Fender Champ Mod Question

[Paleo Pete]

I was reading through some of the old threads, and ran across This One that got me curious. I didn't want to revive the older thread, just a quick clarification, to make sure I understand this.

Post #1 says "and have heard that lifting the resistor to ground on the bass pot will effectively take the tone stack out of the circuit, and result in such an increase. Is this the case?"

And was said to be correct in post #2, which I don't doubt.

I have a 74 Champ, don't want to seriously mod the thing, but a bit more gain might prove interesting, so if I understand this correctly, it's just saying desolder the resistor on the bass pot from ground, and that's it? I know it won't hurt the amp to leave it in place but disconnected, that's not the question, but will it work to begin with, if disconnected in this manner. I don't want to do anything until I know what to expect...

I won't be even looking at it for a few days, right now I'm fighting a case of this year's strain of flu, I don't even want to look at it, much less open it up. Considering the voltages in there, it's just a bad idea...

And another clarification, if you guys would be so kind. I've tried to check this, and I find no voltage so...before ever opening up my Super Reverb I wasn't sure how to go about draining the filter caps, which is a must, but ran across some info that says if you unplug the amp while playing, pull the plug, NOT flip the power switch, and keep it playing until it makes no more sound at all, it should drain the caps. That's how I drain the power in computers (very similar anyway) when I need to drain them to work on them, I know it works for that purpose, but that's not 400VDC or more...computers run on 5 and 12VDC at best. When I do this with my amps I can't find a volt anywhere, so apparently it does work, but I'm still wondering if I'm just missing it somewhere. I've been really careful, never been shocked, and have put my multimeter on both Champ and Super Reverb every time, can't find any voltage at filter caps or anywhere else. Is it really drained though??? I've checked both terminals to ground and both terminals across the caps, one cap to the next, every way I can think of, never a thing. No voltage at any point in the amp.

Or can anybody explain in plain English how to do it with a resistor and jumper wire. I've seen that many times, but none of them seem to speak redneck... One of my biggest problems is I don't understand a lot of the terminology. It took me a week to figure out when someone said "lift it from ground" it meant just disconnect the damn thing from the chassis...I finally found a site with a good explanation of what the basic amp sections are, I had no idea what the "phase inverter section" was until I found a good site with 8x10 color glossy photographs with circles and arrows and a paragraph on the back of each...no, wait...

OK I'm all better now...

A site with a board layout and each section marked. But I still have to refer back to that, it's not all memorized yet, so it's still baby steps here...but will that actually do a decent job of draining power? Or have I Just been lucky...

[MWJB]

"Post #1 says "and have heard that lifting the resistor to ground on the bass pot will effectively take the tone stack out of the circuit, and result in such an increase. Is this the case?" - Yes.

Well your bass pot won't work any more, tone will get thicker, headroom reduced, you'll only know if you like it or not when you try it. Having this tweak, or the tone stack bypass that Bruce suggests, switchable would be a good idea, then you can A/B test the mod with the original circuit.

How about a tube screamer if you just want more gain & want to keep a functioning tone stack & not mod the amp?

Super Reverb cap draining - unplug from the wall & keep strumming technique will work, but your Super Rever is already wired to allow you to drain the caps via the first stage filter cap's 220K smooothing resistors. Power down normally, unplug from the wall and now flip the standby back in to play mode - this will drain your caps.

Champ/any amp cap draining - What I do is use a simple jumper wire (no resistor - we will use the amp's own power supply & plate resistors to work for us) and connect the plate of a preamp tube (pin 1 or 6) to ground (the chassis). This drains the caps in about 30 seconds, plus you can visually check that the jumper is there whilst working on the amp & reassure yourself. Remove the jumper before powering up.

[stokes]

"Remove the jumper before powering up".You will most likely only forget that one once,the resulting snap,crackle and POP! will stay with you a long time.

[Paleo Pete]

"Well your bass pot won't work any more"

I remember that from the other thread, wouldn't be an issue, it stays on 10 anyway. That should be the same as out of the circuit I would think.

"tone will get thicker"

Excellent. This amp sounds great, but I'm thinking I might like it even better if it could gain some weight and grow a little more hair...and this would be a matter of just soldering one end back on to get it back stock. Quick and painless, no major changes. Switchable would be nice but if it works out I'll probably just leave it.

I have an overdrive pedal, Marshall Bluesbreaker, attached to a board with 4 other pedals but never use anything through this amp, it's a practice amp and I normally just consider it "plug n play", just a guitar. Flip the switch, plug in a cord, I'm jammin...and this should leave it nice and clean at low volume, it starts to break up a bit around 5-6 now, which is fine. I'm already using the hottest preamp tube I can find...

