I am not sure I agree with what you are saying. A lot of guys have said they love the MV in a Rocket amp. The Rocket's gain structure is almost identical to a Marshall ( Gain Stage/Cathode follower/Tone Stack/PI). In the Express you have a Gain Stage/Tone Stack/Gain Stage/Gain Stage/PI . Most of the distortion in the Rocket/Marshall is going to be overdriven preamp tubes and not Power tubes unless you crank the amp full tilt. In the Express you got distortion from the power tubes with the Vol control set to less than 12:00. At that point the distortion you are hearing is power tube distortion. If you put a MV in the Express to be used as a mod to reduce overall vol you will loose a lot if not all of the power tube distortion. The fact that the Express preamp can produce a lot more gain than a Marshall or Rocket has little to do with it. You have absolutely changed the structure of the amp and the way it was designed to operate. My only point in my last 2 posts was to try and make the OP aware that he is comparing apples and oranges. The MV and the VVR do two absolutly different things in an amp. You may love the sound of the MV in an Express but IMHO, if you install one you don't have an amp that works, resonds or sounds like an Express anymore at volumes reduce by the MV. I'm not saying it is better or worse just different. There are a lot of mods you can do to any amp and some people will like them and some won't. That is subjective. I am just trying to point out the differences. Take it or leave it.rooster wrote:Dana - OK, I get the design idea behind the VVR and I know that you are describing it correctly. And no, I am not trying to take anything away from the VVR. This said, please keep in mind that the Express has MORE gain in the preamp section than a Plexi Marshall. So when you concede that such a MV (LarMar type) might work in a Marshall, then it will certainly work even better in the Express. I also concede that preamp distortion alone is different than preamp distortion combined with output tube distortion, not a question. But in the vein of controlling an amp's output, an amp like the Express for one - at a gig or in a bedroom - I think the LarMar MV is a very good tool, and yes, easily a better tool than the airbrake.
The LaMar (LarMar) PPIMV in an Express
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Re: The LaMar (LarMar) PPIMV in an Express
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Jackie Treehorn
- Posts: 236
- Joined: Thu Jan 20, 2005 4:54 pm
- Location: New Orleans, LA
Re: The LaMar (LarMar) PPIMV in an Express
The Express, like a lot of fixed bias amp, has an onset of distortion which occurs when the positive signal swing from the phase inverter equals the bias voltage. The impedance of the power tube grid drops and it begins to draw current which the AC coupled long tail pair can't source, so it clips.
You really can't get any significant power tube distortion with this topology, in the sense that you put a clean waveform on the grid of the power tube and the plate is distorted. It's a phase inverter distortion, really. So, there's two ways to clip the phase inverter, on the output when it's pushing up against the bias voltage, or exceeding the headroom on its input. Most amps are made with sufficient swing in the driver to exceed the bias voltage in order to get full wattage, of course. If one sticks a post phase inverter master volume in there, you could still clip the PI on the input. The question is, does it sound any different with that clip?
I've been thinking about how great it would be to have a master volume Express, so it's something I definitely want to try. However, I'm not going to do it under the constraint that it's stock with the master dimed. I'll probably not use global negative feedback, but go with local feedback on the phase inverter and the power tubes (either ultralinear with maybe a Dynaco OT, or use a wreck output transformer with cathode feedback). I also think 220k is still too big a value for the pot. It's too much series resistance when dialed down, imo.
But, the post phase inverter master has a lot of advantages including that it does maintain all the operating points you choose; you're simply clipping the phase inverter in a slightly different way.
You really can't get any significant power tube distortion with this topology, in the sense that you put a clean waveform on the grid of the power tube and the plate is distorted. It's a phase inverter distortion, really. So, there's two ways to clip the phase inverter, on the output when it's pushing up against the bias voltage, or exceeding the headroom on its input. Most amps are made with sufficient swing in the driver to exceed the bias voltage in order to get full wattage, of course. If one sticks a post phase inverter master volume in there, you could still clip the PI on the input. The question is, does it sound any different with that clip?
I've been thinking about how great it would be to have a master volume Express, so it's something I definitely want to try. However, I'm not going to do it under the constraint that it's stock with the master dimed. I'll probably not use global negative feedback, but go with local feedback on the phase inverter and the power tubes (either ultralinear with maybe a Dynaco OT, or use a wreck output transformer with cathode feedback). I also think 220k is still too big a value for the pot. It's too much series resistance when dialed down, imo.
But, the post phase inverter master has a lot of advantages including that it does maintain all the operating points you choose; you're simply clipping the phase inverter in a slightly different way.
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Cliff Schecht
- Posts: 2629
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- Contact:
Re: The LaMar (LarMar) PPIMV in an Express
+1Cliff Schecht wrote:Great post Jackie!
What I know is that my express' tone sounds terrific even when the MV is dialed down. Still maintains touch sensitivity as well. I'll try to make some clips with various setting on the MV.