MV in Plexi build. Supposed to sound like this?

[shocki]

I just wired up a La Mar MV in my plexi. It sounds ok with gain on 12 o clock and the MV quite low (i mean bedroom volume). With more gain it sounds really fizzy. It gets better when I turn up the MV.
I am pretty sure this is quite normal? With just one tube I cannot expect a JCM800 kind of sound and the distortion will get ugly at lower volumes?

I am wondering if the "one wire" mod would be nice. But I have a Super Bass not a Super Lead. So it might get muddy. I want to keep my Plexi pretty much stock. Don't want to mess with the basic sound. But I would like to try out some things.

[Chuck H]

I'm confused. Are you operating a push/pull amp with one tube?

[shocki]

Just one preamp tube While the JCM800 has two preamp tubes cascaded. So what I am doing with the MV is cranking the volume from this one tube and lowering the output at the PI. I pretty much get PI distortion. And I think this cannot sound good at higher gain stages.

I think it is really nice for getting a crunch sounds at lower (but not bedroom level) volumes and then boost it with a pedal.

[Chuck H]

I haven't heard the problem your having in any PPIMV amps. The effect I hear most with most types is the tone becoming compressed and muddy at low volume settings. This affect becoming less pronounced as you increase volume. Regardless, I don't think you should be hearing other differences. That isn't to say that the MV won't sound fizzy. They often do. But it should be most fizzy at the lowest volume and gradually less so as the volume is increased.

Check bias voltage at different MV settings. Check AC signal voltage at the power tube grids at different MV settings too. If anything is miswired these checks should show it. Can you provide a drawing of your actual circuit? Not just a link the the schematic you used.

[shocki]

But it should be most fizzy at the lowest volume and gradually less so as the volume is increased.

Yes that is the case. This is the layout I used. Here you see the MV also.

http://www.ceriatone.com/images/mods...rontCeriat.jpg

[Chuck H]

I think I misread your original post. Your experience sounds normal then. PI distortion IS fizzy. The power amp is normally clipping before the PI so without the MV some of the fizziness is filtered. Making things worse is the possibility that the power amp, if biased hot, may actually operate close to class A at low drive levels. That means your also hearing the OTHER clipped side of either PI drive signals that is normally hidden during the power tubes cutoff cycle. A scope is the only way to see what's happening so that it may be identified and minimized.

I dislike master volumes for everything but uber gain type amps. They're an essential component in that sort of amp. But for amps that generally include power tube clipping as part of the tone, and usually the predominant clipping stage, master volumes of any kind just can't preserve the tone with any integrity at reduced volume. That's why I use attenuation instead. But that's just my preference. There are many players that use their master volume controls effectively as a power amp clipping control. No volume reduction with this method. But it does allow players to choose the amount of power tube clipping to balance with the preamp tone. There are no rules for this. If it sounds good, it is good. Even if it's just the players opinion. So based on your expressions I would think that you don't particularly like master volume circuits for classic style amps either. Maybe it's time to build an attenuator. Maybe try one out (if you can find one) to see if you prefere the affect.