francoix a écrit :
Vu l'amabilité dont tu fais preuve dans tes réponses, je passe mon chemin et m'en vais aider des personnes plus sympathiques et moins hautaines.
Diminuer l'ESR sur le circuit de bias!! Haha! je crève de rire... (c'est désagréable hein!)
...
..merde !
bon moi je veut bien te fournir quelques explications sur le ppimv ( le reste je ne connait pas trop désolé )
voici mon opinion:
I I am unsure which type of PPIMV would be best suited for the amp.
On one hand, there is the type ala Bill Collins (Mission Amps? No idea of actual origin of this PPIMV, citing one place I found a schematic). PDF diagram of this type
On the other hand, there is the "LarMar PPIMV inspired by the Ken Fischer ver2 (sort of)" type, discussed HEAVILY over at the Metro Amps forums. Diagram Here
There is a third from over there, slight variation: Diagram Here
Marshall 1959 100watt schematic
Carvin Legacy schematic (pdf)
While I could try a few types and choose, I would much rather try the most *appropriate* one first, and hopefully avoid a second try. Amp work is very difficult for me now (physically; wahhh for me). Any thoughts?
Although there are other MV types, and more PPIMV types besides these, they were my initial choices.. I am open to other suggestions. An attenuator is out of the question at this point though (I have tried a few and the power section push is not worth the tonal and response sacrifice(s).. IMO! At light attentuation, some are cool. I need another option for my needs)
One reason I am doing this is because I prefer a PPIMV over the Pre PI MV. Secondly, I use rack mount preamps into the Legacy FX Return, and it would be nice to have an MV after the return (tube buffered- nice
).
Thx!
(Oh! I hope those metnioned do not mind, and even more so.. Rockstah/Mark and Metro- I hope you do not mind my referencing your diagrams and stuff. If anything I have linked/mentioned is a no-no, please let me know and I will remove it)
The Larmar ersion kicks some serious ass! It will make you pitch your attenuator out the window.
There are two other versions you didn't show that work very well too.
There is no 'perfect' one.The one that works for you is the right one.
The larmar works very well,but I have used two others that work very well too.
Yeah, this is an interesting one, as the Legacy is.. interesting. I have it narrowed to the LarMar or Hasserl right now, and ordered the parts to try both if I want. Once the chassis is out and the PCB's pulled, it will be ALOT easier to see if the LarMar is even feasible with it.
I guess now it comes down to which would/will sound better. I have heard a LarMar on a Marshall, never heard any PPIMV with a Legacy, and it differs just enough to make things difficult.
prefer the version using a second set of coupling caps, like mission amps, but disagree with his choice of capacitor values.
You really want the second set of capacitors to be 10x the stock set. So for example, if your stock PI coupling caps are 22nF your second set would be 220nF, and they need only be 160V (or better of course). That's going to leave the total capacitance at 22nF, just as in stock. Trying that against the larbear version, you will find it to have less noise issues and be less sensitive to lead dress. For example, you could then place the control right next to the phase inverter on the front panel without any squeal!
Keep in mind that any PPIMV is going to produce a dirtier tone than non-PPIMV when you lower the amp volume, as the NFB loop is opening up, so the PI pushes harder. For better or for worse, some people are starting to expect that sound and are somewhat amazed that their amp is cleaner when full volume than it is using that Master. FWIW - I am one of those that prefer the sound, dynamics, and feel of the fully cranked tone over a disproportionately cranked phase inverter. That being said, short of controlling the output power tube voltages, a PPIMV alone is a good compromise.
If you were power scaling the amp, and scaling the phase inverter too, you might try an improved bootstrap master instead on your PI; it also uses a dual ganged potentiometer, replacing the two 1m PI grid resistors (typical schmidt values), with the ground of the pots connected to the junction of the tail resistor and cathode bypass resistor.
With this setup as the phase inverter scales down the drive to the PI can be scaled and the drive can remain constant, with the NFB tracking proportionately too. Without scaling it, it will decrease the drive of the PI, reducing the drive to the power amp - you probably won't enjoy it as much, as the tone would change... unless you add the PPIMV back in.
IMHO - the ultimate non-power scaled solution would be a PPIMV combined with the above bootstrapped master, giving total control of the PI distortion and NFB while controlling volume.
PPIMV is by far the best way to attenuate your amp. If you use .22 caps instead of .1's on the Mission schematic you'll have the same sound as .1's with no PPIMV.
Actually, with the .1's on the Mission MV the caps are in series with the result of .05 total capacitance.
--ctually, with the .1's on the Mission MV the caps are in series with the
That's right, that's why I was suggesting using .22s instead so that it would work out to .1
I just read about an interesting MV. It involves basically placing a pot in series between the PI cathodes and the low-value PI cathode resistor (which is typically somewhere around 470-820 ohms). Basically, you're raising that cathode resistor value to limit PI current-flow.
Might not be so smooth with amps using NFB/Presence, but amps without NFB/Presence circuits (like the basic 18-20W cathode-biased EL84 amp) should be an interesting experiment.
