Opinions on pre and post phase inverter volume controls

[interloper]

Anyone have any experience with these.

My 80 watt 4x10 Traynor amp needs to be cranked way too loud to achieve any saturation. Would these mods produce a more saturated tone and a nice smokey blues distortion at lower volumes?

 

[JohnnyCrash]

Yes.

I choose what MV based on the amp.

Most times though I prefer the post phase inverter master volume (PPIMV). The main difference is the PPIMV also gets the extra gainstages of the phase inverter tube in there as well. Instead of just controlling the amount of preamp gain that hits the PI, you're now controlling how much preamp and PI gain hits the power section.

On high gain amps it depends. You could even do both on one high gain amp

If your amp is similar to a nonMV plexi or Bassman, I'd go with the PPIMV.

It's essentially a dual ganged pot and two new resistors (about $5-$10 in parts and 20-90 minutes of work) - you'll have to pull the gridleak resistors from the PI to the power tubes and put the PPIMV circuit in its place - if it's a fixed bias amp, the pots' "ground" will go to the negative bias voltage feed (and not to ground).

Be sure to read up on safety and discharge the filter caps before opening any tube amp.

There are detailed, photo instruction all over the 'net - it's one of the easier mods to perform.

 

[interloper]

How effectively would these mods, (done separately or together) cut down volume levels and still keep tone? For example if I were to get a good saturated tone at a certain db range how much quieter would these mods make my amp while still achieving that same tone? Just a rough estimate of course.

 

[JohnnyCrash]

How effectively would these mods, (done separately or together) cut down volume levels and still keep tone? For example if I were to get a good saturated tone at a certain db range how much quieter would these mods make my amp while still achieving that same tone?

The tone from an overdriven amp is not as simple as that.

Power Section
When an amp is cranked the power tubes can saturate as well, with either MV mod your power tubes are, at most settings, not even breaking a sweat.

Attenuators let you cook the whole amp, but they step down volume between the amp and speakers without messing up the power tube/output transformer's impedance issues. But then...

Speakers
Your speakers are not moving as they were when an amp was pummeling them at full blast. The speakers just won't sound the same with attenuated signals barely hitting them.

In addition, most attenuators have a sort of linear response to the give and take of the power tubes/OT (although the Weber MASS uses a speaker motor to duplicate this dynamic give and take).

In short, you will not be able to duplicate how your amp sounds cranked 100% by any of these methods (OD/Dist pedal, MV, or attenuator).

--

I usually like MV mods because you can dime the MV and the amp is back to "stock," it is more portable than OD pedals or attenuators (it's built into the amp hehehe), and requires no batteries or cables.

The easiest and probably cheapest route would be to shop around for a good overdrive/distortion pedal. I used to be a tone snob that refused to use solid state pedals... I've built about a dozen of my own tube amps and now about a dozen+ of my own pedals... I can attest to the cool sounds of all-tube, as well as tube amps mixed with FX pedals.

For reference, I have the following:
1. A pre-PI MV on my Marshall JCM800 clone and a dual 6L6 type combo I'm designing/building.

2. A PPIMV on my plexi50 clone, and Vox AC30 clone, and 40w BF Fender type amp build, and an 18watt "Marshall" variant.

3. I've built an attenuator.

4. I have a variable voltage regulator on my tweed Deluxe clone. This can run the tubes at lower voltages (sort of like a safe Variac).

5. 5 watt amp builds.

6. As mentioned a mountainous pile of OD/Dist/Fuzz pedals I built.

Depending on the amp, sound I need, and amount of volume I need to reduce, I can generally find a decent sound with any of the attenuation methods... everything that attenuates volume colors the sound somehow.

 

[Ronsonic]

I pretty well agree with Johnny on this. The PPIMV is a lot better at giving a cranked sound at lower volume than any other sort. It will not sound exactly the same at living room or even garage levels, but will be so much closer you'll be happy with it. There's no replacing the impact of that air movement or the electricity of 500V tubes at their extreme. But it completely not suck.

Even an attenuator doesn't get exactly the same sound if you use it for more than just taking the edge off.

I'll quibble with his cost and time estimates being optimistic.

Be aware that power tubes have a spec for the grid resistor that you are replacing with a pot. EL34 don't want much more than 100K there.

