Ampeg SVT-VR Output Level dropping on stage

[nevetslab]

I hope the Guitar Dept sent over the correct Ampeg SVT-VR that was reported to have been changing output level on stage in a recent show. Swapping the amp cured the problem, so now I’m trying to find the cause. The Guitar Dept wasn’t successful in recreating the problem, so it's now here. Thus far, I also haven’t succeeded in getting the level to drop. Though this week, I’ve been hindered by American Idol occupying both a couple studios down the hall, as well as having set up in what was our Show Dept office right next door to my shop…it NOT being an acoustically isolated room, so I can’t crank up the SPL and chase this with a speaker. I did, though, set up the amp to drive it with 30hz Sine-Random output into an Ampeg BXE-115HL4 bass cabinet in my shop, producing 106dB SPL peaks at my test bench, though outside the closed doors, you can’t hear it, but can hear their activities down the hall clearly. From my impedance plots of the cabinet, the box tuning looks to be about 35Hz, and sits at a 4 ohm trough in the impedance response.



The power output wasn’t sufficient to cause any level drop while watching the SPL meter for a while, so I changed over to driving a 4 ohm dummy load, and cranked up the level to show periodic clipping, driving at 40hz. I started at 30hz, but was finding the output waveshape looking quite triangular which I wasn’t seeing at 40hz & above.



I was monitoring AC Mains on my power analyzer showing AC Mains Voltage, Current and Wattage, set for Averaging as the waveform is randomly varying a good 8dB min/max. I added a Bruel & Kjaer 2606 Measuring Amp to monitor the SVT-VR output, which can show Slow, Fast RMS levels, Impulse, Impulse Hold (both 35mS rise time) and Peak Hold (50uS rise time). I also clipped on my Current Probe (Tek A6302) onto the AC Mains cord (it having a section with the insulated leads exposed for such use), so on the scope, I can see the Sine-Random output signal as well as the AC Mains current @ 5A/Div.

I let this run for about 3 hrs, while never getting the amp to drop in level, though I did succeed in getting the power xfmr and heater xfmr’s too hot to touch, and the output xfmr uncomfortably warm to touch. AC Mains average current began at 5.1A/499W, though dropped in levels as the xfmrs heated up.

I was going to try again today, but hadn’t photographed any of the set-up, so I went thru that chore, and meanwhile, the client showed up, again working next door in the office being used as a rehearsal space, so again, had to forgo making myself heard.

I had thought about getting our drum dept to send over one of the Porter & Davies Vibration Drum Stools to set the amp on, so I could apply some g-force while trying again, both with bass, burst sine and Sine-Random test signals to try and get this amp to misbehave. And, have our guitar dept verify they sent the CORRECT amp over that was acting up at the show. Nothing like not being able to recreate the problem.

[nevetslab]

Further Investigation, having confirmed I have the amp in question

Having used all three Output connectors under full power burst & random-output level testing on this amp, I have yet to experience any output level dropping. I did at least find Ch 2 had reverted to producing nasty 60hz Buzz from picking up the field from the AC mains wires to/from the Standby Switch, and cured that with a fabricated brass container, listed elsewhere.

I changed signal sources, using bass, but, still having clients around me, I had to do that driving a dummy load. Then changed to a special tone burst generator (Sound Tech 3100B), where you can generate two different levels (a 0dB level and an attenuated level, of whatever duration on both levels you want), and set it up for 40hz 8 cycles at 0dB (200mS) and 1.8sec @ -20dB. During that test, I was seeing the AC mains current going quite high during that 8-cycle burst, and dropped it to 4 cycles (100mS). Still had the same AC Mains peak current level. I found once the client next door had moved into one of the isolated rehearsal rooms down the hall, I changed to my 4 ohm test speaker (Ampeg BXE-115HL4), and drove that at full output for a couple hours. Still, never had the amp randomly drop in output level.

