i blew my amp up. can you help?

[ode2no1]

i have a fender champion 600 that i modded...basically bypassed the bassman style tone circuit to get raunchier tones and a bit more volume. so today i was playing it cranked through a 12" speaker and all was well....then i kicked on a fuzz. no bueno. it sort of cut out for half a second and then sounded darker. i turned the fuzz off and just strummed a bit to see if it sounded ok. then i thought maybe i'm crazy and maybe it's all good. turned the fuzz back on (which was set pretty tame) and then it just cut out completely.

it still powers up, tubes lit and all, but no sound. i swapped the preamp and power tubes and nothing. i've blown a fuse on an amp....blown a speaker on an old vibro champ by running a big muff into it, but never actually blown an amp. what should i be looking for? nothing seems obviously fried on the inside. i don't have a ton of experience working on amps, but my gut tells me maybe it's the output transformer? any opinions? how would i go about testing the transformer itself?

thanks so much in advance.

daniel

[PRR]

> my gut tells me maybe it's the output transformer?

Don't listen to your gut.

It is almost never the OT.

The OT (and PT) is the *last* thing you "want" to replace. Aside from the costly metals, the shipping is brutal.

Put your ear in the speaker. Is there ANY hiss/hum? If so, it ain't dead, and is likely minor.

Replace the power bottle (6V6 IIRC). Repeat ear-test.

Jiggle wiggle all connectors, specially speaker.

Try another speaker (use alligator-clips if it isn't plug-in).

A 600 is small. Kick it across the room. If that makes it work, you KNOW it is a bad joint and you KNOW it will crap-out again, so you gotta fix it.

(Since you have already been inside) Open it up and look for a problem. With this history, I would cast a very sceptical eye at your tone-stack mods-- the solder, any connectors, the solder, and especially the solder joints.

[ode2no1]

it is dead silent...nothing coming from the speaker. i tried it both thru the 12" i was originally playing thru and its own internal 6" speaker. nothing. replaced preamp and power tubes and nothing. so yeah...next i'll start wiggling things around. i really feel like it was a result of the fuzz pedal overloading it though, rather than something shaking loose, especially since i was running it thru an external cab, but i'll go ahead and reflow the solder joints tomorrow.

bypassing the tone stack bumps the volume up quite a bit...could that have any ill effects on any part of such a simple amp design? the only reason i blindly suspect the OT is that it's a $150 made in china 5 watt amp. i'm assuming they're not using top notch transformers.

[R.G.]

On several amp forums, I've noticed a tendency for posts that go like:

"My amp is [whatever symptom it has]. It's the output transformer, isn't it?"

It happens often enough that I'm thinking of coming up with a syndrome name for it.   

The urge to believe that an output transformer is dying in an amp is baffling to me; perhaps it's related to Intern's Disease.

[Gus]

Do the tube heaters glow?

[midwayfair]

+1 to Gus's question.. If you know how to SAFELY measure the voltages in the amp, all the critical voltages are in the schematic, which is on Fender's site.

[GibsonGM]

On several amp forums, I've noticed a tendency for posts that go like:

"My amp is [whatever symptom it has]. It's the output transformer, isn't it?"

It happens often enough that I'm thinking of coming up with a syndrome name for it.   

The urge to believe that an output transformer is dying in an amp is baffling to me; perhaps it's related to Intern's Disease.

Maybe because it's one of the more expensive parts, which we've heard such doom and gloom about??  Jumping to the worst outcome?

"We're all doomed, I tell ya!"   

Altho, if it always WAS the OT, it would make for a LOT easier debugging!

[aron]

Maybe it's a bad solder joint from your mod. You cranked it, the vibration made the solder joint fail and now it's quiet. Investigate the area of the mods.

[ode2no1]

On several amp forums, I've noticed a tendency for posts that go like:

"My amp is [whatever symptom it has]. It's the output transformer, isn't it?"

It happens often enough that I'm thinking of coming up with a syndrome name for it.   

