SVT-CL Classic - loud noise coming out of standby

[lylelwr]

I have an Ampeg SVT-CL head on my bench that's making me scratch my head. When taken out of standby it has a loud buzz that lasts for about 2 secs then fades away. The amp plays fine and is quiet until the next time it's taken out of standby. Makes no difference if the volumes are at min. I put a plug into the main amp input jack to eliminate the preamp section but it still buzzes loudly at turn on. The only thing that will make it turn on quietly is pulling out the phase inverter tube. The outputs are all matched and bias up with two green lights. Has anyone even seen an SVT-CL do this sort of thing?
Thanks for your help.

[Chuck H]

Well, let's consider what we know... It doesn't happen if the PI tube is pulled. So the noise is generated in the preamp right behind the PI or in the PI itself. And...
The noise is related to the standby switch and fades away. So it's some phenomenon that happens in moments as B+ is applied.

I'd check the filter circuit for the PI. It's possible that that filter cap charging up is part of the formula, though not absolutely the problem.

Have you tried replacing the PI tube? Have you checked for loose pin connections or bad solder joints in this part of the circuit?. I don't know how the peripheral monitoring circuits are set up in that amp, but if any power supplies that are part of current monitoring systems in this part of the amp are not filtering properly that may be injecting noise momentarily.

Does it happen if you flip the amp to standby and right back to play again? if not I would really suspect the noise is related to a charging capacitor somehow, though it may not be the capacitor is the bad component.

[hodgy]

Is it an electrical type buzz or more of a 'Sproing!' ?

[Enzo]

V1 is the PI and should be a 12AX7. V2,V3 are buffer and driver stages and should be 12AU7s. Make sure all three sockets have the correct tubes in them. More often than I would wish, I find them wrong when amps come in.

And the one tube I find bad most often in the SVT series is the driver 12AU7. It wouldn't hurt to take a few seconds and pop a different one into V3 just to see.

But as CHuck suggests, if pulling the PI stops it, and isolating things from the preamp don't, then first step ought to be a different PI tube.

[lylelwr]

Ampeg SVT-CL

Thanks for your replies -- I did try a couple of different 12AX7s in the V1 position but they all buzz at power up. I figure if it doesn't buzz with V1 removed then I've eliminated my plate, screen and bias supplies as the noise source as well as V2 & V3. I tried adding an additional 50MFD to the 345 volts that supplies the plates of the phase inverter but it still goes raaazz for about 2 seconds coming out of standbye. I still have the 1/4 inch plug in the main amp input jack to eliminate the preamp so I'm down to the phase inverter section which is just a simple split-load type. This amp is tough to troubleshoot since it only lasts for a second or so each time. I have seen standby swithes arc sometimes but that is usually more of a random event. This does the same thing every time.

[Enzo]

I've eliminated ... V2 & V3.

NEVER think up reasons NOT to check something.

Circuits interact. Pulling V1 does not simply eliminate the tube, it also eliminates certain voltages in the circuit.

Now i don't of course know this to be happening, but follow this reasoning as support for at least checking V2/V3 and their circuits. And sometimes it is not until we replace the faulty part that we can go back and determine why it was causing the symptom.

The second section if V1 is the split load phase inverter. 68k plate load and 68k cathode load. The current through them all is the same, so since there is a 95v drop across the plate load, we can expect around 95v on the cathode at pin 8. From pin 8, C9 couples the signal over to the grid of V3 at pin 2.

WHen V1 is present, those voltages are there, but when V1 is removed, the voltage at pin 8 V1 remains zero. Well, what if C9 is a little leaky when first charging up? Without the V1, there is no charging of that cap. That points to the cap rather than V3, but we still don't really know.

I suspect C8 less, since the voltage across it is higher without V1, but by my own rule, it bears checking.

