JMP Master Volume

[smcgov]

Just bought a JMP hand wired clone 100 watt 1968 specs, and god damn it needs to be loud to get some good gain...any idea on how hard to add master volume and is it a good idea?

here it is....

100 watt Plexi hand wired - eBay (item 280337471754 end time Apr-28-09 08:27:29 PDT)

I might have to sell, the JTM 45 I have is much more versatile...

Thanks

 

[janarn]

Why should you add a master volume?
Buy a Hot Plate or a Ultimate Attenuator.

 

[smcgov]

because they cost mucho $$$

 

[janarn]

To add a mastervolum is how mucho $$$?
And the amp will not sound like it is supposed to do.

 

[ThreeTone]

A master volume will allow you to control the volume while still driving the preamp as much as you want. But at low volumes you won't be pushing the power section, and that's where the really sweet tone comes from.

An attenuator is the best option to get power tube saturation, touch dynamics and power tube harmonics at low volume levels. A Ho's Electronics or Ultimate Attenuator is what I'd recommend.

 

[The Mick]

Weber

 

[HAMPAMP TUBE AMP SERVICES]

There is another option that I've yet to do myself, so I can't tell you if it is expensive. POWER SCALING is a mod that reduces your amps voltages internally, kind of like using a variac without destroying the amp. Like I said, I haven't tried this myself so I can't tell you how well it works, how it affects the sound or how costly it is but I've heard some good things about it. Might be worth checking in to. I've done some builds where I've kept the preamp B+ voltages at a lower level (which works well) but POWER SCALING deals with the power amp as well.

 

[AtomicRob]

As I understand it (which is not very well!!!) power scaling is fairly easy to implement on cathode biased amps, because the bias will scale automatically with the power. But on fixed bias amps like a JMP or most other Marshalls, it's a lot more difficult because you have to scale the bias circuit as well to keep it biased properly as the power level changes.

 

[HAMPAMP TUBE AMP SERVICES]

That is correct, everything is scaled except the filaments. More costly than a master volume also. I guess you could also try something like THD yellow jackets to tame it down but that's probably costly too. Let's face it, you're probably better off selling it.

 

[smcgov]

anybody have an idea what a mv install would cost??

 

[HAMPAMP TUBE AMP SERVICES]

Well, I can't speak for other techs cause everybody charges differently but if you were local to me, it would cost you about $35-$40 plus parts, I haven't priced parts for awhile but I'd say less than $75, but a lot of shops charge that for an hours worth of labor so I really couldn't give you any guess at all. Also depends on which master you want, most of them only require a standard pot, a couple require a dual-ganged pot. Check your local shop and see how much they charge for labor, multiply that by 3 and add another 15-30 for parts and you might be close.

 

[PaoloJM]

Ask the tech about the Lar Mar version of the post phase inverter master volume. These sound great in JMP type amps. Check out a guy called Rockstah on soundclick for some of his sound samples.

The sweet power amp distortion comes from driving the phase inverter hard, heavily overdriven power valves don't actually produce that great a tone.

A normal master volume limits the signal to the phase inverter so you don't get any of the PI tone. That's why people prefer attenuators.

The LarMar allows you to drive the PI hard and limit the signal to the power valves instead, which sounds great IMO.

 

[janarn]

You can't get the sound of EL34 distortion from a 12ax7 in V3 (PI).
The only way to get that is to play loud, or use an attenuator.

 

[PaoloJM]

El34s don't produce distortion in a Marshall.
The more signal you give them the more sustain, warmth and responsiveness you get but you don't get more distortion. Power amp distortion is produced in the PI.
If you don't consider the PI to be part of the power amp then I agree with you, but traditionally it is considered to be and that is what people are refering to when they talk about power amp distortion.
The combination of PI distortion, power valve sustain/dynamics and a lot of air moving from the speakers is the cranked Marshall sound.
You get that same warmth and sustain even with less than full signal on the EL34s. Granted, a certain point you do start loosing it and you wont get it to bedroom levels. Attenuators don't do bedroom volume either.
A PPIMV let's you get the PI distortion and the sutain/dynamics at far reduced volumes and works great for gigging and even rehearsals with a drummer.
If you want to hear what EL34 distortion sounds like then disconnect the negative feedback from your Marshall and see. I doubt you'll like the results though. The presense control kinda does this for higher frequencies and most people find it a bit harsh after about 4 on the dial depending on the model we're talking about.

 

[janarn]

Well I have tried different master volums on Marshall modells,
and my ears tell me that they don't sound as good as an original Plexi.

I think mastervolum only works on amps with more gain stages in the preamp,
than on the classic Marshall four holers.

That's why the early 2204 amps didn't sound good until they cascaded the preamp.

Models based on preamp distortion works well with MV, old Plexis don't.

 

[PaoloJM]

I agree with you, there a lot of very bad master volume mods out there.
For some reason some techs think you put a 2203/4 stlye master volume in a 1959 type circuit. As you point out, they do not sound good.
Many post phase inverter master volumes don't sound good in a plexi either.

Lots of people seem to think that a master volume is a master volume. This, however, is not the case.
There are an estimated 60 variations of master volume type out there today. Different types work well in different amps. Plexi amps definitley need the master volume to be installed after the phase inverter.
The guys over at Metroamp came up with PPIMV variation called the LarMar (mentioned in my previous post). When installed well properly in a '68 spec plexi it absolutley nails the early Van Halen tone, once it's turned up past three on the dial. I think most would agree that the tone on Van Halen I is thee cranked plexi tone.
When used with an attenuator you can approach bedroom volume this way.

 

[HAMPAMP TUBE AMP SERVICES]

The Metroamp PPIMV kit is nothing more than a Ken Fisher master volume circuit in a kit form, it's been around since the 70's and it is in my opinion the best master volume circuit to use. I've even taken the standard master out of 2203's/2204's and installed this one instead and they sound much better. Good advice and recommendation.

 

[PaoloJM]

It's a bit different than the Ken Fisher version. Fisher used a 100k dual pot.
The LarMar uses a 250k with a 2.2Meg between the wiper and V bias.
This has two advantages; the 250k with 2.2Meg = 220k when dialed right up. This is the exact same as the stock circuit without the mod.
Secondly the 2.2M resistor keeps V bias on the grid even if the pot wiper fails, which prevents tube melt down.
It works on the same principle as Ken Fishers but with two important enhancements, isn't this how all great ideas come about!!

 

[HAMPAMP TUBE AMP SERVICES]

Yea ,I always used the 250k instead of the 100k because I've always installed them on EL34 Marshall's and the early problems with this circuit was with pot failure.

 

[smcgov]

this??

Metroamp Parts Store

is all I need? I am so confused....