Ampeg SVT-3 Pro Tube Gain Issue

[MeestorEno]

I recently purchased a SVT-3 Pro off of eBay for $400. It is an older model and I think there may be a tube out, but I am not sure. When I turn the Tube Gain nob it does not seam to do much besides add very little sustain. And when I played other ones (they may have just been newer) I would get that cool tube overdrive if I cranked it up. This one does not do that. Also it seams as though the nob has been taken off and put back on. The line does not start and stop when it is turned in the same location as the others. So maybe the nob could have a bad connection. But there still is the short power loss when the nob is turned, but it does not seam to effect the tone much. How can I tell is there is something major like a bad tube without having to bring it to a professional that could cost a lot of money?

Either way I am going to contact the seller and request that I get a partial refund of around $100. Does that sound fair? Other than that the amp sound freekin killer!

[paganjack]

Well, a bad tube in an SVT-3-pro will cost you about $10 (each) to replace. It's a hybrid amp, so there are a few preamp tubes (IIRC: 3x preamp and 1x driver tube) but a SS power section. Frankly, taking it to a tech to replace the knob is probably more expensive than replacing a couple tubes. It might just be the knob cap that's loose too. That's an even easier problem to fix-just pull it off ( unless there's a screw to loosen) and re-place it!

Changing preamp tubes is easy. Search around- there's plenty of info on how to do it.

[kranahan]

take off the top and see if the tubes are looking alright. look at what color they are when the amp is on. there should be 2 driver tubes in the back left of the amp and 3 preamp tubes in the front left of the amp.

[Low Sound Love]

just to prevent misunderstanding: the 'tube gain' knob is supposed to sound most tube-y when turned hard CCW. In Hard CW position, you should get a more modern sound.

From the SVT 3 Pro manual: At “10” the voltage is at maximum, providing a dynamic, highly responsive tone. At “0” the voltage is at minimum, offering a thickened, more compressed tone. This tone can also be distorted, depending on volume level.

I gave my SVT3 Pro to a tech for a complete check-up and tweaking (I bought it second hand too), it wasn't cheap but it was well worth it, since now I have the sound I wanted (I have my tube gain at zero, for the tube drive sounds).

[MeestorEno]

So how much of a tone difference is there supposed to be via the Tube Gain nob? Because as I said there is very very little tone difference, or maybe even none and I just think there is. The other newer ones I played were able to get a tube overide tone. I took off the nob and turned it directly and still no difference. Could it be the tubes? Because there is no microphonic noise, or any negative noise really. Or is it a bad connection with the nob?

[prd004]

The tube gain knob on the SVT-III Pro is decieving. I have the first generation model and they didn't even call it tube gain, it's labeled plate voltage. It's more of a natural compressor, it's not going to add "Grit" or tube distortion to your sound. When you open it all the way up what you'll really notice is more volume. When you dial it back it will sound kinda compressed with less overall top end volume and less peaks.

The input gain is where you will get that "Ampeg Sound"

[JimmyM]

Originally Posted by prd004

The tube gain knob on the SVT-III Pro is decieving. I have the first generation model and they didn't even call it tube gain, it's labeled plate voltage. It's more of a natural compressor, it's not going to add "Grit" or tube distortion to your sound. When you open it all the way up what you'll really notice is more volume. When you dial it back it will sound kinda compressed with less overall top end volume and less peaks.

The input gain is where you will get that "Ampeg Sound"

+1. crank the gain HARD!

[MeestorEno]

But is it a noticeable difference? Because I can not hear a difference at all on mine. Is there something in the amp that could be causing it to not function properly such as a bad connection or a bad tube?

[vin*tone]

Congrats on the few amp man. I'm not sure there's anything wrong with it at all actually....the tube OD is pretty subtle and different units seem to behave a little differently. There is very little effect unless you're pushing some air. Maybe you could try a new tube in the driver position,...... not sure about that one.

- It's unlikely the pot is loose, my guess is the knob is just on a little crooked. If you remove the knob turn the pot hard left and put the knob back on straight.

- The little silence as you turn the knob from zero is totally normal. You'll hear silence, then a bzzzzzzzzzt then bass. This is actually mentioned in the manual as well.

I would not ask for a refund. With respect - the seller would probably think you are nuts/ trying to claw back some cash.

They're beautiful units man. Enjoy.

[mdogs]

Agree with everything that has been said. I just got one of these in a trade for my SWR SM-400, there are a million tones in there and it takes serious play time to figure them out. I don't think you have a problem...