Naturally, the problem could be elsewhere, but the essence of troubleshooting is to isolate the problem. You have an elusive noise, and we can track it down. Unplug J10 or otherwise disable the NFB line. That way the amp won't spread noise from the output back through other parts of the system. V2 and V3 are buffers and drivers, V2 for the top, and V3 for the bottom - one for push and one for pull so to speak. You noise could be throughout, or it could be in just one half of the stage. You can pull V2 and power up, then replace it and pull V3 and power up. either missing tube have an effect? If one stops the hum, then it was the path for it. Or related at least.

get out the scope. regardless of the source, the hum signal is coming through the amp. I assume it will be on the power tube grids. But let's stay in the V1-3 area. Watch pin 2 of V2 and V3 - the grids. See hum signal at either or both? And if you do, how does pulling V1 change it? And watch those grids on DC scoping. There should be little or no DC offset. But watch especially while the hum exists. Is there any little DC change taking place? Does one side take the extra couple seconds to settle?

What if R27 has gone way high?

[MarkusBass]

As Enzo says:

... the essence of troubleshooting is to isolate the problem.

Are you sure that this is not related to the preamp? You can simply check this by disconnecting the preamp. I'm asking about it because I had a similar problem (but not exactly the same) and it was caused by a broken ground track in the preamp. The amp worked almost fine.

Mark

[Enzo]

He's been plugging into the power amp jack to disconnect the preamp. That doesn't eliminate coupling through the power supply, but usually it is enough.

[rf7]

I'm confused, does it buzz on power up or when you flip the standby switch? I just had one with the relay chattering on power up for a few seconds. Could it be that?

[EVR]

Loud noise coming out of standby

I have an Ampeg SVT-CL head on my bench that's making me scratch my head. When taken out of standby it has a loud buzz that lasts for about 2 secs then fades away. The amp plays fine and is quiet until the next time it's taken out of standby. Makes no difference if the volumes are at min. I put a plug into the main amp input jack to eliminate the preamp section but it still buzzes loudly at turn on. The only thing that will make it turn on quietly is pulling out the phase inverter tube. The outputs are all matched and bias up with two green lights. Has anyone even seen an SVT-CL do this sort of thing?
Thanks for your help.

Hi there. Curious if you were able to diagnose and solve this problem. I just bought a used SVT Classic, and have the exact same issue you describe.

Thanks!

[Amp Kat]

Since the fault light allows you to operate them that rules out pretty much the voltage section because if there is a problem with any power relates issues the fault circuitry will not allow the amp to come on. The front preamp board has been most of the problems I've seen in this amp. Since the heaters are not part of the fault system I would pay close attention to them and make sure all the preamp tubes on that front board are operating at the correct voltage. Pull the blue /white connector and see if the noise goes away. There is also a green wire running through that clear plastic protector that can come loose so check it good. Pulling the PI tube stops everything so basically the noise is just being cut-off there not necessarily the culprit as most amps will go quiet when this tube is pulled especially hum. Another problem I've seen with this amp is the thing is so damn loud and vibrates so hard it can back the power transformer screws out and cause a vibration hum so make sure they are tight. Had one back the screws all the way out and it fell on the main board and wreaked havoc. Look for vibration problems as this thing is a beast and can rattle something loose very easily.

[schwari1]

SVT CL Noise out of Standby

I have experienced the same noise and troubleshot it down to a ribbon cable in the preamp section that is not keyed and can go on 180 degrees out of what it should be. I can't recall the actual plug/jack number but it was top middle of the board as viewed from the front. I disconnected and flipped it over and the sound was gone.

[emuso]

Thanks, schwari1!! That did the trick for me! I was in too big of a hurry to reassemble the amp. I know now!! All of the Best.

[bob p]

It's hard to imagine that the amp would even work with the ribbon cable on backwards.

[Chuck H]

I was thinking the same thing! I suppose, since we're reading about a repeat performance of this issue, that it isn't even visually obvious for some reason. But I'm reading here that it happens! Or at the very least it happened twice. So word to the wise.