PPIMV - why not standard on all PP amps?

[dennyg]

I just finished my first build-from-scratch PP amp - Tubenit's TOS.  I liked the PPIMV so much that I added to my 18Watt Marshall clone with awesome results. Got me thinking - since a PPIMV basically provides an extra gain stage (the PI) to be over-driven by the preamp level pots, I'm curious as to why all amps don't use PPIMV vs standard MV.  If you want PA tube distortion, then just dime the PPIMV and crank the level pots until your ears bleed, right? Other than using a bit more hook-up wire and a dual-gang pot, is there any downside to PPIMV or advantage of a standard MV? 

[sluckey]

bottom line... $$$

[Ed_Chambley]

There are times where it changes presence.  It is according to the type if Post Phase inverter Master too.  Some work well and sometimes they do not.  For instance, I put one on a 2203/JCM 800 along with the pre master and no contest the Post master changed the amps tone a lot. There are different types too.  For the most part the one Referred o as the LARMAR with dual 250K I have found to work on a lot of amps, but you can expect a dulling of highs.


There are way to gain that back and some are effective with some circuits.  But you are correct, when it works well it is difficult to imagine why all amps do not share this.  A lot do.


There is also the VVR to control plate voltage and folks like this too.

[92Volts]

Preamp distortion often comes from cold-clippers or DC-coupled cathode followers which are carefully designed for good overdrive tone. The phase inverter isn't necessarily "supposed" to be overdriven.

Now, some overdriven PIs might sound good! But the designers aim for a specific sound, not maximum flexibility (otherwise you'd have a gain control between every stage to control where distortion happens... etc)

Obviously the cost of adding it make it less likely to be included. But there's are a lot of adjustments and other features not included because they aren't part of the sound the designer wants.

[dennyg]

But there's are a lot of adjustments and other features not included because they aren't part of the sound the designer wants.

ah but the cool thing about this hobby is you can experiment! adding the PPIMV to my 18watt'er turned it into a crunch machine.  and on the TOS, the PPIMV turns the OD level pot into a gain control vs simply balancing with the clean channel.  I was actually surprised by the compression/gain that overdriving the PI can create.  I posted a demo clip of the TOS running the OD level at 1:00 and it's right up there with some gain monsters like Mesa - not quite a 5150 but more than I think Tubenit & Geezer had in mind when they designed this thing. 

[SILVERGUN]

There are times where it changes presence....
 
... but you can expect a dulling of highs.

Ed has some good points there...
 
 
While you are experimenting, I would suggest clipping some bright caps onto the pot lugs (similar to any Fender bright cap, just x2) to compensate for the lost highs at lower output settings on the PPIMV

[dennyg]

While you are experimenting, I would suggest clipping some bright caps onto the pot lugs (similar to any Fender bright cap, just x2) to compensate for the lost highs at lower output settings on the PPIMV

Interesting - may be why the PPIMV works so well on the TOS - it's a very bright amp - I think because the clean channel also provides the first 2 (2.5?) stages for the OD - so to dial in a chimey clean tone also feeds a good bit of treble into the OD - and the amp sounds ideal to my ears with the PPIMV below half.  I'm experimenting around with grid stoppers and caps in the OD section to tame some high end w/o damping the signature blooming/sustain of the TOS

Untangled quote; Willabe

[jjasilli]

Ditto to Ed & 92.  Some players feel that MV's harm tone, particularly a loss of hi's.  There is no free lunch, or singular solution to overdrive at lower volumes.  (BTW: I believe it's Kevin O'Connor who recommends a 100K resistor with a short lead following the MV wiper lug to address this issue.)

[2deaf]

It's not standard on all PP amps because it doesn't sound like the amps that the Rock gods used for decades.

[92Volts]

Are highs lost due to the resistance of the pot in series with the input (Miller?) capacitance of the tubes? Or is something more complicated going on?

I have a 1 Meg MV pot in a single-ended build and noticed a HUGE loss of high frequencies when I installed a 6L6GC in place of a 6V6. I was surprised because the rated input capacitance is like 10pF which "should" roll off 15kHz+ with a 1 Meg resistor which I didn't think would be noticeable. The even weirder thing is those specs don't appear to be very different from the same specs listed for 6V6s on datasheets. So what's going on there?

[jjasilli]

http://www.londonpower.com/all-kits/london-power-master-volume


KOC maintains that a 100K resistor in series with the wiper (output lug) of the MV pot preserves tone over the entire sweep of the pot.  N.B:  His MV's tend to be pre-PI in keeping with his position that the power amp should not be overdriven.  I.e., all tone shaping should be done in the preamp, with the power amp providing only a clean boost. (I'm just the messenger, not advocating this.)

[SILVERGUN]

Since jj brought up KOC, there is this...
http://el34world.com/Forum/index.php?topic=21173.0

[jojokeo]

I usually always have to add 100pF bright caps to both pots to preserve the high end when turned down, I don't use resistance as this is mainly the cause in the first place (keeping things simple).

Eds comment on Presence being affected is correct. That is because the amount of feedback voltage changes and is lessened when the MV is turned down effectively lessening the Presence control's effect.