I am wondering if a tube swap would change the "Tube Gain" control response on my SVT3-PRO. I've rolled the tubes when I bought it (decent quality, not necessary hot/aggressive). Does anyone know which tube sockets have the greatest influence on the Tube Gain control?
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Tube Gain pot on a SVT3-PRO
- Thread starter therumour
- Start date
What is it exactly that you're hoping to change about the response you're getting now?
Any response, really. I've read that you have to push the amp for the Tube Gain to react. I'm using one 8 ohm cab, and playing at a "moderate" volume.
So my question was can tube selection (and placement) cause the Tube Gain to react at lower volumes.
So my question was can tube selection (and placement) cause the Tube Gain to react at lower volumes.
I wasn't that thrilled with the tube gain control in it either.
Don't be afraid to push the gain (regular) on the 3pro. That'll give some saturation
Don't be afraid to push the gain (regular) on the 3pro. That'll give some saturation
I've run some rough numbers and graphs for V1B with a 5751, 12AU7, 12AY7, and 12AT7 with the existing values of Ebb (300 V), Rp, and Rk. The curves show none of those tubes will do what you want *and* still sound halfway decent. One solution would be a hotter biasing of the V1B triode so its grid will draw current at "some TBD" high-ish setting of the Gain control. However, this might be a drawback for situations that require a very linear/clean tone, and/or it might make the gain control "touchy" with high-output instruments. So, there might be that loss of flexibility.
If a tech really wanted to be creative, a variable bias arrangement with a front panel control could be devised for V1B, to permit the user to change that triode's bias at will, and make its grid more or less apt to draw grid current. But such a modification would cause a total loss of the amp's safety certification. As a (retired) tech I would never accommodate such a request, for liability reasons. And the resale value of such a modified amp might drop considerably, if it could be sold at all.
I think your best bet is to hit the input with a stronger signal. But even then, I believe you'd have to hit it with at least 1.5 volts RMS before you heard any (desired) nonlinearity. At least that's what my numbers tell me.
If a tech really wanted to be creative, a variable bias arrangement with a front panel control could be devised for V1B, to permit the user to change that triode's bias at will, and make its grid more or less apt to draw grid current. But such a modification would cause a total loss of the amp's safety certification. As a (retired) tech I would never accommodate such a request, for liability reasons. And the resale value of such a modified amp might drop considerably, if it could be sold at all.
I think your best bet is to hit the input with a stronger signal. But even then, I believe you'd have to hit it with at least 1.5 volts RMS before you heard any (desired) nonlinearity. At least that's what my numbers tell me.
Based on the description from the manual and a brief look at the block diagram and schematic I would say perhaps focus on the 12AX7 PI and 12AU7 driver if you’re planning to tube roll and see how the voltage control changes the feel. I wouldn’t expect much change.
13. TUBE GAIN: The tube gain control
varies the high voltage supply to the
preamp tubes. This allows a variety of
tonal response characteristics from
the power amp and replaces the limiter
found on typical solid state power amps.
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 may also be distorted,
depending on volume level. In between
settings are best for preventing harsh
distortion when driving the power amp
to its limits. The effect of this control
increases from moderate to dramatic
as the power amp is driven harder
13. TUBE GAIN: The tube gain control
varies the high voltage supply to the
preamp tubes. This allows a variety of
tonal response characteristics from
the power amp and replaces the limiter
found on typical solid state power amps.
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 may also be distorted,
depending on volume level. In between
settings are best for preventing harsh
distortion when driving the power amp
to its limits. The effect of this control
increases from moderate to dramatic
as the power amp is driven harder
This.
Tube gain will affect your headroom to some extent but the real key to driving this head is to set your input gain hot. Aim to have the red clipping light on every time you hit a note and then back it off barely at all. That’s where the III Pro sounds best.
In my experience, tube rolling doesn’t have much impact on the behavior of this head. The gain knob does. The tube gain has an effect but not a big one. Also, the gain and tube gain are somewhat interactive.
If you want to make that kind of change take it to a tech and explain what you are after. They may or may not have an idea? Not to mention the possibility of damaging the amp??
Played an SVT3-PRO (with a PR-410HLF cab) for many many years. Never found that the Tube Gain knob did much, but it did add a bit of what I call "smoothness" to the tone...a warm, buttery, goopy, drippy smoothness. Played with a drummer/bassist who had the B2R (all solid-state) amp, and occasionally left my rig at his place for rehearsals. Compared to his B2R, he absolutely LOVED the SVT3-PRO. So what I thought was "not much," he adored. It cost quite a bit more to have that one knob, but I was glad that I did have it.
I did replace the tubes once, but just went with JJs according to the recommendations and placement (4-12AX7s, 1-12AU7), simply for reliability. I did bias the tubes once, as I was having a low volume distortion problem. If you need to do this, be very very VERY careful. The adjustment potentiometer was very difficult to turn, and you only need to turn it a you-know-what hair. You have to use a small flathead screwdriver, and I had to attach vise-grips to the handle of the screwdriver for better leverage as I could not turn it with my hands. If you're not extremely careful, once the pot breaks free to turn you can easily turn it too much, which will blow the amp up (from what I understand). I was successful adjusting the bias correctly (you also need to use a multimeter to keep track of something while you do the adjustment...it was a long time ago so I really don't remember the specifics anymore), and solved the distortion problem.
