Marshall JMP50 PPIMV master volume

[yann]

Hi Guys,

A customer asked me if I could mod his 1973 JMP 50 to add a PPIMV.
I am not doing mods very often so I would like to have your feedback about the mod I plan to add :



How is that going to affect the amp tonewise ?
Is it going to affect the bias ? I think it would be better to re-bias the amp when this master volume is all the way up.

Thanks for your advices.

Cheers !

[alexradium]

bias is not the problem here,for sure you end up with a variation in tone in 2 ways,one for the feedback and one on the phase inverter due to impedance changing and the fact that any voltage divider there will decrease HF without compensation.
But it is pretty useful if you use it down to half on a standard audio pot.
By the way the typical scheme used on Metroamp and others is a 220k pot with 2M2 security resistor.
Another system i found to be clever is the Frondelli MV,which is wired like a 5E3 Fender volume.
Anyway i would add a 250k pot in series with the stock feedback resistor.

[Chuck H]

I think the variable feedback pot is a good idea. I might also add the following changes to the schematic posted for reliability and stability. The screen resistors are probably actually in the amp, though they may be 470R. 1k is a safer choice and probably won't detriment the tone, though you can always change them back if you're guy doesn't like them. The 1.5k grid resistors should go right on the tube socket lug. They won't noticeably alter the tone, though some say they do. The 2.2M "safety" resistors are a good idea because they provide bias in the event of a pot wiper failure. The 250k dual pot is now replacing the bias resistors. The 250k dual pot and 2.2M safety resistors in parallel as they are provide a resistance of 224k from grid to bias circuit. Very close to the stock value of 220k. So no real change in tone when the master is full up. Changes/additions in red:

[yann]

Thanks a lot ! I will try the mod with the modification suggested by Chuck and see how it sounds like.

[Helmholtz]

There are two major issues associated with PPIMVs.

The first is the pot itself. It is extremely hard to find dual ganged pots with a synchronicity/symmetry better than 20%. The divider symmetry is typically worse than the the full resistance differences of the pot units.
This results in asymmetric drive, including a DC current component in the OT primary which may degrade bass response.

The second is that the MV is within the feedback loop. Turning it down reduces open-loop gain and consequently the feedback ratio. This has several effects. At higher MV settings the feedback will counteract the desired volume reduction. At lower settings the amp's output impedance increases and its frequency response changes. In particular the effect of the presence pot is strongly reduced as it depends on the feedback ratio.

Increasing the value of the series feedback resistor increases overall gain (again by reducing the feedback ratio) but seems counterproductive with respect to restoring the amp's behaviour without the PPIMV.

This said, it may still sound O.K.

[yann]

If there are other solutions than PPIMV that can be easily implemented I can have a look at the option. I don't know why he asked me specifically for a PPIMV mod but I can ask him when I got him on the phone.

[Helmholtz]

If there are other solutions than PPIMV that can be easily implemented I can have a look at the option. I don't know why he asked me specifically for a PPIMV mod but I can ask him when I got him on the phone.

All PPIMV solutions "suffer" from the effects described above.

A normal MV before the PI will not change the behaviour of the power stage. But achievable distortion will be less and different especially as the PI doesn't contribute.

[alexradium]

If there are other solutions than PPIMV that can be easily implemented I can have a look at the option. I don't know why he asked me specifically for a PPIMV mod but I can ask him when I got him on the phone.

if he's a paying customer,don't ask any questions,just do it the best way you can,he asked ppimv then you give THAT to him,my 2 cents of experience!
A guy had an amp with this MV and said to me "because i can get power tube saturation!", actually it is the contrary,you can clip the PI and that can produce a similar effect but the power tubes run clean.
So don't expect "artists" to give you technical explanations.

[Chuck H]

if he's a paying customer,don't ask any questions,just do it the best way you can,he asked ppimv then you give THAT to him,my 2 cents of experience!
A guy had an amp with this MV and said to me "because i can get power tube saturation!", actually it is the contrary,you can clip the PI and that can produce a similar effect but the power tubes run clean.
So don't expect "artists" to give you technical explanations.

