To power scale or not to power scale...

I had a Jtm45 power-scaled and I was very, very impressed. I really could get cranked tones, but at virtually any volume. No discernable tonal change between different amounts of powerscaling either - completely unlike attenuators.
As for the controls, you can have them placed wherever you want. You'll need two extra knobs, a power-scaling knob and a MV - probably a Pre-PIMV (also sometimes called a "drive compensation" control). I had one of these controls installed in the low gain input of the normal channel, which I wouldn't have used anyway. The other knob can be fitted almost anywhere. I had mine on the back of the amp.
Make sure you really get someone who knows what they are doing to install it though. A lot of the magic is in the quality of the installation. I have been told that almost no two installations are quite the same - hence the need for an experienced and trained tech. Try and get some recommendations of appropriate techs for power-scaling near you. London Power have a list on their website of recommended installers.
In the UK, having power-scaling installed costs about the same as a new Hotplate. I'm not sure if that would be the same, where you are, but check it out ! I would thoroughly recommend it, if it is for an amp you expect to keep for a long time.
One of the benefits of an attenuator is that when you sell the amp, you still have the attenuator - obviously not the case with PS installed in an amp.

 

i have a few hotplates (i use attenuation a lot) and like the versatility and transparency of them (as long as heavy attenuation isn't applied) and my pet peeve is the rather large jumps between clicks. the airbrake is better in that regard, but isn't always as good sounding (totally amp dependent).

i have power scaling in the JMJ30 and orange retro 50. as long as only moderate amounts of attenuation are applied, it is very transparent sounding and i love the continuously variable control over volume.

this is gonna sound very weird, but i often find a combination of power scaling and/or attenuation or master volume to be very effective. attenuation will keep the artifacts of output transformer saturation intact, whereas power scaling will inhibit OT saturation. it's nice to be able to mix/match/choose.

 

[Does power scaling work very well with a NMV amp?]

Yes. It works very well if you scale the whole amp and don't expect it to sound wonderful sub 2-3W. If you just want to scale the power tubes, you need a post PI MV to keep the preamp from killing the power tubes at lower voltages, which is a more invasive mod for an amp like yours.

[Do I have to lose something from the stock amp i.e. give up a control or jack or can I just punch a hole in the back and have the control there?]

If there is room on the back panel, a single pot can be added there.


[Since the reissues have already settled into the $1000 used range (where I bought it), will this kind of modification hurt or help its value?]

I imagine this mod would be fairly neutral in terms of impact on value.


[How much does a typical power scale retrofit cost for an amp of this type (ballpark)?]

I can't answer this one without running afoul of the mods other than to say you can buy a kit from LP and pay a good tech to install it or you can use someone who already builds this type of circuit and maybe save a little money.

 

I don't understand this part ? So what happens to the power tube without a post PI MV ?

-g

 

Well, as you scale the power tubes down, you tend to overdrive the grids of the power tubes if you don't scale them down at the same time. This can lead to early distortion and eventually crossover and/or blocking distortion. The idea is to set the voltage for the volume you are trying to achieve, then adjust the master to get the tone right. Not my cup-o-tea personally, but some people like the approach.

 

Whatever you scale down has to have some sort of 'master volume' before it in the circuit because when you scale down the voltage you also need to scale down the signal hitting that scaled stage. So if you only scale the power tubes you must have a post-PI MV...or if you scale the PI + power tubes you need a MV before the PI. If you don't have the MV, then when you scale down the first stage that is scaled gets pummelled by the now-too-large signal hitting it and the tone quickly turns to mush...you can even go so far as to make the stage pump/breathe/gate the signal (with a lot of noise along with it...when the 'gate' is open lol).

 

FWIW I had a Suhr Badger with power scaling. Although it worked pretty well down to about half way on the dial I really didn't like how the tone changed much below that. It's way above bedroom level by then. Probably still pushing close to half power. I was really dissapointed since I really liked the video demo of the amp. After viewing the video again I don't think it gets turned down much lower than that on the demo either.

I think there are a few of issues with the use of it in an amp. One is the obvious operating point of the power tubes. They were not designed to run at such low plate and screen voltage and get pretty far off their sweet spot when doing do. Not much you can do about that except don't turn it down very low. Another is that when you turn it down the preamp voltages increase quite a bit since there is much less load on the power supply. You go from that nice low to mid 200 volt sweet warm harmonically rich Marshally preamp B+ voltage to a more sterile sounding 300 or so hifish Fendery B+. That could be fixed but not that easily. Also the fact that the drop across the cathode resistor changes considerably which changes the bias shift properties that we all love about class AB cathode biased amps.

