VVR in JCM800 50W?

[cddesign]

Has anyone installed a VVR in a JCM800 with good results? I am contemplating doing so and wanted feedback from others.

Did you scale the whole amp or just the power tubes? with or without PPMIV?

question questions questions

[dass101]

I did it on an 1987 MK2, scaled the power amp and the PI and installed a standard MV (pre PI). It works great, but keep in mind that you will need serious heat sinking for the FET, I blew a couple of them before installing an aluminium heat sink outside the chassis, and it still gets really hot. Ideally you would want at least 10x10 cm heat sink, with a fan if possible.

[cddesign]

Thanks Dass! It seems like the only thing i've heard so far is that heat will kill the mosfet in time.

[Reeltarded]

Before you do that just put a 220-270k resistor between the treble and master and parallel another 100k over the CF for 50k.

Surprise! The amp now sounds as nasty as you will ever want, as clean as you'll ever need, and the same from 0 to 9 and a half. As soon as the guitar can hear itself it starts crying like a Dumble fan on his 10th build.



haha I made a funny

PPIMV was my favorite thing.. until I tried it myself. Marshalls NEED NFB, and less isn't really better.

[roberto]

Before you do that just put a 220-270k resistor between the treble and master and parallel another 100k over the CF for 50k.

Really? Not 220k between MV and PI? Something around 56-68k as CF is quite good for that pourpose. I like the 100k on 4ohm tap NFB configuration.

[vibratoking]

+1 on the heatsink if you install VVR. I fried several MOSFETs in an Express and Tweed Deluxe until I installed heatsinks. Yes, it took 2 amps for me to learn the lesson.

[cddesign]

so i am researching heatsinks for the mosfet- any suggestions? I would mount it outside the chassis as well.

I still haven't heard if its worth all the trouble to even install the VVR on an 800.

[Colossal]

I still haven't heard if its worth all the trouble to even install the VVR on an 800.

I think that question is best answered by asking how you choose to intend to use the VVR. If you want to dime the amp to get sick cranked tone and then back off the VVR just a little so you can play a dimed amp in a club but still have it singing and loud, then yeah, a VVR might be a great strategy.

But if you are looking to dime an 800 in the bedroom and dial back the power down to bedroom levels, then you might be better off just adding a post-phase inverter master volume and be done with it. At a certain point where you are reducing volume, the tone is going to change because you stop moving air with the speaker. You are going to perceive that as a loss of low end, clarity and definition, increased fuzz, less sparkle and dimension, etc as the operating points in the amp change with reduced voltage. That key point for any attenutation method whether a PPIMV, a VVR, an attenuator or combination of any of these is when the speaker stops moving your pant legs.

To make it more unclear, some guys insist the VVR does a better job than a PPIMV (and that is true depending on the amp being VVR'd), others insist that a PPIMV is a great addition to a master volume Marshall and is all that is needed. It all comes down to what volume level vs. tone you are trying to achieve. The VVR does a good job of preserving the overall character of amp distortion but if you will not get that high-on-musical-crack feeling that a dimed amp gives you at bedroom level. Maybe run in stereo with a fat delay and stand in the middle.

I use a PPIMV in conjunction with the preamp gain and preamp master to find a sweet spot that loads the PI for more compression (and reduced output swing) and lowers volume down to reasonable levels where I am satisfied and inspired with my tone but not walking out of the practice room with ears ringing.

Hope that helps you somewhat.

[cddesign]

thanks for that . Yeah, i've tried the PPIMV and am still on the fence with it. I feel the sound gets way too compressed and doesn't sound like a jcm800 at all when it's used. I like having presense and negative feedback/"depth" control.

So i guess my question is will the VVR keep my "cranked tone" better than the PPIMV ?

[Colossal]

thanks for that . Yeah, i've tried the PPIMV and am still on the fence with it. I feel the sound gets way too compressed and doesn't sound like a jcm800 at all when it's used. I like having presense and negative feedback/"depth" control.

At low PPIMV, yes, the sound will get rather compressed, but you can compensate by using your JCM800 master. Ease off the master slightly; try to find a balance between the increased PI load (as you back off on the PPIMV) and decreasing the preamp signal going to the PI (normal master volume, post tone stack). You can edge up the gain slightly too if you don't get enough sing. I find this works comfortably with my modded marshalls for lower level jamming.

I run my controls thusly:

PPIMV: 8 (takes the edge off)
GAIN: 7
PREAMP MASTER: 3

Backing off on the PPIMV slightly does give a little more crunch but you still keep the feedback controls working. Yeah, the big downside to PPIMV is that controls in the negative feedback loop lose their effectiveness. I too use and highly value my presence and resonance controls. I find the Preamp Master to be highly effective with this setup different combinations of PPIMV and preamp master yield different volumes and tones at those volumes. I really try to keep the gain moderate. Too much and clarity suffers easily. I play 7 strings in standard tuning so I like that low string tight. I want to hear every note in a wide interval chord but I like that wide/fat low end. I think I've really found a pretty happy medium with just using the Gain, Preamp Master, and PPIMV in JCM type topologies in terms of being musically satisfied and not shredding my hearing.

So i guess my question is will the VVR keep my "cranked tone" better than the PPIMV ?

I've put them in a few different amps (all low to mid wattage Marshall type amps) and they work very well. It's like dialing your sound right down. There is a point though that the tube operating characteristics change dramatically enough and coupled with the point where you physically stop moving air. You will hear your distorted tone, but the 3D shimmering quality that we enjoy at volume begins to fade and it sounds more like listening to an mp4 of yourself; kind of monochromatic. Still good enough for late night jamming.

It will also depend on if you scale the whole amp or just the PI and power section. Part of the high gain tone often comes from high preamp voltages. So if you opt to leave the preamp operating at full voltage scaling only the power section, then you have to reduce the signal entering the PI so you will need to compensate by reducing the preamp master volume anyway otherwise the PI would be hard clipped. It would probably sound very fuzzy and juicy but muddy. Great for Sabbath riffs for sure.

You can also scale the whole amp.

[John_P_WI]

+1 on what Colossal said. Also, search here for either a bootstrapped master or improved bootstrapped master. These will help keep your tone over the existing master as you dial things down.

Power scaling / vvr is another tool, you may find that trying Reels master volume and CF advice above (or one of the masters I mentioned) may take the edge off enough that you don't need power scaling.

I use power scaling and combinations of pre and post masters depending on the tone and "tube" coloration that I am looking for.

Good luck.

[Reeltarded]

Before you do that just put a 220-270k resistor between the treble and master and parallel another 100k over the CF for 50k.

Really? Not 220k between MV and PI? Something around 56-68k as CF is quite good for that pourpose. I like the 100k on 4ohm tap NFB configuration.

CF at 50k or near it with a 220-270k from TREB to MASTER. Try that. Rawk.

Oh, add: i used to like 100k on the 4 tap, but I love 100k on the 16 way better now.

[kdmay]

Just buy an Alex and be done with it. Balance the attenuator and the PPIMV and you will get killer tones without upsetting the family or neighbours.

I am very happy with the transparency of the Alex