Crackling sound from Ampeg SVT CL

[IronLung1986]

Does the noise go away completely with the master volume off?

So, the crackling noise when I first take it off standby, before I play anything, which happens in that last video, DOES still happen even if the Master is all the way down. If I then start playing with the Master down after those first few seconds, there is no sound or issue at all.

 

[agedhorse]

Ok, if you are lucky, it may be a failing preamp tube.

My suggestion is to take your amp to a qualified, experienced service tech that has the reputation of being honest.

 

[baxter_x]

it may be a failing preamp tube.

If it's just that, why not swapping the tubes?
No bias, and cheap

 

[agedhorse]

If it's just that, why not swapping the tubes?
No bias, and cheap

It’s a possible path, but it’s a fair amount of work in this amp and the other part of this is not to create additional problems.

 

[IronLung1986]

Thank you @agedhorse @Wasnex and everyone else who took the time to help me out on this. I'll be taking the amp to a tech next week. I'll give an update for anyone who comes across a similar issue in the future. Hopefully it's not too bad, fingers crossed.

 

[bassguitarist85]

My money is on a flaky socket/pot somewhere in the preamp. My SVT-CL was dropped in shipping transit to me. Had to glue repair the wood case, fix a broken solder joint on the power tube board, remount the fan, V1 was microphonic, still had a terrible background hum. Fixed the hum when I replaced the aluminum shielding in the wood case that was ripped apart in shipping. After all that repair work plus the standard maintenance deoxit F5, cleaning etc, dead quiet. A tech will get you sorted out I'm sure. These amps are a fair amount of work in general, but worth it.

 

[IronLung1986]

Just tried another little test:

Switched on the power (for first time today so it was completely cold).

Let it sit on Standby for a full 30 minutes before doing anything.

Switched Standby up to play.

No problems at all. No scratchy sound right at the beginning, no crackling when I play. And I played for a half hour or so, pretty loud and it worked perfectly. There was a small swishing sound when I turned the Master, that's all.

Could it be related to it being warm enough?

I'll still be taking it to the tech, just curious what this might mean.

 

[Wasnex]

Could it be related to it being warm enough?

I don't think so.

Some tubes may crackle and pop as they warm. But after they stabilize at operating temperature, it won't make a difference.

Letting the amp run in standby applies power to the heaters. The heaters warm the tube cathodes. The cathodes have to be hot before they can emit electrons.

With an SVT CL, the high voltage power supplies remain off off when the amp is in standby. I think a 15V power supply is activated, which is used to power IC chips. Also I believe the -180V bias supply for the output tubes is activated.

When you set the amp to operate, the high voltage power supplies are turned on and all operating voltages are sent to the tubes so current begins to flow from the cathodes to plates. Pretty sure the tubes will only reach full operating temperature after the tubes actually begin pulling current.

 

[IronLung1986]

I don't think so.

Some tubes may crackle and pop as they warm. But after they stabilize at operating temperature, it won't make a difference.

Letting the amp run in standby applies power to the heaters. The heaters warm the tube cathodes. The cathodes have to be hot before they can emit electrons.

With an SVT CL, the high voltage power supplies remain off off when the amp is in standby. I think a 15V power supply is activated, which is used to power IC chips. Also I believe the -180V bias supply for the output tubes is activated.

When you set the amp to operate, the high voltage power supplies are turned on and all operating voltages are sent to the tubes so current begins to flow from the cathodes to plates. Pretty sure the tubes will only reach full operating temperature after the tubes actually begin pulling current.

Ah ok. Yea I wonder why just letting it run for a bit before playing avoided the issue. At least it's nice to know the issue has a workaround for the time being.

 

[JMacBass65]

Ah ok. Yea I wonder why just letting it run for a bit before playing avoided the issue. At least it's nice to know the issue has a workaround for the time being.

Eh, it avoided it for now. If you are gigging I would make damn sure I had a backup at the ready. I have had similar issues with an old (1976) SVT went away for no apparent reason, only to return at a later date, for no apparent reason, except to say that the original cause was not identified and resolved. Still need to get that head to a tech.

 

[IronLung1986]

Eh, it avoided it for now. If you are gigging I would make damn sure I had a backup at the ready. I have had similar issues with an old (1976) SVT went away for no apparent reason, only to return at a later date, for no apparent reason, except to say that the original cause was not identified and resolved. Still need to get that head to a tech.

Oh yea, no doubt about it. I'll be taking it to a tech next week. I could never just be ok with an issue like this.

 

[B-string]

Eh, it avoided it for now. If you are gigging I would make damn sure I had a backup at the ready. I have had similar issues with an old (1976) SVT went away for no apparent reason, only to return at a later date, for no apparent reason, except to say that the original cause was not identified and resolved. Still need to get that head to a tech.

If your 1976 is still using a 7pin 6C4 in the preamp finding an acceptably "quiet" one can be a real chore. None seem to be really quiet for the last couple decades.

 

[JMacBass65]

If your 1976 is still using a 7pin 6C4 in the preamp finding an acceptably "quiet" one can be a real chore. None seem to be really quiet for the last couple decades.

Yeah, that tube was pointed out to me by several people here. I went and got a couple new ones. It's been a while but I believe I tried them both, and that might have contributed to the "sometimes it works fine, other times not" view. In other words, the symptoms didn't really change, so the results were inconclusive. The last time I used it was at a gig, started giving me trouble a few songs in, I had my trusty 400RB sitting on top of it, ready to go. Made the switch and haven't fired up the SVT since.

I need a trusted tech in the DC/MD/NoVA area (or even Richmond), if anyone has a name, that would be awesome.

 

[B-string]

Yeah, that tube was pointed out to me by several people here. I went and got a couple new ones. It's been a while but I believe I tried them both, and that might have contributed to the "sometimes it works fine, other times not" view. In other words, the symptoms didn't really change, so the results were inconclusive. The last time I used it was at a gig, started giving me trouble a few songs in, I had my trusty 400RB sitting on top of it, ready to go. Made the switch and haven't fired up the SVT since.

I need a trusted tech in the DC/MD/NoVA area (or even Richmond), if anyone has a name, that would be awesome.

Just a little background. Before I left the land of granola (SoCal) I bought into an auction that included "NOS" tubes, chances are they truly were NOS, Included were 28 6C4's of RCA and GE mix. I wound up out of the 28 with 5 "good" (read as very low microphonic) tubes and additional 8 "acceptable. The rest should not go anywhere near an SVT. All fresh that "test" as such, but not for a circuit like the SVT uses.
Being basically 1/2 a 12AU7 I made a few 7 to 9 pin adaptors (all in service) but the retainer that had to be added was pretty ugly. It was not mod for people looking to retain a pristine value. You most likely need to purchase a large quantity to find a mechanically quiet one or buy from a service that "weeds" them out and provides a guarantee of service. Groove Tubes of over two decades ago did have quiet ones and TAD had a few but gave up on the idea quickly. I don't know current re-branders as I stopped offering repairs a few years ago. Loss of a matched pair of RCA 6L6GC's to a nonpayer erected the prepared tombstone.