If an Ampeg SVT-4 Pro was left on for the duration of a session (like 4 hrs.), could the heat from the tubes potentially add a bit of unwanted hiss on the direct out?

Not that I'm an expert, but having played tube amps most of my Rock life, leaving a tube amp on that long is usually considered a good thing. Ample time to warm up and all. It's repeatedly turned on/off in a short ammount of time that can cause weirdness. Hiss and hum (in my experience) has had more to do with stuff like bad grounding, dying tubes, shitty power strips and stuff to do with capaciters, faulty conections and other more exotic innerds type things. Not having a fan blowing on the back of the amp can also cause problems with overheating. Not sure if hissing is one of those problems.

First thing though,
I would check for snakes.

Motherfucking snakes...

Dude anything is possible when you're recording a session. When we were recording "Without" we spent like 2 hours trying to identify this almost wet, slapping sound in the background of one of our softer songs. It was driving us nuts. Finally Bart fessed up and admitted he had been masturbating in the room adjacent to the vocal booth during the take.

See?

First-On-The-List,

Check for snakes...

damn filthy motherfucking snakes.

Hmmm...we did actually check for snakes, but no go. The hiss wasn't present at the beginning of the session, and the only anything that changed was the temperature of the tubes. So. Thanks as always, gents. Anyone else? Sid?

My bass player in my last had an SVT-4 pro. He had nothing but problems with it. Wierd given the Ampeg name, that amp only has pre-amp tubes, he had to replace them constantly, he also had to have it repaired once for buzzing, aparently he moved it when it was hot and broke some solders, I do know those heads get real hot, given the amount of wattage, (1200 or something right/), anyway, the guy at Lesmans knows Ampeg pretty well, and was able to fix it(constantly). Hopefully I may of helped.

If not, Sid probably can, I know he just had that problem a few weeks ago and changed the tubes out.

why are you recording with the direct out and not using a mic? from my experience (with guitar amps) the tone on the direct out usually is pretty sucky

Sucky isn't how I'd describe either tone, bass and guitar. There were a lot of reasons we're going direct. We can record "silently" when my daughter is at home (asleep, etc.). Last time we tried to do some recording, we just couldn't duplicate Lance's sound with a mic (or 4 for that matter). So basically we went with what sounded best to us on my rig. We try not to get sucked into the whole "do-this-because-that's-how-it's-done" mindset. We're lucky that we're not paying anyone and we've got the time to experiment.

I agree with Webmonkey, try micing and see if the hiss is there, then direct boxing it and compare. Might be a bad cable for the DI-happened to us. One of the most useful tools in the studio is a cable tester. Might be a bead DI box too.

I do appreciate the suggestions. Maury, take it as a given that we tried everything and the hiss is definitely coming out the DI on the back of the amp. We DI'd the bass straight in to make sure it wasn't the instrument itself, and it wasn't. So back to the original question: Could heat off the tubes potentially introduce a hiss out of the DI out? (And actually it's line out, and a cool thing about the Aardvark Q10 is that you have 3 different input operating levels including line, so we don't even have to introduce the sound of my Whirlwind Imp to the signal.)

In a word, Mike, yes. Tubes produce some noise. Because the tubes handle an electrical current, they produce some hum, just like a light bulb does. Usually it is inaudible. Somehow the low level noise of the tubes has found its way into your output signal and been amplified to an annoying level.

Which vexes me thusly, because this is definitely a "hiss" as opposed to the usual inaudible warmth added at like 200-300Hz or so with a tube, but I'm not very experienced with tubes. Only enough so to know that they can be moody. Anyways, we're tracking again manana, so I shall definitely keep everyone updated. And yes, we really are taking every electronic instrument--direct, line level.

When a tube is going out and becomes microphonic, it will start to "hiss" or squeal. Here's how to test it: take a pen and, while the amp is on and the tubes are fired, tap on them and see if you hear the sound of the tapping coming out of the speaker. If you hear the tapping, then you have a microphonic tube. If you don't hear the tapping, then your tube is ok (probably) and the sound is coming from elsewhere.

Thank you, sir. It will be the first thing we do tomorrow. So I'm guessing that the heat off the tubes won't affect signal on the DI?

The heat itself, no. But tubes can become noisy for many reasons, as can amps. Sometimes tube amps hiss because the filter caps have worn away over time and need to be recapped. There are just so many variables. In all honesty, most of the tubes being produced and sold right now are crap and have a very short lifespan. They just don't make them like the old RCA's and Sylvania's.

btw the amp is brand new. about 2-3 months old. a few gigs, but nothing major. we got one song done in the first 3 hrs, but in the third hour is when this hiss appeared. no changes were made.

if its a problem with the DI, chances are, the board pres are causing the stress, not the amp.

needless to say, mc's right. leave a tube amp all fucking day. it can only get better. the same can't be said for the board that's receiving the DI.

also, those svt's have really hot DI's. the only way to tame them is to use a good active DI box, ie. Countryman or similar (FET electronics). Or, use a really weak signal on the board to minimize hiss and pump it up afterward if needed.