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"Rich-Mod" PPIMV in Plexi - Occillation issue


 
12/17/2005 2:36 PM
Jeff Miller
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"Rich-Mod" PPIMV in Plexi - Occillation issue
Admittingly, I am very green when it comes to building & troubleshooting amps. The amp in question is a Metropolous 1959SLP Kit I assembled.  
 
 
I have my PPIMV control in the lower input of the "normal" (bassy) channel of my Metro Plexi.  
 
I remember when I installed it (the first time) that I didn't have any issues - went in like a champ, and worked well.  
 
I finally got around to getting the correct value pot (dual 500K, vs. the dual 1M I installed) - which I ended up not installing, but had removed my 1M dual pot in prep for installing the D500K - I decided the values were too far off on the D500K in comparison to what I had with the D1M. So, I re-installed the D1M, and also changed the bias-splitter resistors so that it would be "stock" with the PPIMV dimed - at least, ideally. (I could be wrong - lol)  
 
Re-installing it, I ran into some occilation problems - which I finally got resolved by luck (IMO). Well, my luck has ran out. With the PPIMV operating at around "8" on the level - I get some ringing/whistling, then a thump, and loud whistling. Moving the wires going to the D1M pot does change where the occilation starts occurring, but the "best" I've been able to manage is around 8 on the control. Prior to this, it was operating quite well and I could dime everything and it was as expected. I thought maybe it could be tube related - but swapping out V1 didn't have any change in the results. Plugging into the low-input on the bright channel results in the control being able to go higher (roughly "1" more) and plugging into the normal/bassy channel's high-input results in a similar (though different sounding) result as the high-input on the bright channel  
 
I've also noticed that if I move the bright channel's input-wires around, it also changes when it starts occillating. I'm thinking of removing the PPIMV to test for any other issues with occilation as well - but that would be a last resort IMO, or, to yank out and put in shielded wire (lol)  
 
Right now, I have no shielded wire in the amp, and, I do get a bit of hiss and hum at higher levels. (maybe shielded wire would cut that down as well?)...  
 
Other than the PPIMV and the different Bias-Splitter Resistors, it's as stock as the kit can get. I also have the bright caps at 100pf on both channelsm, vs the 500pf the kit calls for on only the bright channel...  
 
If anyone has some suggestions - I'd greatly appreciate it!  
 
Thanks!  
 
PS: I know pots can sometimes go microphonic, but I think this might not be the case - since it moves around to different frequencies. Also of note, I can simply place my finger in certain spots of the amp, and it will reduce or induce occillation. Talk about frustrating!  
 
Thank you for any help/advice.
 
12/17/2005 4:18 PM
Tom Phillips
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Jeff,  
Good description of the symptoms.  
I believe that your oscillations are caused by your overall wire routing which brings the signal from the phase inverter to a location near the input jack. This is not a good practice in a high gain circuit. The high level signal from the PI is coupling into the sensitive input stage. Or it may be coupling into one of the other stages as it makes its way back to the front end of the amp. Maybe both.  
 
You have already seen the effect of slight changes in wire dress. You are "living on the edge" with the setup you described and I'm not surprized at all that it's giving you trouble. I believe that you need to either shield the wires to the MV pot, or better yet, mount the MV pot somewhere closer to the PI. If you don't want to drill another hole you could mount the MV pot in one of the speaker output jack holes.  
Regards,  
Tom
 
12/17/2005 10:07 PM
Jeff Miller
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Wow - thank for the reply Tom!  
 
I'll give it a shot with some shielded wire first* then if that doesn't pan out, then I'll go to using one of the speaker jacks. I'd prefer to have it where it is currently located (convienience, reversable, etc)  
 
 
*Regarding the shielded wire. I have some that has two conductors plus a shielding conductor.  
 