I did replace the tubes once, but just went with JJs according to the recommendations and placement (4-12AX7s, 1-12AU7), simply for reliability. I did bias the tubes once, as I was having a low volume distortion problem. If you need to do this, be very very VERY careful. The adjustment potentiometer was very difficult to turn, and you only need to turn it a you-know-what hair. You have to use a small flathead screwdriver, and I had to attach vise-grips to the handle of the screwdriver for better leverage as I could not turn it with my hands. If you're not extremely careful, once the pot breaks free to turn you can easily turn it too much, which will blow the amp up (from what I understand). I was successful adjusting the bias correctly (you also need to use a multimeter to keep track of something while you do the adjustment...it was a long time ago so I really don't remember the specifics anymore), and solved the distortion problem.
Heed what @JoeyFab says about the sensitivity of the bias adjustment! Yes, you can instantly toast the output transistors by turning it too far, which is much less than you might think it is. I recommend leaving it alone unless you hear a continuous background fuzz tone when playing at lower volume. Read https://www.talkbass.com/threads/svt3pro-bias-setting.437406/; note that Jerrold Tiers was a design engineer at Ampeg and very familiar with the design (he might have designed the 3Pro, I don't remember).
My experience with my SVT-3Pro is that the input Gain control is the most important. I'd suggest turning it up all the way; if that's a little too much (possible with a hot bass) back it off until you like it. The Peak light is more like a "signal present" indicator, it should have been green instead of red.
I've never noticed much change from adjusting Tube Gain, probably because I don't push the amp loud enough for it to have much effect. It's possible you'd hear some difference from replacing the 12AX7 next to the 12AU7 with another compatible tube. But I'd leave that alone, crank the input gain, and enjoy the thickness and saturation from the preamp.
My experience with my SVT-3Pro is that the input Gain control is the most important. I'd suggest turning it up all the way; if that's a little too much (possible with a hot bass) back it off until you like it. The Peak light is more like a "signal present" indicator, it should have been green instead of red.
I've never noticed much change from adjusting Tube Gain, probably because I don't push the amp loud enough for it to have much effect. It's possible you'd hear some difference from replacing the 12AX7 next to the 12AU7 with another compatible tube. But I'd leave that alone, crank the input gain, and enjoy the thickness and saturation from the preamp.
The svt3pro is one of the best amps, but less than straightforward
The input gain gets syrupy and tubey at or past 3 o'clock.
Ignore the red light and go by your ear.
Often the red light being almost steadily on sounds better than just below it being on.
As far as the tube drive control,
It's counter intuitive.
All the way up and it sounds like a solid state - very responsive, quick, and clear.
All the way down and it's pillowy, tubey, and vintage.
Try different combos of input gain and tube gain to find your sound.
Oh yeah, and max out the post-eq gain slider all the way to the right of the graphic eq. Even if u don't use the graphic eq, engage it, keep the graphic eq flat, and max the post eq gain slider. That pushes the last stage of the preamp harder.
Really, there's no need to mess with the amp and tube-rolling doesn't have a big impact like with other preamps.
It is, however, an easy amp to fry by messing with it.
The input gain gets syrupy and tubey at or past 3 o'clock.
Ignore the red light and go by your ear.
Often the red light being almost steadily on sounds better than just below it being on.
As far as the tube drive control,
It's counter intuitive.
All the way up and it sounds like a solid state - very responsive, quick, and clear.
All the way down and it's pillowy, tubey, and vintage.
Try different combos of input gain and tube gain to find your sound.
Oh yeah, and max out the post-eq gain slider all the way to the right of the graphic eq. Even if u don't use the graphic eq, engage it, keep the graphic eq flat, and max the post eq gain slider. That pushes the last stage of the preamp harder.
Really, there's no need to mess with the amp and tube-rolling doesn't have a big impact like with other preamps.
It is, however, an easy amp to fry by messing with it.
I know when I started playing mine, I may have been of the same mindset as the OP thinking it would be like a tube drive control. It adds harmonic tube content and gets you that just at the edge of breakup when set right. Like everyone said, have the input gain higher than anything else. I set it high and then manage the master volume and tube gain from there. I think most Ampeg amps are like that.
The SVT-3 Pro is my favorite amp ever. I have so much fun every time I play it. The SVT-7 Pro I have has a lot of power and sounds really good, but with almost the same controls, it just doesn't have that something something to the tone and response that the 3 does. The 7 will push easily into breakup unlike the 3. I can get that 7 sounding really dirty, but I don't use that as an always on tone thing, so I prefer the 3.
The SVT-3 Pro is my favorite amp ever. I have so much fun every time I play it. The SVT-7 Pro I have has a lot of power and sounds really good, but with almost the same controls, it just doesn't have that something something to the tone and response that the 3 does. The 7 will push easily into breakup unlike the 3. I can get that 7 sounding really dirty, but I don't use that as an always on tone thing, so I prefer the 3.
Oh, yeah! I loan my 3Pro as backline for a small local festival every summer. One bass player who used it was getting a sound I really liked - he had the Gain control dimed and the red light was on solid!
This. I misunderstood the control for the first two decades of ownership and never understood why it didn’t make the sound I wanted. The third decade of ownership was much more fun.
I’d never heard this tip before. Can’t wait to try it.