I talked to our general manager, who was at the show and both heard the amp dropping in level periodically, changed out the amp with another, and the problem went away, and he identified this amp (having a tape label on the rear vent grille) as definitely being the one in question. So, it's been fun in the shop while being hoodwinked by the amp refusing to reveal it's alternative behavior.



Running at 40Hz tone burst into the test speaker, it's no longer at the lowest impedance point on the speaker, so we're not seeing as much peak current flowing as in the burst driving the 4 ohm dummy load, though not by much. Probably has more to do with it being a reactive load than a pure resistance load. Those reference lines on the plot are those of the 8 x 4 ohm/500W Dale Load Bank.



At this point, if I go any further, it will be to pull the chassis out of the cabinet, extract the output PCB to look af the solder joints, as well as on the main PCB. Last fully serviced in May 2018. It did change in it's Ch 2 Buzz since then, so, who knows. Its' been more fun trying to induce it to start coughing than more surgery.

[J M Fahey]

Thanks for letting us peek inside a real , fully equipped Pro lab and very logic battery of tests, applied by a very experienced Tech.

Ok, thatīs the suspect/culprit head.

Canīt you roll it to some enclosed space , or (momentarily) free rehearsal room, together with the same cabinets it was used with when problems appeared?
Playing it full blast of course.

I would go as far as to sit head on cabinets the same way it was on stage.

We might have an impedance or mechanical problem here, none of which are tested in your Lab in normal circumstances.

Would even try to get and use same cables used onstage, go figure.

[nevetslab]

Thanks for letting us peek inside a real , fully equipped Pro lab and very logic battery of tests, applied by a very experienced Tech.

Ok, thatīs the suspect/culprit head.

Canīt you roll it to some enclosed space , or (momentarily) free rehearsal room, together with the same cabinets it was used with when problems appeared?
Playing it full blast of course.

I would go as far as to sit head on cabinets the same way it was on stage.

We might have an impedance or mechanical problem here, none of which are tested in your Lab in normal circumstances.

Would even try to get and use same cables used onstage, go figure.

Normally, when faced with one of these, and the lower-power shop speakers that can't handle full power, I'll roll in one of the 810 cabinets. That used to be convenient when the equipment depot was right across the street, but last year, we purchased another building a block and a half away, so anything I need that's large/heavy has to be brought over with one of our trucks and staff that are so directed.

I haven't yet grabbed one of the Buttkicker stool transducers to build a vibration table that I could set any amp/combo amp atop and excite it so as to test for mechanically induced problems. Granted, you can only get so much conducted vibration from the speaker cabinet being driven into the amp that's acting up, which MAY be a coupled vibration problem. That HAS been on my mind, and I'm sure there's a spare Buttkicker transducer I could have allocated to such a torture device. Far cheaper than a B & K 500+ lb vibration tables to put an amp into serious G-Force.

I'm pulling that amp apart today to have a look at the power supply caps, the output connector PCB, other usual suspects that have always been found to cause intermittent behavior. Though, apart from NOT having replaced the supply caps in May 2018, it did get thorough service then, and found all of what usually cause these faults back then. But, of course, it's been traveling again on the road, which does frightful things inside from that sort of life. More to come, I'm sure.

[nevetslab]

No surprises found after pulling the power amp chassis out for inspection. I extracted the output PCB, all solid there, same on the AC mains PCB. All solid on the power amp PCB, all of which I had addressed back in May 2018. I looked at the ESR of the power supply caps, injecting 200mA 5khz square wave into them in-circuit, looking at the step-size on the scope, and calculated the ESR. All looked much the same as I measured on the bench with brand new caps this morning. And I never experienced any sudden drop in output during the prior power-output measurements, so it's back together and will be sent back into service.

So, either outwitted by something that will only show up in normal use, or, maybe oxidation inside the NL-4-equipped speaker cables that got wiped and improved while swapping out amp heads. I had exercised the NL-4 connections here on the bench, and used my adapter (NL-4 to T/S Jack) to mate with the test speaker and dummy load).