NEW Ampeg SVT-CL came back in Protect Mode

[nevetslab]

One of the two new Ampeg SVT-CL amps I had just set up for my client came back last Thursday in it's new road case, with a Marshall JCM 800 case sitting atop it. I didn't yet see which SVT-CL they sent over, but hearing it was in the flashy-red light mode......cool....I do need more $$ to make it thru March. When I moved the cases back into the shop at the end of the day, my heart sank as I saw it was one of the two amps I had just set up and returned to them earlier last week!

I put the amp up on the check-out bench yesterday morning while I was getting my coffee made, plugged the mains and speaker in, switched on to S/B while I fetched the water. Came back, switched into Operate, watching for fireworks in the power tubes while seeing the Power Analyzer out of the corner of my eye, with Bias LED's in view if it comes up. It came up just fine, no fireworks, both GRN LED's lit, AC Mains power draw 220W @ 120VAC....Nominal where I bias them. Happy and sad. Cycled it on and off from S/B to Operate, waiting about a minute between cycles. I didn't have signal connected....just turned up the Master & Gain, cranked up the Treble to verify I had signal (residual noise present). I left it on in S/B mode most of the morning and into the afternoon, getting up and cycling it into Operate and back every so often. Turned it off all the way, waited, turned back on, waited, then into Operate, did that often.

Finally having finished my month-end billing, I moved the amp to the test bench, moved the Sound Tech 3100B atop the amp, powered up the audio analyzer & scope, set it up for 4-cycle 40Hz Burst (100mS on @ 0dB), and -20dB @ 1.9 Sec Burst, with it repeating at 2 sec rep rate, and dialed it in on the scope, set in triggered storage mode, and cranked it up to moderate clipping on the 0dB burst. Shakes the building and pulls full power for that short burst. It's been running flawlessly.

When I had it just on & idling, in operate mode, I've pounding on the amp, lifted both ends up a couple inches and let it drop back to the bench top to jolt it many time. So far, I haven't seen any hint of failure/Protect mode.

[mozz]

Possible they are using it with a cabinet and the amp is set to the wrong impedance?

[nevetslab]

Possible they are using it with a cabinet and the amp is set to the wrong impedance?

No....impedance is still set to 4 ohms, which is how I'm using it.

[nickb]

... set it up for 4-cycle 40Hz Burst (100mS on @ 0dB), and -20dB @ 1.9 Sec Burst, with it repeating at 2 sec rep rate, and dialed it in on the scope, set in triggered storage mode, and cranked it up to moderate clipping on the 0dB burst. Shakes the building and pulls full power for that short burst. It's been running flawlessly.

By way of bringing some relief, I was doing pretty much the same thing with a just done Mackie Thump 18S. This is a brute of a powered subwoofer (1.2KW) that's a three foot heavy cube. I was amused to see it slowly walk across the floor, 1/8in every cycle. I could push it back again with just my little finger...

Anyway, back to reality. I really don't know what the answer to such situations is. I really wish I did for my own sanity. If you can't reproduce the fault there is no way you can fix it with confidence. In this case I''d either sent it back with a caveat that if it happens again the power tubes get replaced, or if a failure is not an option, fit new ones, retest and then send it off.

I wish you luck.

[TimmyP1955]

Folks don't always use SVT heads with 4 Ohm or 2 Ohm loads as they should. We've had folks run them with 8 Ohm cabinets, which I think might sometimes make the amps unhappy.

[Leo_Gnardo]

Possible they are using it with a cabinet and the amp is set to the wrong impedance?

That ^^ and never neglect the possibility those who rented it may be using a shorted or open speaker cable.

Or a very crappy guitar cable. That was the word amongst the local teenage experts when I was just getting started with amps - if you don't have a speaker cable, just use your worst guitar cable. Heh, heh . . . because that always works, right? 50 years later, same old stuff.

[nevetslab]

Folks don't always use SVT heads with 4 Ohm or 2 Ohm loads as they should. We've had folks run them with 8 Ohm cabinets, which I think might sometimes make the amps unhappy.

I heard back from the manager of the rental company, who told me the amp went into protect when they had switched back into Standby from Operate, and it took turning it off and back on again to get out of protect. I've already gone thru that exercise repeatedly at the start of the search, and never once saw Protect Mode. In one of the recent SVT-CL threads (https://music-electronics-forum.com/...ad.php?t=48527), 65ampman had stated he once had seen new a Sovetk 6550 power tube arc, but t it never occurred again. After 14 hrs of running, power & S/B cycling, and slamming full power bursts thru the system shaking the building while smoothly filling the shop with deep low 40Hz tone between bursts, I finally shut it off and packed it back up.