One thing I like to do with these is use a pot larger than the value I want and bypass it with a similar resistor from wiper to the bias line. Say a 500K Pot with a 470K resistor. Smooths out the taper and ensures that no matter how bad that pot gets it will never lose bias on that tube.

 

[interloper]

Thanks for the info guys I still have a lot to learn.

 

[lookah]

Hi Folks. I have a 40 watt, 6l6, fixed-bias amp with a PPIMV similar to the Trainwreck Type 2 MV.

I once tried to "red-plate" my tubes to find their real max dissipation and I couldn't get the plates to glow (although the blue glow did increase). Is this due to the way the PPIMV interacts with the bias circuit?

Interestingly, the amp maker suggests 35ma bias point per tube... regardless whether they are 5881's (23ma plate diss) or 6l6GC's (30ma plate diss).


Any thoughts?

 

[figaro]

I prefer a ppimv.

But, also remember that it's not all about amp distortion. At higher volumes, the speaker also contributes to the distorted sound. When you lower the volume, the speaker doesn't break-up as much and affects the distorted sound. This is why many amps don't sound as good when you turn the master volume down!

 

[Ronsonic]

Hi Folks. I have a 40 watt, 6l6, fixed-bias amp with a PPIMV similar to the Trainwreck Type 2 MV.

I once tried to "red-plate" my tubes to find their real max dissipation and I couldn't get the plates to glow (although the blue glow did increase). Is this due to the way the PPIMV interacts with the bias circuit?

It simply doesn't have the range to allow that much current with that set of tubes. Not a bad thing necessarily.

Interestingly, the amp maker suggests 35ma bias point per tube... regardless whether they are 5881's (23ma plate diss) or 6l6GC's (30ma plate diss).

Both are 6L6 class tubes and close enough. The amp really doesn't care what the static plate diss of the tube is. Being a piece of stupid old electronics it cannot read a tube manual. There are these ideas out there that tubes need to run hot to sound hot or that you need to bias to some percentage of some number in some book or that a couple of mils either way are critical and none of that is true.

Any thoughts?

You like to experiment, try biasing by ear. Some amps don't get cold enough to make this interesting, but let's give it a go. Leave your bias measuring gadget hooked up, we're going to use that to make sure we don't melt anything.

Set up the amp clean channel, master volume up, normal tone settings, gain/channel volume just loud enough to clear its throat, like 1.5 - 2. Set the bias dead cold, far over as it goes.

Strum a big open chord like a G or E. Let it ring and die. If it's way cold you'll have a thin reedy sound with a wretched sounding decay and poor sustain. Now start bringing it up a little at a time. At some point it will all start to snap into focus and the amp will get fat and the sustain sweet. Take a look at your meter, probably see something in the low 20s. Now crank up to stage level and see what happens. You'll find a similar weakness to strength transition as you ramp it up. Yeah, this will get loud. Take a look at the numbers.

You now know more than you did.

 

[lookah]

Thanks very much for the feedback. I'm familiar with biasing and indeed have experimented with different settings and found one that works for me (usually ends up being 60% or so of max dissipation).

But I guess I'm still confused if it's the PPIMV's circuit that doesn't allow the amp to "have the range to allow that much current with that set of tubes". Or, in my case, is it just that i have a low plate voltage amp? (i measured about 400v-405v)

 

[JohnnyCrash]

Thanks very much for the feedback. I'm familiar with biasing and indeed have experimented with different settings and found one that works for me (usually ends up being 60% or so of max dissipation).

But I guess I'm still confused if it's the PPIMV's circuit that doesn't allow the amp to "have the range to allow that much current with that set of tubes". Or, in my case, is it just that i have a low plate voltage amp? (i measured about 400v-405v)

What is your lowest range of available bias? What is your highest?

Most likely it's the bias circuit itself that is setting the sweep limitations. Keep the tubes happy... why would you try to redplate them?

 

[lookah]

"What is your lowest range of available bias? What is your highest?"

Well, that's exactly what's odd and why I asked these questions in the first place...with the 5881's and plate voltage of 401v, the lowest setting i can get is 18ma but the highest is 75. so 23ma/401v=57ma for 100% plate dissipation. so there's no way those babies shouldn't be red-plating when i put them at 75ma right? yet they don't. same thing with the 6L6GC's.