#1
I have a Fender Roc Pro 700 hybrid amp. just recently, something went/is going bad.

I replaced the 12AX7, the only tube in the thing about a month ago., and it's been good, tighter sounding.

Before i replaced the tube, every so often i'd hear this constant very high pitched squeal lightly in the backround, and i'd just tap on the amp, and it'd be fine. i just figuired it was a ground loop, and i didn't know if the outlet was grounded or not anyway.

today, jamming with buddies, i realize when i get there that i have very little gain and volume. I figured that before i got there, my buddy turned on the amp, didn't let it heat, and maybe blew the tube. But then I'd get no sound. Fuse is fine, because i get power. Transformer has always heated up rediculous amounts, if that is normal. But now, before i plug anything in, it's doing the same sqeauling as it's heating, so i didn't even play through it.


sorry for wall of text, i can answer any more questions specifically. but do you guys have any idea from what I'm describing?


Thanks a lot!
Quote by Invader Jim
The questions people ask here makes me wonder how the TS's dress themselves in the morning and can shower without drowning...
#2
I f you still have it, pop the old 12AX7 in and you'll at least be able to tell if its the tube thats the problem
#3
alright, both tubes are glowing just fine as of last night.

also, yesterday i remember that i was also getting plenty of noise with my amp that i never had before. That points me kind of to resistors, capacitors, and large aluminum/electrolyc redial high voltage capacitors, right?

Well when i was checking tubes, i noticed that several pieces of hardware were heating up to extreme temperatures, the point where i couldn't touch, after just a few minutes, no playing. and i was getting a little bit of smell that i never noticed before..

what was heating up was the wire wound resistors. one looked like it may have harmed the PCB also, although i couldn't see the bottom of it. hopefully no traces are damaged.

those resistors are wire wound 10%, 5W, 470 ohm. Now would these go bad because of age, bad high voltage caps leaving a spike of power to these.., bad transformer, or the new tube too powerful, or what? i can get anyone a schematic.

I don't know where to start.

thanks.
Quote by Invader Jim
The questions people ask here makes me wonder how the TS's dress themselves in the morning and can shower without drowning...
#4
For what it's worth, this is not a tube amp.
If there's just one tube in there it's a hybrid, and uses a solidstate power section.
Get a new amp, it's likely nonreparable, at least not cost-effectively.
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#5
huh? I came right out and sid that it's a hybrid, didn't i? It sounds amazing, it really does. even the solid state cleans sound like tube. the tube drive channel is tube..

I don't see what isn't repairable on it. It doesn't have a million IC's everywhere that'd i'd have to check.. It has a few TL072's that are just boosters on the clean side of things, and from what i see, they're fine.

I didn't post a thread asking if my amp sucks, or if i should get a new one. so before you post, please make sure that you have knowledge in the area, and your post pertains to the subject. if you don't know how to give me advice on what is wrong, please refrain from posting mindlessly at all. thank you, from much of UG.

NOW, does anyone have advice or thoughts on what could be wrong? would it just be age burning up these wire wounds, or the eletrolycs failing, or a bad tranformer, or a tube sucking too much voltage? I'm seeing what your opinions are. thank you all.
Quote by Invader Jim
The questions people ask here makes me wonder how the TS's dress themselves in the morning and can shower without drowning...
#6
Quote by THE Saint Jimmy
alright, both tubes are glowing just fine as of last night.
also, yesterday i remember that i was also getting plenty of noise with my amp that i never had before. That points me kind of to resistors, capacitors, and large aluminum/electrolyc redial high voltage capacitors, right?

Well when i was checking tubes, i noticed that several pieces of hardware were heating up to extreme temperatures, the point where i couldn't touch, after just a few minutes, no playing. and i was getting a little bit of smell that i never noticed before..

what was heating up was the wire wound resistors. one looked like it may have harmed the PCB also, although i couldn't see the bottom of it. hopefully no traces are damaged.

those resistors are wire wound 10%, 5W, 470 ohm. Now would these go bad because of age, bad high voltage caps leaving a spike of power to these.., bad transformer, or the new tube too powerful, or what? i can get anyone a schematic.

I don't know where to start.

thanks.

Didn't you say there was a single tubes?

Haha that's all amps are made or pretty much....resistors, caps, tubes and transformers.

Explain, you are not being clear. The resistor would fail if it say more voltage than is was rated for (yes resistors have voltage ratings) or it was seeing more than its set power rating (5W).

You talked about squealing and noise and such...hitting the amp stopped it. Your issue is not a ground loop. It really sounds like you have a microphonic tube.

My thoughts on what has happened are limited... No offense but you haven't made things crystal clear or gave a ton of info. Thought pics are probably useless you might as well post them just in case someone here can spot something.
#7
i know i'm not really clear, i do apologize.

It does only have one preamp tube, but i had two in my posession. I tried them both, both glow fine. So i didn't think tube would be a problem

i just a multimeter today, so i gotta tear apart this damn PCB and start tesing stuff. if all ohms are fine, that means i've got a spike of power going through the reistors then. i guess then i'll test electroylc. if those are fine, then i guess go to transformer?

microphonic tube could be it, because i pulled em out, moved them around, and now it's quiet when i turn it on with nothing plugged in of course.


i know things aren't clear, i'll get some pictures, try to gether my thoughts and all, i'll post again soon.


thanks so much chris!
Quote by Invader Jim
The questions people ask here makes me wonder how the TS's dress themselves in the morning and can shower without drowning...
#8
Quote by truespin
For what it's worth, this is not a tube amp.
If there's just one tube in there it's a hybrid, and uses a solidstate power section.
Get a new amp, it's likely nonreparable, at least not cost-effectively.


I agree with this guy mostly. Solid State amps will be really hard to fix due to the amount of ICs and transitors. Just because it has a valve doesn't mean the valve is the problem. I would like to suggest something to try but am not sure what. If you begin to check stuff with a mutlimeter, do you know what you are looking for?
#9
yes...did you read any of my last posts. ?

the very few IC are simple OP amps. like TL072's.

Not a million transistors either.

I never said the valve is the problem. I know it's something on that channel though, perhaps you're confused.

i have a copy of the ****ing Fender service manual. I trust myself completely. I have a layout and schematic of everything. All parts, all values.


EDIT: I know it isn't too complicated to replace four TL072's either. and not pricy.

I'm not the average pit monkey, not to sound uptight.


but now that amp is running okay..The wire-wound resistors and a few diodes are still insanely hot, which makes me uneasy. but maybe microphonics were the issue. If i find anything else, I'll update this. thank you chris, you were a help!
Quote by Invader Jim
The questions people ask here makes me wonder how the TS's dress themselves in the morning and can shower without drowning...
Last edited by THE Saint Jimmy at Aug 24, 2008,