[Jerrold Tiers]

Originally Posted by paganjack

Well, a bad tube in an SVT-3-pro will cost you about $10 (each) to replace. It's a hybrid amp, so there are a few preamp tubes (IIRC: 3x preamp and 1x driver tube) but a SS power section.

Almost, but not quite correct....... And it affect this question...

The power amp is hybrid.... NOT SS.

The two tubes in the power amp do all the drive for the mosfet output stage, so ALL the "gain" is tube, and the mosfets are basically functioning like an output transformer, "repeating", or "matching" the tube output to the speaker.

The "tube gain" (actually a plate voltage control) works ONLY on the power amp tubes, it does nothing to the preamp rubes whatsoever.

So...... if you set it low, which is more 'tube clipping" it still won't do anything unless you are pushing the power amp. In that way it is very much like many 100% tube amps, it requires the power amp to be driven hard in order to have a difference.

[Verminaard]

If it has the original tubes in it, you might want to just spring for a nice new set from Eurotubes. The JJs do make quite a difference in the 3 pro. And it's only $50.00 for the set. The original tubes aren't of the best quality and the upgrade is well worth it in my opinion.

[Foz]

upgrade is well worth it in my opinion.

+1

also well worth the effort to have the bias checked and set on the mosfets - this amp comes alive when these are set properly - nice bit of kit.

and do like Jimmy sez and push the input gain hard - if that little red light aint lighting up - u r doing it wrong.

[stiles72]

Originally Posted by MeestorEno

So how much of a tone difference is there supposed to be via the Tube Gain nob? Because as I said there is very very little tone difference, or maybe even none and I just think there is. The other newer ones I played were able to get a tube overide tone. I took off the nob and turned it directly and still no difference. Could it be the tubes? Because there is no microphonic noise, or any negative noise really. Or is it a bad connection with the nob?

+1 to what Jerrold has posted

I have two 3-Pros, and unless you are running your input gain hot, it's very possible you won't notice a significant difference in sound the tube gain (plate voltage) knob. Took me a while to figure it out. What I notice is that as the knob is turned clockwise, you start to get a little more "presence" or definition around 11:00 on the dial. It's very subtle - and it's not like the drive control on the SVT-2Pro which actually gives you a guitar like distortion/fuzz. I use the tube gain around 9:00, but in live settigns if I need just a little bit more bite I dial it up to 11:00 - 12:00. I used to run it wide open to get more level - but that was before I understood about how to set that input gain.

[mdogs]

One of the issues with the amp are the recommended settings that Ampeg put in the manual. The settings sound good, but they DO NOT tell you what this amp is all about...

[Verminaard]

I also find the tube gain dialed at 9:00 sounds best and most tubey. Lower than that seems to sap some of the presence/ girth of the tone. Input gain set to when red light is on about 75% of the time or so balanced out with the main volume knob, because they work reactively with each other.

[fuadramsey]

Since most things are now digital I've always had the feeling that you were clipping if the light was coming on too much. But since it's a tube preamp are you actually driving it harder versus "clipping?"

Is there an issue if you drive it too hard, or run the preamp all the way open? Or is it a tailor it to your sound deal?

Originally Posted by JimmyM

+1. crank the gain HARD!

[rickdog]

Originally Posted by fuadramsey

Since most things are now digital I've always had the feeling that you were clipping if the light was coming on too much. But since it's a tube preamp are you actually driving it harder versus "clipping?"

The "PEAK" light comes on long before you hit hard clipping levels. Assuming you like classic Ampeg tone, this light comes on to show you the signal level is sufficient to reach the sweet spot. Like JimmyM said, crank it up!

The light is driven by sensing each tube's output, so it will come on when the signal level gets high enough anywhere in the preamp.

Originally Posted by fuadramsey

Is there an issue if you drive it too hard, or run the preamp all the way open? Or is it a tailor it to your sound deal?

It's tubes, man, not delicate little transistors! If you push it too hard, it will sound bad long before you're going to damage anything. Just turn it up and listen for where it sounds good to you. If the light worries you, put a piece of black tape over it

Keep in mind that there's one tube stage before the Gain control. If the light comes on a lot and you hear unpleasant distortion even with the gain all the way down, turn down your bass or use the -15 dB pad switch.

[NKBassman]

I keep the gain on my 3PRO between 2:00-3:00. The "clip" light is pretty much on continuously while I'm playing.