Ac30cc2 problems

[punkrockmax]

Well I've had my ac30cc2 for a couple months and recently I got a case of the rice krispies. basically my amp was makin a lot of unwanted noises. i decided to take a look inside and see what the problem was. Not sure if this is the cause but I noticed that capacitor c36 was missing. Also I don't know why but the resistor right next to it (r51) is just a straight bridge with no resistor. I know it's not supposed to be a bridge because the other side, c37 and r52, has a full on resistor and the schematic also says there should be a resistor there.

Obviously I need to replace these components even if that's not causing my snaps crackles and pops. One problem I'm running into is that I'm not 100% sure what type of capacitor I'm missing. I've theorized that they are metallized polypropylene film due to this picture Metallized Polypropylene Film Capacitors - Box but am not 100% sure. They are definitely not ceramic or electrolytic. They look like small yellow boxes. Anyone out there know for sure what I need to order?

When I do these fixes I am planning on ordering some new preamp tubes because my guess is if the capacitor is not my problem they are. The stock tubes in this amp seem to be of poor quality especially the rectifier which I've already had to replace. One more problem I need to solve is what tubes are what in the preamp section. I have a theory that v2 is the phase inverter and v1 and v3 are the tubes for each separate channel but have not verified this on the schematic. Does anybody know for sure what is what in there?

Thanks for any advice, it's greatly appreciated!

[Enzo]

I'll pretty much guarantee the missing parts are not your problem. In fact replacing them would BE a problem. You will only cause trouble for yourself if you start replacing parts you think are missing without knowing what they do.

Those parts are grounding reference/isolation components. They are there as part of your grounding. They are also involved in hum reduction. You replace the link with a 47 ohm resistor, and likely as not, your hum will increase. You might get lucky and it doesn't, but it won't be a good idea. They replaced the resistor and parallel cap with a wire.

As to the cap, type doesn;t matter, these would most likely be film caps.


V3 is the phase inverter tube. The input stages are in V1, then V2 appears to be extra gain stages for boost.

[jrfrond]

Shot noise, or "Rice Krispies" is more likely due to thermally-defective resistors than anything else. Crappy tubes, which are common nowadays, can also produce similar noises. There's also a possibility of one of more contaminated tube sockets. Could be a lot of things, and caps probably are not among potential issues.

[punkrockmax]

Brutal! Here I thought I had sufficiently solved my problem and was definitely a genius. Maybe if I describe my problem more it will help me out diagnosing. I will also describe my current skill level in electronics to see if my best bet is to just take it to the tech.

First off when I turn the standby switch the amp runs fine no noies/hum. Sometimes when I plug in a guitar cable this will set the amp off into making clicks and pops and such. Sometimes though I'll plug in and there will still be no noise yet. Once I start playing the pops and clicks will definitely start and will continue to get louder and more intense the longer I play. Once the pops and clicks have started they will not stop even if I unplug everything and turn all the knobs including the master to 0. That being said if it's not that capacitor I'm pretty clueless as what it could be besides bad tubes.

Now my skill level is pretty amateur so here's where I am at. When I first started a few years back I was just screwin around and shocked myself so I decided I would look up some safety precautions. Since then I've learned how to drain the power caps and to look but not touch in most cases. I do own a DMM but am not sure really of any tests to run or how to run tests.

This being said I am definitely at your mercy. Thanks for all the help!

[Enzo]

Well, welcome to the forum, we don;t mean to be brutal, just honest.

Noisy tube, noisy resistor, both likely suspects. Noisy cap, less so. That particular missing cap, not so much.

Turn any reverb off and ball up your fist and whack the top of the amp hard. Does that affect the pops and crackles? If so, there is a loose connection - either a cracked solder joint, something inside a tube, a poor contact in a tube socket, etc. And if the amp reacts to the hit, then that demonstrates a mechanical sensitivity. And vibration from the speaker shaking the parts can easily be involved.


Once the amp starts making noises, turn each control up and down. If ANY control has ANY effect on the noise, then that control is after the source of the noise. COntrols that have no effect are before the noise source.

[Euthymia]

Shot noise, or "Rice Krispies" is more likely due to thermally-defective resistors than anything else. Crappy tubes, which are common nowadays, can also produce similar noises. There's also a possibility of one of more contaminated tube sockets. Could be a lot of things, and caps probably are not among potential issues.

JR, have you not had a cathode bypass cap be the cause of the krispies? I don't see it very often, but I do see it. For that symptom, once I've figured out what stage is doing it, I check tube, socket, plate resistors, cathode resistors, and cathode bypass caps, in that order.

[punkrockmax]

I didn't mean to say the responses were brutal. I'm just bummed that this is going to be costing me money I don't have right now and that I may have to shelve this amp with little option for replacement. Having said that I do appreciate the information I've been getting here o so much and this forum seems to be one of the better corners of the internet.

Anyways, I ran the bang your head test today with little results. I had nothing plugged in and the knobs all on 0 and beat on it for a while. The pops kicked in but the beating didn't seem to have to much effect on the amp. There was some noise but only if I hit it pretty hard and not necessarily every time I hit it hard. A loose connection does still seem like a possibility though, most of the time when I play this amp it's part of a two amp setup I play bass with. I usually set this amp on top of my 4x10 which would definitely increase vibration.