So, we'll see if this comes back again.

[Leo_Gnardo]

One last try - I had an SVT CL that exhibited similar behavior on stage in Montreal. Turns out the AC power was over 130 volts, and the amp's self protect circuitry kept the hi voltage off, just as well because if it hadn't we might have had a smoking wreck. The bassist wound up playing through the DI box, heard himself in the monitors, and audience heard him through the PA. The show went on successfully without anyone knowing the amp wasn't on. (After the show a big wink & a smile between muso & tech: "we fooled 'em again, didn't we Chief?") Next day in Ottowa, no problems at all.

Most amps won't, but this amp does prevent itself from damage when it senses conditions aren't right.

[bsco]

One of the two new Ampeg SVT-CL amps I had just set up for my client came back last Thursday in it's new road case, with a Marshall JCM 800 case sitting atop it. I didn't yet see which SVT-CL they sent over, but hearing it was in the flashy-red light mode......cool....I do need more $$ to make it thru March. When I moved the cases back into the shop at the end of the day, my heart sank as I saw it was one of the two amps I had just set up and returned to them earlier last week!

I put the amp up on the check-out bench yesterday morning while I was getting my coffee made, plugged the mains and speaker in, switched on to S/B while I fetched the water. Came back, switched into Operate, watching for fireworks in the power tubes while seeing the Power Analyzer out of the corner of my eye, with Bias LED's in view if it comes up. It came up just fine, no fireworks, both GRN LED's lit, AC Mains power draw 220W @ 120VAC....Nominal where I bias them. Happy and sad. Cycled it on and off from S/B to Operate, waiting about a minute between cycles. I didn't have signal connected....just turned up the Master & Gain, cranked up the Treble to verify I had signal (residual noise present). I left it on in S/B mode most of the morning and into the afternoon, getting up and cycling it into Operate and back every so often. Turned it off all the way, waited, turned back on, waited, then into Operate, did that often.

Finally having finished my month-end billing, I moved the amp to the test bench, moved the Sound Tech 3100B atop the amp, powered up the audio analyzer & scope, set it up for 4-cycle 40Hz Burst (100mS on @ 0dB), and -20dB @ 1.9 Sec Burst, with it repeating at 2 sec rep rate, and dialed it in on the scope, set in triggered storage mode, and cranked it up to moderate clipping on the 0dB burst. Shakes the building and pulls full power for that short burst. It's been running flawlessly.

When I had it just on & idling, in operate mode, I've pounding on the amp, lifted both ends up a couple inches and let it drop back to the bench top to jolt it many time. So far, I haven't seen any hint of failure/Protect mode.

I remember that you had helped me out on a previous SVT repair...mine was always in protect and the problem was a shorted IC chip back in the rear of the amp....near the back panel....is it possible that the chip might be intermittment but then again, you had pretty much beat the crap out of it.....with no issues....go figure....

[nevetslab]

I remember that you had helped me out on a previous SVT repair...mine was always in protect and the problem was a shorted IC chip back in the rear of the amp....near the back panel....is it possible that the chip might be intermittment but then again, you had pretty much beat the crap out of it.....with no issues....go figure....

There are any number of possibilities to induce one of these to go into protect mode. The client had apparently switched back out of Operate mode into S/B mode, and the pilot light began blinking (protect mode). It wouldn't switch back into Operate mode, once it's in that mode (circuitry won't allow that). He had switched it off, then moments later, turned back on, it was back in S/B, then back into Operate and was in Operate mode, without it reverting back to Protect mode from Operate Mode (I believe.....I'm inferring that from what little they said). Having covered all of those variables at first, before even applying signal, it never did that for me. I didn't run the variac up and down to see it at low line and high line. I do know if you try to slow start one in S/B mode with a variac, it will enter Protect Mode....not sure why off the top of my head.

Anyway, the client's driver picked it up along with a couple other amps of theirs, so we'll see.

[drewl]

Wasn't there an update several years ago that prevented false protect mode tripping?

You might want to check to make sure even though it's a newer model.

[nevetslab]

Wasn't there an update several years ago that prevented false protect mode tripping?

You might want to check to make sure even though it's a newer model.

Yes, there was. Adding a 0.1uF across IC2B pins 6 & 7 (it's set up as a comparator with 220k across pins 7 & 5). I've been adding that to units that don't have C38 on the board. It is in later schematics.