Ampeg SVT-3 Pro POP PROBLEM

Weshunter413 · #1 07-01-2011, 04:42 PM

I have an Ampeg SVT-3 Pro USA with an issue. When I turn it on and wait till the tubes warm up it make a HUGE pop sound in my cab. I know it's not the cab because I have used multiple cabs and speakon, and 1/4 cables. I believe it's a problem in the power amp because it doesn't pop when plugged into the DI out, only the power section. Everything else works great and it only happens at first when tubes warm up.

Thanks for any help you can lend.

AwkwardLoudness · #2 07-01-2011, 04:57 PM

Are all of the volume knobs turned down when you hear a 'pop' through the cab? If yes, then your amp almost certainly has a bad solder connection somewhere and the circuit is not properly grounded. I have repaired a couple amps with this issue.

Weshunter413 · #3 07-01-2011, 05:20 PM

Yes no matter what knobs are turned which way it pops, I'll try checking the connections. Should I check any where specific?

Jerrold Tiers · #4 07-01-2011, 09:13 PM

Quote:

Originally Posted by AwkwardLoudness

Are all of the volume knobs turned down when you hear a 'pop' through the cab? If yes, then your amp almost certainly has a bad solder connection somewhere and the circuit is not properly grounded. I have repaired a couple amps with this issue.

yes, decently likely.

Somewhere in power supply or power amp, probably in the power amp tubes area. Could even BE a tube problem, which is easy.

if you try this yourself, be careful, I don't recommend it unless you are familiar with electronics and soldering, etc. Tech is better.

AwkwardLoudness · #5 07-02-2011, 09:50 AM

Quote:

Originally Posted by Jerrold Tiers

Somewhere in power supply or power amp, probably in the power amp tubes area.

There are no tubes in the power section of a SVT-3PRO. It's a hybrid amp with tubes in the preamp.

Jerrold Tiers · #6 07-02-2011, 04:02 PM

Quote:

Originally Posted by AwkwardLoudness

There are no tubes in the power section of a SVT-3PRO. It's a hybrid amp with tubes in the preamp.

Ah, but there you are wrong......

Preamp of the SVT-3PRO is indeed mostly tube. But the poweramp is not pure SS at all.

There is a 12AX7 and a 12AU7, which provide ALL the voltage gain in the poweramp. The mosfets are ONLY "matching" the tube output to the speaker. The other SS parts in the power amp provide current to the bias circuit, and overcurrent protection.

Perhaps you are thinking of the SVT-III. it has a tube preamp and a solid state power amp.

rickdog · #7 07-02-2011, 06:38 PM

The SVT-3PRO has a relay on a timer that doesn't connect the power amp output to the speaker until everything has had a chance to warm up and stabilize. Mine makes a soft but audible "pop" when it finally kicks in.

Do you know if your big POP! is happening when this relay closes? Or does it happen after that?

And how does that amp sound once it's running after the pop? All ok? Distorted?

Where I'm going with this... there's a DC balance circuit that is supposed to ensure that the power amp output doesn't have a DC offset. If that circuit isn't working right, or if there's a problem in the solid-state part of the power amp, when that relay closes there's gonna be one heck of a big pop as the DC gets applied to the speakers.

Jerrold Tiers · #8 07-03-2011, 08:27 AM

The delay is a few seconds, or should be. I suppose it depends on what "wait for tubes to warm up" means..... I assumed that meant a considerably longer time than I expect for the turn-on delay, but that doesn't necessarily have to be so.

A DC offset would surely do that. The DC corrector* really should keep that down, unless the DC problem is pretty significant. I HAVE seen a unit where there was a problem, and the DC corrector couldn't fix it all the way even when it was at max. That could be the problem also, if the pop is only a few seconds after turn-on.

You can see if the speakers are pushed-in or popped-out more than normal. Watch them as you turn on. Either they move and then return to normal position, or they move and "stick" at a different position. If the latter, or if you have a meter and measure a DC on the output, then the DC is the issue.

*The DC corrector is necessary because the tubes directly drive the mosfets, with no DC connection. There is no corrective feedback, and the DC corrector provides that just for DC, not for "signal", to drive the DC to zero.

rickdog · #9 07-03-2011, 09:12 AM

Quote:

Originally Posted by Jerrold Tiers

The delay is a few seconds, or should be. I suppose it depends on what "wait for tubes to warm up" means..... I assumed that meant a considerably longer time than I expect for the turn-on delay, but that doesn't necessarily have to be so.

Well, my SVT-3PRO (purchased new in 2002) has a delay of "a few seconds" for a sufficiently large definition of "few" ... it has always taken several tens of seconds.

Apologies to the OP for temporarily hijacking this thread

Jerrold Tiers · #10 07-04-2011, 08:29 AM

"A few" would be around 15 seconds or so......

if the pop is after a considerably longer time than that, a minute or more (as I initially assumed from "warmed up") , then it is probably a tube, part, or connection issue.

if it is around 15 sec or a bit more, then in may indeed be DC and the pop occurs when the relay closes.

Typical reason for the DC offset is a problem in the parts that supply current to the bias circuit. IC1, Q3, Q4 and associated parts.

best to take to tech.