Thanks for the info on cap draining, I wanted to double check it. I've seen your jumper wire method before, butI don't think anybody bothered to mention which pins the plate are...now I know, that helps. Still learning here...I'll try to remember to remove the jumper wire, I won't say I will...someday I'm sure to forget one...I just hope it's less "eventful" than last year when I put a battery in my jeep, tightened down the hot side and the ring on my pinkie touched the ground post while also touching the wrench...sparks weren't the only thing flying...do you know how hot a small piece of white gold gets when 12 volts DC hits it for about 1/100 second? The blister healed in a few weeks though so it's ok now...

Thanks guys I really appreciate it, this just sounded like something quick and painless that might get a little more raunch out of the amp at fill tilt. I know it won't be any louder, that's not the idea anyway. Or I don't think it will be...but I'm not looking for loud, it's fine as is in that department, plenty loud for around the house and still gets good tube raunch when cranked. If this will give it a bit more raunch, I'll like it. Might try for a switch for testing before I put it back in the chassis, do an A/B test, that wouldn't be difficult...

This thing just sounds great already, if this turns out to be a good improvement I'm not sure if I'll be able to live with me... I'll let you know how it turns out in a few days, after I recuperate, right now I'm not even playing anything but acoustic till I get over the flu.

Thanks again...

[Alex R]

Cap draining - I think I've seen the source of the dangerous and false idea that once the sound dies down the caps are drained. It was in an article in a large-circulation Brit guitar magazine. A customer mentioned it to me as I drained his caps via a method that works. He said is that true? I said no, and he looked at me snake-eyed and said 'so Guitar Porn Monthly is wrong and you're right, eh?'. I said, well there's an easy way for you to prove me wrong... there's the amp, there are the caps, there's the switch...

I meant to write to them and point out that their amp expert isn't one before someone's widow's lawyer does, but forgot. Anyone remember which magazine?

[Paleo Pete]

Hoffman Amps maybe? I'm not sure really, it's been a while. I've never even seen a British guitar magazine, much less read one...

I'm not going to argue with an amp tech, I'm certainly not one, but I do know I've never found any voltage using this method, and due to the very stern warnings on tube amp sites I was particularly careful about it the first few of times I opened mine up, checking with a multimeter, AC and DC, everywhere I could think of, especially around the filter caps. The Hoffman site says

"Once the sound dies down, the voltage in the filter caps will be below a level that can hurt you. There will still be a small voltage present in the caps but it is not enough to give you a nasty shock. Typically there is less than 20 volts is in the filter cap after draining it this way."

I'm learning, I asked because I want to know, not start an argument, but I also question what I see as questionable. Hoffman seems to have pretty reliable info generally, I didn't accept this cap draining method without checking for info that also agrees in general with most other tube amp info sites I browsed. (Again I'm not sure it was Hoffman I found this, but did some looking around since it was questioned.)

Maybe I've just been lucky, that's why I'm asking - I'd rather not depend on luck, but I've replaced every cap in my Super Reverb, half the resistors or more, power transformer in the MX, cap job and some resistors in the Champ, never got even a tingle much less a shock. But if it's been just luck, I'd rather know. I'll use the jumper method described above in the future now that I know exactly how, probably in addition to the unplug method, but so far unplugging has seemed to do pretty well. I check for voltage every time, and never touch anything inside while it's plugged in except with meter probes and the plastic TV adjuster tool I use for the "chopstick test". And then I still worry...

Anyway I asked because I want to be safe, not lucky...but I'm seeing conflicting info here so I question it.

[Alex R]

That's slightly different from what I saw/heard in the Brit mag I've forgotten about. He doesn't claim it applies to all amps. He's relying on the voltage being allowed to drain back through the standby switch and a de-energised power transformer secondary I guess. But the method depends on conduction back through the rectifier doesn't it? Or am I missing something?

I'm going to conduct some experiments on the next few amps I have on the bench. I'll let you know the results. In the meantime I imagine someone will know more than I do about this; it was a new one on me when I read it in the mag and it still sounds very dodgy indeed to me as a way of not getting electrified. Educate us, someone!

[stokes]

Most amps will drain the caps with the "let the sound die method",but why take chances.I was taught many years ago to drain them with the resistor in series with a jumper wire,I keep one handy on my bench and use it every time I open a chassis,and each time I power the amp up while it is on the bench.I was zapped once,it wont happen again.There really is no reason for argument or discussion on this point,use the jumper wire,either with a resistor wired between 2 aligator clips or thru the plate resistor of a preamp tube.To advise someone that "letting the sound die" is good enough is just bad advice.

[Paleo Pete]

Thanks for the comments and explanations, I'll be using the jumper wire, but wanted to ask, just because I like to know. The only way to know is to ask.