My hats off to Mr. O'Conner on thinking outside the box but to me the implementation of it isn't worth the bother. To be honest I think a good PPIMV works much better overall.

 

Have you used your JMJ with a Hot Plate and the power scaling?

 

Are you sure about this? Based on my understanding of the circuit, this should not be the case.

 

When you do the whole amp, to me it starts to get weaker and weaker as you scale it down, as the preamp tubes now get less and less voltage, so they lose their gain and drive.

 

i prefer the airbrake to the hotplate on the JMJ, but yeah.
the JMJ does not have drive control (i suspect that matt choose to not include drive control because it would have compromised the signal path) and so, as noted by others, there begins to be artifacts at the lower settings on the power scaling. i kinda like the fuzzy sound in a weird, bizarre sort of way, but using a bit of power scaling and attenuation allows much lower dB and 'bedroom' levels w/o the typical drawbacks of massive amounts of attenuation when using only one method. conversely, using both power scaling and attenuation allows one to vary the amount of output transformer saturation to taste. for recording, in particular, this makes the added cost of both strategies more than worthwhile.

i do the same w/ the retro 50, as well.

 

Thanks Zero. I was curious what kind of tweakability one could get mixing and matching attenuation with power scaling. Now I'll have to try it.

 

KOC evens says that with a power scaled amp, you have to change your expectations of how the amp should sound when attenuating.

Another non-invasive approach is to use smaller, less efficient speakers. Here at the house I have an old Gibson GA 5 cabinet with an 8" Utah and use it with my tweed deluxe clone and TT15. It scrubs several decibels off. It mics up really well.

MikeY

 

Well, my amps are SE, so no PI, but they do have independant PA MV. When I dial down my power brake, the pre-amp has more than enough gain to saturate the power tubes, which, I thought, was the whole purpose. I don't see the crossover distortion, which should be more of a function of cathode bias; or the blocking distortion, which should be more of a function of the series grid resistor.



-g

 

I use a different technology. The Guytron amp has an internal 20 watt tube power section slaved to a 100 watt tube output section. By the way Guy Hedrick does like power scaling and designed an amp years ago for someone using it but he still prefers the slaved aproach in that you are still getting more of the real thing. Having said all that I see here on TGP people trying to get this huge sound at whisper levels and it just won't happen. It's physics. Be it power scaling, master volume, attenuators, slaving what have you there is still no subsitute for moving air. This is why SRV, EJ, Bard Paisley, Keith Urban, and countless others use multiple amps.

Most guitar speakers are not designed to sound good at 3 watts nor are the guitar cabinets they are put in designed to sound big and full at that low level. There is really no easy answer than to realize part of being a pro-player is dealing with the multitude of situations you are presented with. You also have to be honest about the type of gigs you are doing. If your just a part timer playing weekends in the local bar these are not professional venues that are designed for live sound. Usually the band is on top of one another, your drummers cymbals are next to your ear, your guitar amp is on the floor with you standing directly in front of it and your next to a loud bass player that's sucking up all the low end your cab can produce etc. It's just part of live sound.

I find I can get a pretty cool sound at a very low level but put me next to a drummer in a small room and people are sitting 10 feet in front of my speaker cab and it's a whole different world.

 

Exactly, the preamp has more than enough gain to overdrive the power tubes...now when you scale down the power tubes (but not the preamp), their headroom goes down and it takes less to overdrive them...so you turn down the MV to get you back to where you were before...but at a lower volume.

 

I did the video- I take it down much lower than halfway in the video.

I think power scaling works really very well, it's pretty hard to quantify exactly what's going on when you take an amp from raging loud down to conversational levels- you obviously aren't pushing air and engaging the speaker(s) in the same way, and your ears aren't going to hear things the same either.

I think it's a waste of time to try and get your 100 watt nmv marshall down to conversational levels- just use a modeler or whatever at that point, for home practice and save the big amp for gigs! Or build an iso cabinet into a closet like i did, mic it and cover it up in blankets, and listen through studio monitors. I like that solution lots.

 

Great post. I agree completely. I spent a lot of years chasing power amp distortion at low levels and owned the original Guytron, as well as too many attenuators to count. I now own a Suhr Badger with Power Scaling.

The comments above about weekend warriors describe my situation perfectly. I don't need perfect bedroom tone (I practice clean 99% of the time anyways) and have always realized it's impossible to achieve due to physics and Fletcher-Munson. But, I do need something that sounds great at small club gig levels and have found that a variety of technologies can give you that.