+1

The guy heard it or read or heard about it and wants it. Give it to them as they asked. It's a common solution to the loudness problem and the easiest one to implement. Other options include, but may not be limited to...

Power scaling - Which also changes the tone and is complicated by the need to simultaneously adjust the bias and the drive signal to the scaled circuits...

Attenuator - Which also sounds different and carries the stigma of being bad for the amp. Which it's not, but the chances of an open load mistake and/or the propensity to crank the amp more often are both detrimental in terms of possible damage and wear.

6V6 power tubes - Which doesn't sound as different as you would think, but still does sound different. It also depends on whether this amp is the 380Vp, 420Vp or 470Vp model of it's type. Only safe for the first two. It also limits clean headroom the the reduced output and only has one (though reduced) volume for overdrive.

Triode wiring - Sounds very different and has the tubes running into a less than ideal load for triode operation. Like changing to 6V6 tubes it's limited one level volume reduction.

Less efficient speakers - Different/less efficient speakers will, of course sound different. Speakers are probably the most important factor to the final EQ balance of an overdriven amplifier. So changing them will sound different and, like using 6V6 tubes, will reduce clean headroom and be limited to a single level of volume reduction for overdrive.

Fewer speakers - If your guy is using a 4X12 cabinet he could reduce volume similar to using less efficient speakers by just using fewer speakers. A smaller cabinet with a single 12" speaker of a similar tone as the ones he is using would be noticeable less loud. The single speaker, as opposed to "less efficient" speakers would also allow for more options in speaker selection. So, say, if the cabinet he has is loaded with 4X Vintage 30's he could use a single Vintage 30 for a smaller difference in the overall EQ balance. A smaller cabinet/fewer speakers also limits clean headroom and is limited to a single reduced volume level.

[g1]

Some manufacturers use a 5 (or more) position double pole switch and resistors. I would think this helps get around the symmetry issue Helmholtz described with regard to lack of precision in the dual pot.
It also lets them call out 'power levels' on the switch which helps mislead buyers about 'power attenuation'.

[Enzo]

Also keep in mind these are just guitar amps. These various issues are more tendencies than problems. And if the system gets slightly imbalanced, well, so what? You get to turn your otherwise already highly distorted signal down some.

[Chuck H]

Didn't Marshall and Sunn both use a screen voltage reduction circuit for "half power". I'm sure I saw it, but now I can't find it.

[g1]

Was that maybe a pseudo triode mode? Trying to recall the model.

[Guitarist]

Te LarMar PPIMV is a good one. re~ a quality balanced pot, you could test a few and get one that is self-matched by measuring the sweep differences.

[Diablo]

I think the variable feedback pot is a good idea. I might also add the following changes to the schematic posted for reliability and stability. The screen resistors are probably actually in the amp, though they may be 470R. 1k is a safer choice and probably won't detriment the tone, though you can always change them back if you're guy doesn't like them. The 1.5k grid resistors should go right on the tube socket lug. They won't noticeably alter the tone, though some say they do. The 2.2M "safety" resistors are a good idea because they provide bias in the event of a pot wiper failure. The 250k dual pot is now replacing the bias resistors. The 250k dual pot and 2.2M safety resistors in parallel as they are provide a resistance of 224k from grid to bias circuit. Very close to the stock value of 220k. So no real change in tone when the master is full up. Changes/additions in red:

A word of caution on implementing this PPIMV. As drawn, it works great. I screwed it up when I installed it in my Tweed Twin 5E8A clone. By mistake, I reversed the leads on the dual pot. I put the leads from the phase inverter caps on the pot wipers, and put the leads that go to the output tube grid stoppers on the first lugs of the pot. While the PPIMV does work this way, it doesn’t work right. When the MV is turned down, the amp distorts badly, even when the preamp volume control is also turned down to 1 or 2. I don’t understand why it doesn’t work properly, but maybe someone here can explain the problem. Anyway, I rewired the pot to match the schematic, and now it is fine - no distortion at low MV settings.