The urge to believe that an output transformer is dying in an amp is baffling to me; perhaps it's related to Intern's Disease.

haha, honestly i don't have all that much experience working on amps. i've done mods here and there and rebiasing, but that's pretty much the extent of it. i don't really know why i thought it was the output transformer...no real logic behind it other than the amp seems fine other than there being no sound.

the tubes are glowing just like normal and the fuse isn't blowing...it definitely sounded like it was directly related to being overloaded with the fuzz pedal...as in i'd never had a problem with the amp in the past and the pedal did seems to make things weird. i won't be able to grab the amp until tomorrow, but i'll definitely check the schematic and measure voltages.

i never thought slamming an amp input would do much more damage than wear the tubes out faster so this is weird.

[R.G.]

You might want to look at this link from geofex.com:
http://geofex.com/ampdbug/ampdebug.htm

[pinkjimiphoton]

hm,mmm.... tone stack bypass, gain ya a good 6 db or so... or twice as much "volume"...
no sound at all, everything lights up.

you didn't say if you tried a different speaker. it is a very real possibility that by bypassing the tone stack you are allowing unfiltered dc thru where it should be blocked.

hitting the amp harder can make the amp louder. if the speaker can't handle the spikes, it can pop fairly easily. put a 9v battery across it's terminals to ensure the speaker survived.

next, check the leads from the amp's output to the speakers. i've seen them melt internally and either open or short.

like everyone else says, take a look at your "mods". 10 bux says that's the root of your problem.

get a copy of the schematic, and for @#$%s sake be careful man, it's a @#$%ing tube amp, and the voltages can be LETHAL. not to be a pompous asshole, dude, but if you don't know your way around inside an amp enough to do more than biasing it, you shouldn't try to mod it. it's not a solid state circuit, and some mods can @#$%in kill you. seriously. like a heart attack. i think most of us who have gotten a good zap across the chest will agree, it's not a great way to enjoy your day.

check the speaker. check the cord. turn the amp off, discharge the caps and leave a drain to ground from the ct of the ot DO NOT FORGET TO REMOVE BEFORE POWER UP and undo your mods. odds are you'll get the amp working again.

draw up a schematic of what you did and how you did it. pictures, etc. and post back here. if you DID let too much voltage thru in some areas you may have cooked some components. not entirely likely, but without seeing what you did and the schematic it's kinda hard to speculate.

good luck man. keep one hand in your back pocket for me, a'ight?

[vigilante397]

I hate to be "that guy," but have you tested the tubes? I've had a couple amps that I worked on that when even one preamp tube is starting to go the whole amp will start to sound like garbage or will make no sound at all. Just a thought.

[davent]

He's checked out the tubes and speakers...

it is dead silent...nothing coming from the speaker. i tried it both thru the 12" i was originally playing thru and its own internal 6" speaker. nothing. replaced preamp and power tubes and nothing. so yeah...next i'll start wiggling things around. i really feel like it was a result of the fuzz pedal overloading it though, rather than something shaking loose, especially since i was running it thru an external cab, but i'll go ahead and reflow the solder joints tomorrow.

[pinkjimiphoton]

actually LOOKING at the schematic helps immensley.

the "tone stack" is preset, and has blocking caps rated at 400v. if you removed this part of the circuit, you had the full b+ voltage of the amp going right into the input of your volume control. seeing as how it should only be seeing AC of only x number of volts, and is probably a 1/4 amp rated pot, i'd bet you melted the mofo inside. without the caps in the "tone stack" there are no blocking caps to protect the rest of the circuit.
so you would have got a huge volume increase, a peculiar weird muffled distortion that would sound way more ballsy than the stock circuit, and as the pot burned out, a tonal change up until  total failure.

take a 400v cap and put it in series with the output of the first stage.... in other words, plate of v1 needs a 400v cap going to the input of  the volume control, and of course REPLACE the volume control... and you should be back in the running, sans tone stack. make the cap something that can handle the voltage, first... worry about tone AFTER you get it running.

i am very grateful you didn't hurt yourself doing this. you can really f yourself up not realizing you're being electrocuted. i have neuropathy in my hands that tends to make them numb  and tingly... sometimes i've mistaken low level electrocution for this.

anyways... here's a schematic:

http://support.fender.com/schematics/guitar_amplifiers/Champion_600_schematic.pdf

as you can see, without those caps in the tone stack, (c 7,8, 9) there is nothing to block the dc b+. so you hit the pot with about 350 volts dc, which is wayyyyyyyyyyyyy above what it is designed to take.

use the .1 cap you have now to block dc. it should be able to handle the voltage. to see if it's the pot, jumper it right from the plate of the tube (pin 1 of the 12 ax7) right to where the output of the volume control goes... in this case, r14 and the amp should fire back up, minust any volume control of course.

if ya look at the schematic, the actual AC audio should be a bit less than 2vac  .... you slammed that beotch with about 100x what it was designed to withstand.

check it out, and hollah back.   i bet that pot is totally toast.

