PPIMV vs. regular MV: a comparison in a Marshall-style circuit

[El Caballo]

One fun thing about building and owning a kit head is that you're not afraid to try different mods. Just dig out the soldering iron and cowboy up!

Test rig: my mutt JTM45/1987 Weber kit head with KT66 output tubes and generic Chinese 12AX7 preamp tubes, switchable "one wire" cascading preamp mod, and other tweaks.

I've been running a Rich mod PPIMV for a while, but I decided to add a regular 2203-style MV (post-tone stack, pre-phase-inverter) to the mix. I know that neither one is completely out of circuit when dimed, but the difference in load is pretty subtle, and the point is to compare one against the other.

All comparisons done at moderate to loud bedroom volumes.

Round 1: PPIMV used for attenuation, regular MV dimed.

Crunchy, crunchy, crunchy. Throttling your volume with a PPIMV basically disconnects your negative feedback loop, and since the speakers aren't working hard, they can reproduce all those sharp little spikes coming out of the phase inverter that get smoothed out at higher volume. It's the opposite of lifeless...too lively, like your presence is on 15. Your presence control, and your resonance, and any other feedback loop mod do absolutely nothing with a PPIMV turned down low.

It works well (if scratchy) when your preamp is dimed, but those "on the edge of breakup" tones get fizzy.

Round 2: Regular MV used for attenuation, PPIMV dimed.

Wow. Who knew that the phase inverter was making so much of your distortion? The tone gets much, much cleaner, and dare I say, somewhat flat. However, you'll be pleased to note that the tone controls actually have an effect, since they're after all the distortion. Cranking your presence up helps, and if you have a resonance mod, crank it up too, and you'll get a lot of liveliness back. Better for clean tone, and less obnoxiously fizzy for "edge of breakup", but also two-dimensional, since you're basically losing a whole gain stage worth of clipping.

Round 3: "One wire mod" engaged.

This mod (at least the way I did it) takes the two parallel input sections (normal and bright) and cascades them, running the Bright channel into the Normal channel. It's more gain than anyone can use, and in fact, out-of-control oscillations ensue at Bright gain above 3 and Normal gain above 8. (Maybe a less microphonic preamp tube will help? Anyone have some hints?)

This mod actually works really well with a regular MV, because the distortion you lose from the phase inverter you get back with the extra preamp stage. Plus the tone controls are now all post-distortion, so you get some neat tone shaping ability, and something similar to a high-gain PPIMV tone, but with presence and resonance that actually work as advertised. I can see why Marshall used this approach in the 2203 Master Volume and later heads. Because of this, a 2203 with the MV low is probably going to sound more like a 1959 than a dimed 2203.

With a PPIMV, there's just way too much gain with way too little NFB, and everything gets really scratchy and splatty. On the other hand, it's cool to get howling feedback at conversational levels, and some people might like the totally out-of-control nature of the sound.

Moral: Neither one will get you cranked amp tone. Both fall short in different ways. I actually like having both, because the regular MV lets me control how much distortion I get out of the phase inverter, and then I use the PPIMV for master volume.

At some point my Weber Mass Lite should show up, and I'll offer my thoughts on that once I've had some quality time with it.

[ChickenLover]

Do you have a bypass cap on V2a?

When doing the one-wire mod...it is too much gain. A 2203 would have an unbypassed 10K resistor on the second stage. What you can do is set up the second stage like that, then just bypass it in 'plexi' mode and use it in '2203' mode. The interstage stuff won't be exactly like a 2203 but it will be set up closer to what some 'mods' suggest.

When doing any thing like the one-wire mod or this 'plexi/2203' mod you have to be careful what you do with grid wires...those are most prone to oscillations. Grid stoppers right on the socket can help with the oscillations and unless they're fairly large won't affect tone that much.

But in the end I agree...Volume...there is no substitute.

[El Caballo]

You're right...the 2203 MV preamp is exactly a cascaded 1959, but with a 10K cathode resistor on V1a to tone down the gain. I could certainly do that, but I'll need a 3PDT switch.

Wouldn't the 10K cathode resistor bias V1a really close to cutoff? That would certainly account for the crunchiness of a JCM800. Ideally it seems like you'd pick a lower value Rp to go with Rk if you wanted less gain.

The 68K mixer resistor for the input is already on the tube socket.



Anyway, back to the MV discussion...I hope this is of use to people wanting to figure out what the different types of MV are good for.

I think a PPIMV works really well for moderate attenuation, because the NFB still does its job. It won't get you down to bedroom volume without dramatic tone change, but it will take it down to normal gig volume from ridiculously loud.

I think a regular MV is kind of useless by itself on 1959 and other NMV heads, but it can work reasonably well if you push it with a dirt pedal to make up for the loss of a gain stage. The exception is this: you can get a good clean jazz tone out of a Marshall! Turn the mids to about 1/2, turn Treble off to darken the tone, and turn the MV down enough so that you're not pushing the phase inverter.

[paulg]

El Caballo: Is there a schematic for this "one wire" mod? Does it actually use one wire?
Thanks for the observations. My experience is MV (any of them) seem good for headroom reduction. Attenuators drop the volume levels even more (in a good way). I'm now experimenting with Power Scaling to see if it provides "bedroom level rock!".

[Bussman]

Here ya go!
http://www.lynx.bc.ca/%7Ejc/jtm45oneWireSml2.gif
Have fun!

[El Caballo]

El Caballo: Is there a schematic for this "one wire" mod? Does it actually use one wire?

That schematic is impossible to read. Here's what's happening: you take the 270K or 470K mixer resistor (depending on which amp you start with) from your bright preamp channel, and disconnect the end where it joins the other mixer resistor. Then you solder a wire between that and where the normal channel enters V1. Thus the "one wire" name.

Result: instead of the normal and bright channels being mixed, the bright channel feeds into the normal channel. Obviously you lose the original circuit at this point, and you get uncontrollable electronic feedback at most volume settings. As ChickenLover pointed out, the thing to do in this case is change the normal channel Rk/Ck to a 10K resistor and no cap, at which point you have basically made yourself a JCM800 preamp.

If you want to make it switchable, it's a lot more work, because high-gain circuits like to oscillate when you run wires all over the place. It's no accident that amps with BREWTAL GAIN almost always use PCBs.