About a half an hour after I ran the test I turned the amp on without hitting it. I just let it set and maybe 30 seconds in the pops started without me touching anything. The knobs have no effect on the pops what soever, the tremolo and the gain sounds don't show up on the pops. The pops are actually quite loud and there is an occasional pop then fizzle.

My plans for the future of this amp are to buy new power tubes and just hope that that's the problem. The stock rectifier was definitely pretty low quality so maybe these stock eh ones aren't of the best caliber. I am interested in replacing the resistors and caps in this amp with higher quality ones but wouldn't really want to do this right now hoping that it would solve my problems. The main reason I want to wait on that is because if I get that deep into it I would want to figure out what parts do what and how to make some modifications to get more of an ideal tone.

Anyways right now some specific questions I'm left with are as follows.
If I do buy new tubes and they aren't the problem will my new tubes be potentially damaged? and what can I replace them with, including preamp tubes to get more of a music-man/fender deep clear tone out of it?

One more thing I need to figure out in the meantime is what to do while this thing is out of commission. The speakers in the cab are rated for 30 watts each but seem like they can be pushed as hard as say a 4x12 and a solid state 300 watt head. If I use a different head on here do I run the risk of blowing the speakers?

Well, thanks for your time and knowledge it means a lot to me!

[Euthymia]

The volume and/or tone controls do not affect the level of the crackle? That can help pinpoint where the noisy stage is. Your last post seemed to indicate that it does it with everything at "0." If that is the case, my vote for most likely candidate(s) would be the phase inverter tube or the 100K plate resistors on the phase inverter tube. After that, maybe the 22K resistor feeding those.

If you pull the PI tube and it still crackles and fizzles, then you are more likely looking at a boogered power tube. Does it happen with the "bias" switch in either position?

[punkrockmax]

I just tested the resistors listed above. All 3 showed up right where they are supposed to be. I also ran a test with the PI tube removed and had the same issues with no change. However I did run another test of hiting the amp and found that pounding on the input side caused pops and an odd noise. If hit just right it would cause the same noise to happen on cue.

More tests I ran included running the bias warm as opposed to running it on the hot setting. When I did this the pops stopped completely. However when I hit the amp near the input I would get a similar but quieter noise. All of the tests I've been running have been with the knobs all at 0 and no signal plugged in. One thing I have noticed is the noise kicks in approximately 1-2 minutes after the standby is flipped regardless of how long the amp has been sitting powered on. Every time I flip the standby it takes the same amount of time for the pops to kick in, only on the hot bias that is.

I know on amps I've had before when a tube goes out completely the amp will still function at half power on two power tubes. Is it possible to test the power tubes on my amp in such a fashion, ie pulling two out and turning on the amp to see if the noise still occurs?

[Euthymia]

If you still got noise with the PI tube out you need to swap in a new set of EL84's. I suggest JJ.

BTW, testing the resistors will not tell you if they are the source of the crackle. Their values can be fine and they can still be noisy.

Your ohmmeter doesn't test them at high enough voltage to induce the arcing that makes the sound go "pop."

If you swap in a new set of power tubes and the amp is still crackling and popping, we'll want to take a look at those cathode resistors (which was the idea behind me having you flip the hot/warm switch), but at this point, I'm pretty sure it's a tube. You've likely got a tube that gets noisy when it gets hot, and cooling it down a little with the "warm" setting helps.

[punkrockmax]

thanks a lot for the advice i really appreciate it! i ordered the tubes yesterday and they should be here soon. i'll post as soon as i got em in!

[punkrockmax]

well i accidentally put the wrong address on the order form so it took a lot longer than expected to get the tubes but i got some new ones and that was totally the problem! thanks so much for all yr help I really appreciate it!

[MWAMP]

Crackling/noisy AC30CC2

Having fixed a few of these I can offer some pointers to trouble spots. Firstly, the coupling capacitors in the preamp stage (small black cylinders) are ghastly - often microphonic and noisy and crackle and carry-on when sprayed with some freeze-spray. Cheap rubbish manufactured in a country beginning with C in all probability. I replace them with 630VW polyesters. Secondly there is a board groundplane -> chassis connection via a screwed pillar on the upper LH corner of the board. Make sure that is in good contact with the board and chassis and that the screws are tight. These are not necessarily the only problems with the preamp stage/s in this amp but they are common-as.

[Prairie Dawg]

I own one and it has not been out of my smoke free home in the last three years since I bought it brand new-it doesn't even get moved around much. A few weeks ago while demonstrating it to a prospective buyer the normal channel quit cold. Just like that. Needless to say the sale went up in smoke. I got the time to troubleshoot it a couple weeks ago and found that pin seven was poorly soldered on the first and second preamp tubes. I was able to resolder them by poking my iron through the tube hole and redoing the joints with a little lead/tin stuff-the kind that wets and flows nicely so that you could solder an anvil to a phone pole if you wanted to.

Problem solved but there you go-something to look for.

[soundguruman]

On these, so far, we re-soldered the boards and the crackle went away. It was not the parts as much as just a bad solder flow problem.