[pinkjimiphoton]

circled in red is the problems as described in my last post.

the brown squares are the stuff you can probably safely remove. i would consider leaving the 15k to ground there.

the green lines show where to connect what. notice the dc voltage is now blocked by the .1 cap, allowing the ac (audio) to flow thru unmolested.

you will def have to replace the volume control... any 1meg audio pot should do.

as you can see, you have about 1.7vac (audio) in the part of the circuit you messed with, and a dcv of around 310 or so after r 8 (100k)....
so ya blasted the pot with 300vdc or so. it could probably take it for a couple minutes... maybe.

but it would have popped. also check the continuity of the leads going between the pcb and the breakaway input board. something in there is fried. guaranteed. most likely the pot.

anyways... here's a graphic.
keep us posted.
again, PLEASE, keep ONE HAND IN YOUR BACK POCKET WHILE MESSING WITH THIS. 300 volts can do a lot more than tickle.



whenever dealing with audio, you HAVE TO BLOCK THE DC FROM PASSING FROM STAGE TO STAGE. there are indeed exceptions, but if you don't, in most cases you're gonna make yourself trouble. i forget the term for direct coupling two stages of a dual triode, but for all intents, that's what you did. handy for driving a marshall hard perhaps, but in this case the root of all your woes.

i really doubt the fuzzbox was anything more than coincidence. peace!

[Luke51411]

How did you remove the tonestack because I know there is a mod out there that just involves lifting one resistor, leaving the rest of the circuitry intact. I believe it is R19. I've done this mod when I had a champion 600. It's always best to describe how you accomplished the mod, exactly what was done to the circuit instead of just a vague statement such as "I removed the tone stack" Which specific components were modified?

[vigilante397]

He's checked out the tubes and speakers...

Ah, must have missed that bit, my apologies. Carry on.

[ode2no1]

hey jimi...i definitely appreciate your concern for my safety and all of your comments about the whole thing. i promise i know enough about amps to not be poking around willy nilly and get electrocuted...i just meant i'm by no means an amp tech. also, i had forgotten exactly what i did to the amp but found a thread i'd posted about it. http://www.diystompboxes.com/smfforum/index.php?topic=109345.0

so i didn't completely bypass the tonestack, but put a big cap across C1. well, i used a switch and had the stock setting and then two bigger caps in parallel to c1 to get 3 different settings. i also ended up putting a bright cap on a switch.

my band played a show this past weekend and i've been busy with other stuff so i still haven't had a chance to mess with the amp, but i'm off the next two days and will do some investigating, but i did want to chime in and explain what it was i did to the amp. thanks to everyone for your comments thus far. if anything comes to mind i'm all ears. otherwise tomorrow i'll be reflowing my solder joints and taking voltage measurements.

[pinkjimiphoton]

sorry dan,
i just worry when i see peeps that aren't used to this much voltage. i've made plenty of mistakes myself, a couple that could or should have been lethal,
and i hate to see anybody get hurt, sorry if i came off like a dick.

rock on bro!

[ode2no1]

don't worry about it man, you totally didn't come off like a dick. so i know it's been a month, but i finally got around to taking voltage readings. all of my actual readings are in RED above the schematic's voltages. the info at the bottom of the schematic says the readings have a 10% tolerance, and what i noticed is that i'm within tolerance on everything but the 3 voltages pertaining to the tubes. i'm getting too much on pins 3 and 8 of the preamp tube and too little on pin 8 of the 6v6. i do have a 12at7 in the preamp slot, not sure if that would affect things, but it was running fine for almost 10 months before it gave out.

i also measured the pot and it seems to be working just fine. nothing is obviously fried on the board...traces or components. both fuses are in tact. i reflowed the solder joints on the tube sockets and disconnected the mods i'd done. tubes still light up just fine but no signal is passing. does anyone have any ideas as far as what to look for next?