 
I have three questions regarding the use of some shielded wire:  
 
 
What would be the minimum wire gauge for use with a PPIMV?  
The wire I have is less than 20ga, but the wire I am currently using is 20ga.  
I assume there isn't much voltage or amperage flowing through the wires, so it may be feasible to use some wire that is smaller than what I've been using - at least, that's my train of thought :)  
 
Would I be able to minimize my usage of the wire by passing the "from board to pot" for one conductor, and "from pot to board" for the other conductor - or, both wire-pairs being the "send" and another wire-pair being the "return"? If so, which would be the ideal method? If not, then I assume that I would need to run one shielded wire for each wire going to/from the board to the pot (total of 5 "sets")  
 
What about the shield - would the shields need to be grounded in this instance, or, should I use the shield to be the conductor for the 5th wire, which goes to the junction of the bias splitter resistors?  
 
Thank you!
 
12/17/2005 11:33 PM
Tom Phillips
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Jeff,  
Here’s my thoughts on your questions.  
 
1) You can use small gauge wire for this application because the current is low. (It’s the current that determine the wire size. Voltage determines the insulation required) 22 or 24 AWG will be fine. Even smaller is Probably OK but it gets harder to work with.  
 
2) I think that either wiring method you listed would work. Method A: Passing the "from board to pot" for one conductor, and "from pot to board" for the other conductor – or Method B: one wire-pair being the "send" and another wire-pair being the "return"? My preferred method would be “B”  
 
3) The shield: The shields do need to be grounded to do their job. Usually the shield is only grounded at one end to prevent adding a ground loop. However, you probably could get away with using the shield as the conductor for the 5th wire, which goes to the junction of the bias splitter resistors. Try using the shield of the cable for the wires "from pot to board". If we are thinking of the same wires, those would be the wires connected to the other side of the bias splitter resistors. By the way, this still allows the shield, which is now connected to the bias supply line, to act as a shield because the bias supply is at AC ground. But only if you have the “normal” Marshall bias supply topology. Meaning that the junction of the bias feed resistors connects directly to one of the bias filter caps without going through a resistor. Connect the other shield to chassis ground at one end.  
 
One other thing to consider. It could only take a short portion of unshielded MV wire getting too close to the input jack wire to cause a problem so pay close attention to the wire routing at the pre-amp input end. Try rotating the pot so that the lugs are farthest from the input jack and dress the MV wires so they stay farthest away from the input wires.  
 
Good luck,  
Tom
 
12/18/2005 12:04 PM
Jeff Miller
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I beleive we are thinking of the same wires :)  
 
"From board to pot" is coming off the .022uf caps (I think these are referred to as coupling caps?) - which connects to the pot on the 1st lug.  
 
The from pot to board connects to the 2nd lug (wiper) to the bias-splitter resistor, and the 5th wire connects to the junction of those two resistors and the 3rd lug on both pots.  
 
If this is correct, then I think I'm ready to go forward with making the nessicary changes :)  
 
Thank you for your time and great responses!
 
12/18/2005 4:34 PM
Jeff Miller
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Tried it, no better (or worse) than what I was at before.  
 
I dismounted the PPIMV pot (still wired) and had it in another area of the amp (still not mounted) and was getting the same result.  
 
I must have something else wrong/screwy...  
 
Oh well - how often do I play my Plexi wtih the PPIMV above half-way - hmm - rarely - lol!  
 
I may still play around with which one is the "5th" wire - since I did it backwards from what you're referring to - I do have both "sends" grouped, same for the other pair, but the send to pot (off the .022uf caps) is the grounded wire (not attached on the other end) and the returns are the ones that have the 5th wire on the "shield" wire.  
 
I may try reversing them, but, given that I move the pot, wires and all completely away from the input jacks (more like over the middle control, kinda "floating - lol) - it didn't make a single bit of difference.  
 
I think maybe I'll call George of Metroplous (kit supplier/builder/owner) and see what he says. I might have to use shielded wire in more places - lol.  
 
But, if I go there, it's mod-time.
 
12/19/2005 6:13 AM
Matt
Jeff,  
Since the Presence control takes it's signal from the impedance selector, I move the Presence control to the back of the amp and put the MV where the Presence control was. This puts the MV down at the PI-end of the chassis and also puts the MV on the front panel. I don't adjust the Presence control very much so putting it on the rear doesn't cause much inconvenience. I also use shielded wire for all the MV wiring except the common 'ground' wire just to be sure. Food for thought.
 
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