That's right, that's why I was suggesting using .22s instead so that it would work out to .1
That's right, that's why I was suggesting using .22s instead so that it would work out to .1
Those 220nF would be huge in the 630V variety - the size of batteries - and you can't use the lower voltage ones for the first set. Good luck finding a usable cap for that! And when you're done, you've changed the sound of the amp by increasing those caps from 22nF to 100nF. Might be a bit woofy, and tend to oscillate.
Actually, with the .1's on the Mission MV the caps are in series with the result of .05 total capacitance.
Indeed. It'll work, but you might be having some NFB LF oscillation issues, depending upon your loop design. Methinks that's why marshall a) decreased the NFB and b) went to the smaller couplers. In any case, it's changing the stock tone. Use your stock cap + a cap 10x the value (and it can be lower voltage as I said).
The two .100uF (or 100nF) in series are .050uF (50nF) as you said, much better... but for a marshall 50W or higher use the .022uF (22nF) in series with the .220uF (220nF) for the ideal 22nF (.022uF), which is exactly the same as stock in 99% of amps.
The goal with the circuit is to have the amp sound as stock as possible with the PPIMV fully clockwise, which is why I recommend using that circuit as described above.
Enjoy,
Well Mickey I've used that arrangement many, many times and I can tell you it works very well. The tone is not changed from stock, and the tone from full up to way down low remains very much the same, it is a very transparent MV design. At least as transparent as any other means of reducing volume in an amp. I've never had any NFB oscillation issues, though there is a reduction in the effect of NFB, so the amp will drive harder and distort sooner, which is pretty much the point anyway, isn't it? A benefit is a smoother transition from clean to distortion.
I understand completely how the PPIMV circuit works - I have been using that design for years myself, and have sold many amps with it, both hand-wired and circuit board based designs, as well as recommending it for use as a limit control for power scaling V1 designs on powerscaling.com.
In brief, I know how it works and what it does, that wasn't the issue, and I agree t's the right circuit to use.
not the circuit I questioned, but your recommendation of cap values to the OP. What I was disagreeing with was changing the value from a total of .22uF to .50uF, nothing else. It's audible, it shows on a scope, and on a good many amps it will destabilize the feedback loop (which are unfortunately at the edge of unstable on quite a number of amps made). Why not do the simplest thing possible, and not change the value?
22nF + 220nF = 22nF
So the 22nF are left alone, the dual gang pot is inserted, and 220nF lower voltage caps are added (which are cheap) to the potentiometer wipers. The math is simple, the caps themselves are cheap and commonly available - and much smaller too. The Xicon MEB 220nF 200V is perfect for this use and dirt cheap.
YMMV, and of course you should do whatever you deem best - just making my recommendations to the OP. Adding two new caps is just better IMHO than ripping out two, to add four, and have a different total value from stock. For the amp in question, it's the best advice I can give.
No offense meant to any - just stating the exceedingly obvious, and assuming you must not have understood me.
ffense meant to any - just stating the exceedingly obvious, and assuming you must not have understood me.
ep, slight miscommunication there. I thought you were questioning the entire MV design. Re the change in capacitance, that was Redman's suggestion, not sure why he suggested it, but I'll let him explain if he wants to. But the Carvin Legacy that the OP mentioned uses .047uf caps stock, so the .1uf caps in the Mission Amps schematic would work fine (and is how I would do it with that amp). Or .047 + .47, as you suggest. But in the case of the Legacy, he'll be pulling the stock caps anyway out of the pcb, it will be easiest to work with new caps when installing the MV rather than stick with the original caps with the short leads. Might as well go ahead and replace them IMO. I've done so many of MV installations in Carvin amps I can almost do them in my sleep. A quad of tubular .1uf caps make for a very tidy installation in that amp.
Actually, I misread that - my bad! I looked at the schematic he gave of the marshall 1959 and was basing my recommendation off of that including my remarks about not modifying amps with feedback that are close to the edge of unstable, or have a tendency to squeal or LF oscillate.
Yes, I agree, in that carvin legacy schematic a pair of 100nF in series are fine, as a 470nF won't be as easy to get.
Yep, slight miscommunication there. I thought you were questioning the entire MV design. Re the change in capacitance, that was Redman's suggestion, not sure why he suggested it, but I'll let him explain if he wants to. But the Carvin Legacy that the OP mentioned uses .047uf caps stock, so the .1uf caps in the Mission Amps schematic would work fine (and is how I would do it with that amp). Or .047 + .47, as you suggest. But in the case of the Legacy, he'll be pulling the stock caps anyway out of the pcb, it will be easiest to work with new caps when installing the MV rather than stick with the original caps with the short leads. Might as well go ahead and replace them IMO. I've done so many of MV installations in Carvin amps I can almost do them in my sleep. A quad of tubular .1uf caps make for a very tidy installation in that amp.
I suggested .22's if you wanted to end up with .1's because .1's usually sound better to me in an amp thats using 220k bias feeds. Also the extra resistance to ground the dual gang pot adds soaks up some bass, especially at lower settings.
tout ca pour dire ( avec le sourire quand meme ) que je suis du meme avis que francoix ! les copier coller direct dans la tronche c'est jamais tres agreable
