Ampeg SVT CL

[bsco]

Hi there...Have an Ampeg SVT CL...Was going in to protect mode....Replaced output tubes...were bad and were old...Also replaced the 12AX7A and both 12AU7A's as well...had a microphonic tube in the front so I replaced that...seeing that there was only one tube left and they are all around the same age I decided to swap that out as well...Amp sounds fantastic now....however I did notice that when it is first turned on and allowed to warm up for a few minutes and switched into play mode, there is a small very low level static sound there from the speaker...even with all the volumes turned down.....sounds like noisy plate load resistors...so I went through the schematic and replaced the resistors for the two preamp tubes, 100K, 1/2W....still there....changed the 100K 1/2W resistor for the 12AX7 in the rear of the amp...sound is still there...am I looking in the wrong place here?? Is it possible for the plate load resistors for the two 12AU7's to cause this?? These tubes are used for biasing right?? Or is there something else I should be looking for??? Comments anyone???

[Enzo]

I haven't bought big boxes of tubes in years, but a long time ago I had a box of 100 12AT7s, GEs or maybe Sylvanias, I forget. They were great tubes, sounded great. But every single one of them made crackly noises as they warmed up the first few times.

Your tubes may be new, but that doesn't mean you don;t have a noisy one. I;d suspect that before the resistors. Yes it is possible the 12AU7s are doing this, The 12AX7s could do it too, as could the resistors for any of them. The thing to do is not to just replace a lot of parts until you hit the bas one, the thing to do is track down the source. Pull the 12AX7, noise stay or go? You have a scope or signal tracer? Follow the signal path and see where the noise originates.

[MarkusBass]

I don't know what you mean by "static sound" but did you match the output tubes when (before) you replaced them?

Mark

[bsco]

Thanks Enzo....I'll check that out today.... Yes MarkusBass...the new tubes are a matched set of 6.....The static noise reminds me of the noise in Fender amps that are caused by the plate load resistors...By the way, Enzo, the noise is there even with all the controls turned down to their minimum...you can still hear it in the speakers.....Thanks for the advice...
Cheers,
Bernie

[bsco]

Hi Enzo. Checked out that amp again this morning....that crackling sound seems to have greatly reduced itself to the point where you have to actually strain your ears in order to hear it...So your hunch about new tubes being noisy for the first few hours could be it....I am going to put the tube retainers back on and just lay it back in the cabinet, power it up and let it idle for a few hours....if everything works out ok, I'll reassemble it back into the cabinet and give it a final check....Thanks for the advice....
Cheers,
Bernie

[Enzo]

Mark, we don't use the term as much as we used to, but at least in the USA, "static" refers to the crackles and pops and white noise like you hear when listening to a distant AM radio station or any channel when the station is off the air. If there are lightning and thunder storms in your region, your AM radio will pick up the lightning as a burst of noise. we call that static too. For that matter on a cold dry winter day, your body builds up a static electric charge, and when you handle a table radio or even just touch a doorknob and feel the spark, you can hear that static through the speaker.

[MarkusBass]

Enzo, thanks for explanations.

Mark

[bsco]

Enzo...put that amp back in the cabinet......let it worm up and ran it for awhile...sounds perfect....
Cheers,
Bernie
P.S. You don't need a set of weights if you own one of these....

[Enzo]

85 pounds of screaming 300 watts.


It could have been the awesome Peavey Classic 400. 93 pounds of screaming 400 watts. COunt your blessings.

[drewl]

I have one that is making a very very faint "thump" noise like every two seconds.
It's barely audible, coming from the preamp.
I'll start swapping preamp tubes, but never heard this before.
You can barely hear the noise, picky owner.

[Jazz P Bass]

My first choice after tubes would be to check the power supply.
Especially the preamp section.
A doofy decoupling cap can do all sorts of bizarre stuff.

[Enzo]

And make sure no cell phones are near it. Does the thump happen with no instrument plugged into the input?

Tune an AM radio to an empty channel and wave it around the area. Does it pick anything up with the same periodicity?