Lar/Mar PPIMV
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Re: Lar/Mar PPIMV
For the shielded cable part - I used RG174/U type cable
I did this type of mod on many marshall 1987/1959 type heads over the past years! Works great!
I did this type of mod on many marshall 1987/1959 type heads over the past years! Works great!
don't grow up - it's a trap!
Re: Lar/Mar PPIMV
Did you connect the shield to the Bias voltage, or to ground?
Do you know why the shield is connected to the bias voltage in the plan/instructions?
Do you know why the shield is connected to the bias voltage in the plan/instructions?
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Re: Lar/Mar PPIMV
Not to ground! The shield carries the bias voltage.
Re: Lar/Mar PPIMV
To where? I only see it connected at one end, to the pots.
Am I missing something? The diagram I have shows the shields connected to the bias voltage at the pots on one end and to nothing on the other end.
Am I missing something? The diagram I have shows the shields connected to the bias voltage at the pots on one end and to nothing on the other end.
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Re: Lar/Mar PPIMV
http://ampgarage.com/forum/download/file.php?id=25515
This is not exactly as I installed one. I have only installed two, and removed one because of the NFB circuit going bye byes.
Ahh so Google PPIMV and look at layouts and pictures of installs. I have a small wire under the board from the 220k junction (removed resistors) that goes under the board and terminates at the left lugs of the dual 250k pot if looking from the back. The blk and red wires from the grid feeds are terminated as shown, and the shield itself terminates to the left lugs just as shown in that layout ^^^^ up there.
This is not exactly as I installed one. I have only installed two, and removed one because of the NFB circuit going bye byes.
Ahh so Google PPIMV and look at layouts and pictures of installs. I have a small wire under the board from the 220k junction (removed resistors) that goes under the board and terminates at the left lugs of the dual 250k pot if looking from the back. The blk and red wires from the grid feeds are terminated as shown, and the shield itself terminates to the left lugs just as shown in that layout ^^^^ up there.
Re: Lar/Mar PPIMV
That's the diagram I have too. I wired it exactly as it shows but I can see how the shielding could be used to convey the bias voltage to the pots. However, since I gave the bias its own wire would it matter if I hooked the shielding to ground instead of to the bias V?
Earlier in this thread I said how, since installing the PPIMV it started feeding back, or some kind of low Freq parasitic oscillation, about 300Hz, when turned half way up. It was fine before but I'm looking for a way to use it in the house without replacing windows. I'm also looking into an Airbrake, but that will take a little longer.
Also, I'm concerned about the NFB problem. Does it remain even when the MV is dimed?
(Btw, Happy New year . . . a little early)
Earlier in this thread I said how, since installing the PPIMV it started feeding back, or some kind of low Freq parasitic oscillation, about 300Hz, when turned half way up. It was fine before but I'm looking for a way to use it in the house without replacing windows. I'm also looking into an Airbrake, but that will take a little longer.
Also, I'm concerned about the NFB problem. Does it remain even when the MV is dimed?
(Btw, Happy New year . . . a little early)
- martin manning
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Re: Lar/Mar PPIMV
Bias voltage is AC ground in that grid circuit, and that is the reason for the shield going to Vbias. Using the shield to carry the bias voltage is an interesting idea that was not part of the original design. Off-hand I can't see a problem with that. Reeltard, did you do one that way? Did it work out?sepulchre wrote:That's the diagram I have too. I wired it exactly as it shows but I can see how the shielding could be used to convey the bias voltage to the pots. However, since I gave the bias its own wire would it matter if I hooked the shielding to ground instead of to the bias V?
I have a Lar-Mar in my express, with the leads unshielded but twisted, and it works fine. See: http://ampgarage.com/forum/viewtopic.ph ... 948#205948 It's a non-traditional layout though, and the MV runs are pretty direct.
The beauty of this MV is that when it is dimed, the circuit is 100% back to the original. The odds are that your oscillation is due to some other lead dress issue. For low volume settings I suppose you could increase the NFB with a smaller FB resistor.sepulchre wrote:Also, I'm concerned about the NFB problem. Does it remain even when the MV is dimed?
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Re: Lar/Mar PPIMV
Yes I did but was also different values from Metro Amp kit
Re: Lar/Mar PPIMV
I put Martin's wiring to work in my recent Express but used single lead coax to keep noise down. There is a lot wire going to one dual pot but it works great and once the chassis was in the head cabinet I had no issues from the presonance control.
Mark
Mark
Re: Lar/Mar PPIMV
I used single lead coax too. Yes, there's a lot of wire going to the pot but they are not too long - 2 to 3 inches.
I didn't have the feedback problem before installing the MV. But I'll check everything again. Like Reel, I have a wire running from where the two 220K joined (now gone) to the left side of the pots. That's also where I connected the shields - kind of a mess with all those single lead coax shields coming together. Maybe I'll redo those, connecting them at the board end to clean up the pot area. It's rather crowded with the 2M resistors and all.
I didn't have the feedback problem before installing the MV. But I'll check everything again. Like Reel, I have a wire running from where the two 220K joined (now gone) to the left side of the pots. That's also where I connected the shields - kind of a mess with all those single lead coax shields coming together. Maybe I'll redo those, connecting them at the board end to clean up the pot area. It's rather crowded with the 2M resistors and all.
- dorrisant
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Re: Lar/Mar PPIMV
While on the Lar/Mar subject....
Does anyone see why a 1M dual pot strapped with 250K resistors wouldn't work in place of the usual?
It is strapped to roughly 224K either way... right?
Also... don't know where to source it yet but we have some shielded single pair twisted wire at work... I think it would be the cat's meow. Only have to comb/pick out the ends of one cable...
Tony
Does anyone see why a 1M dual pot strapped with 250K resistors wouldn't work in place of the usual?
It is strapped to roughly 224K either way... right?
Also... don't know where to source it yet but we have some shielded single pair twisted wire at work... I think it would be the cat's meow. Only have to comb/pick out the ends of one cable...
Tony
Last edited by dorrisant on Mon Dec 31, 2012 2:38 am, edited 1 time in total.
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- martin manning
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Re: Lar/Mar PPIMV
It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.dorrisant wrote:While on the Lar/Mar subject....
Does anyone see why a 1M dual pot strapped with 250K resistors wouldn't work in place of the usual?
It is strapped to roughly 224K either way... right?
- dorrisant
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Re: Lar/Mar PPIMV
Thanks Martin. I think I will try it both ways and listen for the difference... dual 250Ks are delayed in transit.martin manning wrote: It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.
Tony
"Education is what you're left with after you have forgotten what you have learned" - Enzo
Re: Lar/Mar PPIMV
Martin is right and it's worth the wait!dorrisant wrote:Thanks Martin. I think I will try it both ways and listen for the difference... dual 250Ks are delayed in transit.martin manning wrote: It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.
You'll be disappointed with a dual 1M pot and probably the PO will be driving you nuts
Larry
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Re: Lar/Mar PPIMV
Usually I don't use shielded wires by putting in a PPIMV into an amp, I just twist the cables and additional I'm running the twisted bundles close to the chassis - and never have had PO problems.martin manning wrote:Using the shield to carry the bias voltage is an interesting idea that was not part of the original design. Off-hand I can't see a problem with that.
Hence I haven't recommended, using the shield as for carrying the negative bias voltage.
But there's NO reason, not using the shield for this purpose
if you're using shielded cables.
Larry
Larry's Website now with included Pix's Gallery