[Alex R]

Yes it has to be bad advice because of course the number 1 reason the sound dies away when you turn off the amp in any circumstances is not 'HT falling to 20v' but the fact that you turned the amp off (and the heaters cooled). I have a drain resistor on the bench and that does it ok.

[Bruce / Mission Amps]

Yes it has to be bad advice because of course the number 1 reason the sound dies away when you turn off the amp in any circumstances is not 'HT falling to 20v' but the fact that you turned the amp off (and the heaters cooled). ...

Hmmm... partially incorrect.
The reason this can be dubious is because vacuum tubes have to be hot in order to draw current. They are thermionic devices.
If you have an amp with a solid state rectifier (or even a 5Y3GT or 5U4GB) and you turn it on for just a few seconds and then turn it off, there is a VERY GOOD chance the main filter caps will be fully charged with anything from 350vdc to 500vdc yet all the tubes will still be cold... cold means they will not drain of the power supply caps and you will get knocked on your ass if you are grounded and touch the B+ rail. It has nothing to do with the heaters being hot or cold yet.
Try to be safe and always use the test lead method with a 5K to 10K 1 watt resistor across the B+ rail.

[Alex R]

Yep. When I started this off it was to say that a customer had pointed out to me that someone writing in a UK guitar magazine - ok it was 'The Guitarist' - had recommended the 'method' quite unambiguously, saying that you could hear the voltage dying down if if you unplugged it with the amp turned on. I said, no, that's the heaters cooling, HT is likely still there, good way to get yourself zapped making that assumption, and got a slightly suspicious look in return. Like, oh yeah, you're right in your little workshop and this glossy and prestigious publication is all wrong. Like, you'll be telling me next that Cornford amps don't sound good like dey always say dey do in The Guitarist, or the Pope isn't Catholic or somethink. Then I forgot to write to the magazine and point out how exposed they were to litigation by widows of self-fried guitarists.

What I guess happened is that Hoffman's guidance - which is after all only addressed to the people who assemble his own boards - has begun to accrete to itself that strange and often valueless deposit Internet Authority, and to reproduce itself in strange mutated forms. When I saw it here I started to think, hey, am I getting this wrong? But hey, I wasn't.

[Enzo]

And don't trust the standby switch to help anyway. AMps can have the standby switch on the AC side of the rectifier, and the rectifier itself will prevent the transformer from draining the caps. Same with switch between the rectifier and the first filter. In other words discharge won't happen through the rectifier.

[dadada]

Wow!

I happened across this thread a few days ago because my early 70s vibro champ never had much tone. No matter what you did with the guitar settings it didn't seem to make much difference, The amp was just "blah," boring and almost useless without sticking a bunch of pedals in front of it. Well, I made the mods from this thread and the difference ei night and day. Deep, rich thick tone all through the mids, tight bass and clear treble (of course controlled only from the guitar) a range of colors hitherto unimaginable in such a beast. It starts breaking up at 3 1/2 - 4 and just keeps cranking until 10 where a ballsy sustain just sings. Can't wait to hear my tele through it. Wow! $2.34 in parts and a 1/2 hour's soldering. I am thrilled beyond words and want to offer my appreciation. Thanks mucho.
Now, should i put a 10 " speaker in it? WOW!

[Paleo Pete]

Had a chance to try this today and it sounds great. Losing hte tone stack is no problem, and the extra gain makes this already great practice amp just killer. Can't wait to fix my 1x12 cabinet and get it plugged back into the Champ and see what this bad boy sounds like now. I'll have to play with it some, still don't know if I can get a good enough clean sound, but the Super Reverb at low volume does that quite well, so recording at less-than-ear-bleeding volume shouldn't be a problem.

Simple to reverse, I just left the resistor in place so it's practically brainless too if I want to put it back. Even managed to get a little feedback out of it, first time ever for this amp. Lots of harmonic overtones jumping out right at you, good sustain and I think even more raunch than the Super Reverb will do. Tone didn't suffer a bit, it's still THE best sounding practice amp I've ever heard.

Thanks a bunch for the info and assistance guys, this has been an interesting excursion.

BY the way I might have a recording of this little monstrosity before long, I'm starting to work with a local studio/music store doing guitar lessons and repairs and some recording too so I'd be willing to bet I have it in front of a mic before long at all. Right now just working on acoustic stuff, rhythm tracks and so forth.

dadada:

I thought about putting a 10" speaker in mine and decided against it, I can't see cutting up a vintage cabinet, the only way I would even think about it would be to completely replace the baffle board and grillcloth, saving the original so it can be replaced later. I went with a speaker cable and external cabinet, loaded with a 12" Utah I had reconed years ago by a shop in Texas. Just had to make the cable since the Champ has an RCA jack instead of standard 1/4" one.