Strange noise in Sunn amp

[SoundmasterG]

Hello everyone,

I will have a Sunn Sentura 1 amp coming in Monday for a repair that has the strangest problem.....it does this only on the low F, F#, and G on a guitar, and it does it with any guitar or cabinet, so it is the amp that is doing it. It will have a fuzzy mosquito oscillation noise on top of those notes, but it isn't a really high pitched one like some amps that I've heard before.....and it does it when the note decays and only on those notes I mentioned. I had the amp in a couple weeks ago when it had this problem originally and there were a couple problems with some previous mods that I resolved, and all the electrolytics and one bad coupling cap were replaced. I checked several resistor values in the amp, checked voltages, checked tubes, etc. After I worked on the amp a couple weeks ago, the amp didn't manifest it's problem, so I was assuming it was solved, but the problem is back again. At this point I'm not even sure where to begin as I can't see anything obvious on the scope, though perhaps that is because the problem wasn't occuring at that time. Does anyone have any good suggestioned for what to check for or where to start to try to solve this amp's problems?

Thanks,

Greg

[Jack1962]

Have you checked the Output Transformer ?

[SoundmasterG]

No I haven't. The amp functions well aside from the oscillation noise on those 3 notes, and puts out good power....I don't see how anything could be messed up with the output transformer honestly. The amp will be here tomorrow, so I'm hoping I can figure out what to check for between now and then. What were you thinking that could be wrong with the output transformer?

Greg

[The_Gaz]

I'm sure you did, but did you change the tubes? I've had this problem from components inside a tube rattling because of low frequencies. That, of course, only happened when the amp was on top of the cab, and I'm guessing you didn't test the amp on top of a cab in if you shop did you? That's my only guessing considering all the other work you've done.

[SoundmasterG]

Yes I tried different tubes and tested the ones that were in the amp too.....I also purposely tried the map on top of a cabinet to get vibration going.....and it has the problem now apparently but didn't when it was at my shop. Strange....what will probably happen is that he'll bring it over and it won' thave a problem again...haha. Well I'll find out more tomorrow....

Greg

[SoundmasterG]

Well he brought the amp over and this time we were able to make it repeat the sound. It turned out to be a loose cap lead in the reverb/trem board. The vibration of being on a speaker cabinet over time had caused the solder joint to loosen up for the cap lead and it was just enough to cause intermittant contact and with the right notes it would resonate. It was around 180Hz that caused it. I resoldered that lead, and zip tied the caps down to the old cap holders on the board and it is pretty solid now, so hopefully it holds up.

Greg

[The_Gaz]

Interesting - thanks for posting the outcome... and by the by, did you ever get the MIGAC100 up and running?

[SoundmasterG]

Yes the former Sovtek MIG 100U / VOX AC100 clone is up and running and has been for awhile. I initially had a low power problem that was due to my choice of circuit to mix the channels via the phase inverter.....so I've since rewired it to use the stock phase inverter circuit, and the amp is putting out full power now like it should. I have a bit of a high pitched squeal going on with my high gain channel that I added.....it only happens when the volumes and trebles are maxed. I'm trying to eliminate that and not lose a bunch of gain as it sounds REALLY good otherwise. The clean Vox channel also probably has a little blocking distortion going on when everything is maxed that I'd like to eliminate...its really subtle...but I don't want to lose the sound of the Vox AC100 either, even though I've since changed the circuit a bit. I'm getting there though. Once the circuits work as they should, then I need to get the channel switching working correctly, and then add power scaling, and then redesign it all into a new board. So a ways to go still yet, but I'm close. I plan to post all about it with pics and soundclips once I get that done, and also the Bogen CHB100 that I've also been working on. (which also sounds REALLY good so far....this is the one using the 12AV7 that I posted about. It has a high pitched squeal also and some oscillation, and a hum going on too) My brother was just over and played both amps while I fixed his Silvertone 1484. A shielded wire had broken  and was making intermittant contact on one of the channels, so the amp would crackle and had low gain as compared to normal. Never a dull day!

Thanks for the interest!

Greg

[The_Gaz]

Good to hear. I'd like to see those pictures when you get them - I used to have a beautiful (inside only) Mig100u, and I think an AC100 maybe one of the only intriguing high power designs I myself may torn it apart to build 

[SoundmasterG]

Yeah the MIG sounded pretty lame....thin and unappealing. I could have sold it after I fixed it, but I'm happy I did the mods to it. It has a lot of potential once I get it done. Incidentally I do have the stock choke, output transformer, and circuitry from the MIG 100U if you or anyone else might be interested in them? I'd sell them pretty cheap as I don't plan to use them for anything in the future. If you'r einterested in the parts, let me know via PM.

I've been sidetracked on the Bogen lately.....trying to get something I can use for blues jams. It sounds pretty good as is, but is very susceptible to high gain due to the small chassis I think. Though I've read about some people doing 2204 circuits into them. I'm thinking of changing things up again from where I'm at now. It is ok but uninspiring no matter what I seem to do to it. If I get good gain where it sounds good, then it also will have a bit of oscillation when dimed.....if I back off the gain in the preamp, then it doesn't sound as good. Ah well...its a fun project to tinker with and learn new things. At some point soon I'll get back to the Vox project though.

Greg

[chip]

hey SoundmasterG,

I have had the same problem with a recent new build.

".....fuzzy mosquito oscillation noise on top of (only some) notes... and it does it when the note decays..."

How did you figure out which cap had the bad connection?  I'm not sure where to begin.

Thank you,
Chip

[SoundmasterG]

I had the owner play the amp at very loud volumes while I held the amp head on the speaker cabinet upside down so it would vibrate, and while doing that I also poked parts with a chopstick to see if I could make the noise stop and start. It took awhile and was hard on the ears but I found the problem within about 10 minutes. It wasn't very easy or safe to do probably as holding the chassis with one hand while poking with the other has many possibilities for a slip-up, but I took precautions, and it worked out well.

In your specific problem it sounds like you have an oscillation in general rather than something that was vibration induced like what I had in that Sunn. First thing would be to try to isolate the problem down stage by stage. Pull the phase inverter tube and see if you still hear it or not. If you do, it is after the phase inverter....if not, it is before it. You can repeat this stage to stage to try to track down the problem. Sometimes though this doesn't work as without a given stage in the amp the noise won't happen. If you have a scope and signal generator, you can try to sweep the frequencies on the generator and see if you see anything on the scope. Once you know which stage the problem is happening in or around, then you can take steps to solve it. Reduce gain, reduce high end, change grounding or parts orientation. Sometimes a small cap in parallel with the plate resistor or from the plate back to the grid...with or without a resistor in series, or a cap from grid to ground can help. They can also hurt too. The smallest cap needed is usually best as it can ruin tone too in some situations. Another possibility is to put a voltage divider in between stages to reduce gain, and if high frequencies are the problem, you put a cap in parallel with the resistor to ground in the divider. This acts as a low pass filter and shorts the highs to ground. Gerald Weber has some good hints in a couple of his books, the 2nd one especially, so I'd suggest to get it if you don't already.

Greg