go back and read everything, the only change made is that resistor. Everything else is stock values, the bypass cap is now 50V instead of the 25V it had a few days ago, still 25uF. Everything else has been replaced with stock values, the 250pf cap is the only thing in the box that's never been replaced. Only because I didn't have one and never thought to grab one when getting parts when I originally rebuilt it. I also thought the ceramic discs should be ok, the general consensus is they last a good while. That's why the 330pf cap was never replaced until recently. Didn't think it was necessary. I have a 240pf lying around, will that work in place of the 250? Doesn't sound like enough change to be a problem, within 10% if my math is right. But if I'm not mistaken that's in the tone section...also will silver mica be OK? I find loads of those, not many ceramics...

Thanks again for your time and effort guys, I hope to get this thing back in business, and soon as I can get schedules sorted out with some friends, I'm dying to record this thing and let you hear the results of your efforts. We've been wanting to get together and record, but with work schedules, single parents and so forth it's not easy...Right now all I can do is wait, the 620 ohm resistor will be 10 days or so, I think their shipping is slow boat from China...

I'll have to look around, I have some very old carbon comp resistors still new, various impedance, and at least 1 watt. Just over 1/4 inch diameter and 3/4 inch long, heavy leads...I got an organizer box full of stuff at a yard sale including a bunch of the old "domino" capacitors and some tube sockets, some other stuff... Several of the domino caps are still on the original cardboard, new. Probably from the 40's, that's what was in my mid 40's Electromuse lap steel...I talked to the guy who was selling it, some of it is pre 50's...some of the resistors are used, most of them look new, never been soldered. I've put a bunch on the meter, they still read what they should and don't drift around. But none are in the range of what I need here unfortunately...most are much higher, at least 2k, a few under 100 ohm. Might be a 6k in there...I'm not making any changes yet though, I want to try it as is a bit more and see how it goes.

[Steve A.]

6k2 is the European way of writing 6.2k or 6200 ohms and has been adopted by many people. (On printed copies the "dot" in 6.2k is not always clear.) My point was that you can add resistors together in series or in parallel to get the specific value you want.

So you are running your Champ with a 6L6? Doesn't that create an impedance mismatch with the OT?

Steve Ahola

[Tom Phillips]

...and has been adopted by many people...

Which makes me sad and, I think, disturbs the flow when used in a written discussion.

[Steve A.]

Which makes me sad and, I think, disturbs the flow when used in a written discussion.

Only if you move your lips when you read...

[Tom Phillips]

Only if you move your lips when you read...

They probably do sometimes. I also have equipment that use capacitors marked in µµF and the frequency values are labeled CPS. Those are true core values.

[Leo_Gnardo]

So you are running your Champ with a 6L6? Doesn't that create an impedance mismatch with the OT?

Steve Ahola

FWIW I've run Champs with 6L6 and the only thing I notice is the output tube never wears out. Where a 6V6 is near or at red plate, the 6L6 is "just loafing." Sounds fine - try it!

[Paleo Pete]

So you are running your Champ with a 6L6? Doesn't that create an impedance mismatch with the OT?

To be honest, I have no idea. But as Leo said, it sounds great...No more wattage, just a bit gutsier sound. I'm not sure just what the difference is, but when I unplugged the 6V6 and swapped in the 6L6 I liked the sound better. Gutsier is the only way I can describe it.


Thanks for the explanation, I'll poke through my stash and see what I have, I was hoping I'd find 2 at around 1200 so I could run them parallel, but it didn't happen...for now I'll run with the 510 ohm, it seems to be working well. And that's in the range that was recommended earlier in this thread. I have a 620 ohm 5 watt ordered...I'll see what happens when it gets here. Judging by the way it's sounding now, that will probably be an improvement. The main thing is, it seems to be working right finally. I've run it hard several times, no more arcing or fuses blown, no hum, sounds better than it did before too. It just might be fixed, definitely an improvement. If I thought the 510 ohm in it now would hold up I'd run with it...I'm just not sure about that particular one. It does test consistent, same every time and doesn't fluctuate...maybe I'm just being over cautious...

[nosaj]

Just a wild pitch have you cleaned your tube sockets and with what?

Thanks,
Jason

[Paleo Pete]

Jason - Cleaning tube sockets would have been pointless, they still had burnt traces on them from the arcing, which would have been an open invitation to more arcing. Yes they were cleaned before the arcing started, contact cleaner and pipe cleaners about every couple of years to keep them in decent shape. After arcing new ceramic ones were installed, they have a total of about 6 hours playing time on them so they shouldn't be dirty. Actually the tube sockets have been replaced every time it's happened, this is my 3rd set. At this point, I think the arcing has stopped, I'm pretty sure I've played it long enough a couple of times it would have happened if it were going to. Not positive yet, only time will tell, but pretty sure...Thanks for the reply.

Actually I'm glad you brought this up, it's time to open up the Super Reverb and clean those...it's been about 2 years I think since I last cleaned them...

[nosaj]

That wasn't what I was getting at but more or less what you used if residue might've caused arcing.

Nosaj