ME.HarpDoc
100 posts
Jan 25, 2016
3:54 PM
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This may seem like a stupid question, which is why I put it in this forum 'cause i know such questions are ok.
I noticed SB on a post on the main forum talked about removing the volume pot from his CM mic.. What is a volume pot? how does one remove it, add it or replace it? Exactly what does it do?
I've got an Astatic JT30 with a Shure CM element. Does it have a volume pot? What's it look like?
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SuperBee
3294 posts
Jan 25, 2016
4:12 PM
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"Pot" is short for "potentiometer".
It's a variable resistor, commonly used as a volume control. If you have a volume control on your mic, it'll be a pot. The signal from your mic element has to pass through the pot. Rotate the knob one way, there is less resistance, more signal passes through to the amp, rotate the other way and resistance increases, less signal passes. "Signal" is basically AC current.
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Killa_Hertz
344 posts
Jan 25, 2016
7:09 PM
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Actually that's not exactly acurate, but close enough. A potentiometer or Pot is a voltage divider. As you turn the knob 100% voltage from one side starts to split and the remaining goes out the other.
People use potetiometers and tap off of the wiper making a rehostat. A.K.A. variable resistor. It's actually better to use a pot as a variable resistor for volume control as the pot can reach zero with no leakage.
Anyways its a volume knob.
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"Trust Those Who Seek The Truth. Doubt Those Who Say They Have Found It."
Last Edited by Killa_Hertz on Mar 30, 2016 4:06 PM
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MindTheGap
1038 posts
Jan 26, 2016
12:35 AM
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Yes, I always wondered if people wired crystal/ceramic pots as a variable resistor rather than a potentiometer for that very reason. I don't have one - a crystal I mean.
Either way, you end up with a voltage divider - the impedance of the element, the VC resistance and the resistance of the amp input. Some of these things will vary from case to case (e.g. different elements, different amps), which might explain why people's reported experiences vary.
As to how to remove it if you have one - that's a practical question for a mic person! But I'd have thought a simple option would be to leave it physically in place and rewire the mic internally to bypass it, then you can put it back into service if you want.
There's good reasons why you might get strange effects - either desirable or unwanted - using VC with a crystal/ceramic element. Less so with your dynamic element.
Last Edited by MindTheGap on Jan 26, 2016 1:12 AM
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Killa_Hertz
350 posts
Jan 26, 2016
5:34 AM
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Yes u can just bypass it.
The reason you use a pot and wire it as a rehostat instead of just buying a rstat. Is that a rehostat doesnt completely hit zero. It tends to leak. So a pot wired as a rstat makes a better volume control.
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"Trust Those Who Seek The Truth. Doubt Those Who Say They Have Found It."
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MindTheGap
1041 posts
Jan 26, 2016
7:34 AM
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kHz - Do you know for certain that people wire up their mic pot as a variable resistor (rheostat) rather than as a potential divider?
I can see it means you can turn it fully on - so no lingering questions over loss of signal, whereas wired as potentiometer it is always putting a load on the element. But then after that the effect of it (the 'taper' if you like) is dependent on the other values in the circuit - which could be different for different amps for example. So results could vary.
I've heard people describe some of their VC's as effectively an on/off control, i.e. either fully on or very quickly tailing off to nothing. This would explain it. I imagine a 10M 'rheostat' into a 1M amp input for instance. A little tweak of the VC and the signal would drop like a stone. Especially if they used a log-taper pot.
No guessing now :)
Last Edited by MindTheGap on Jan 26, 2016 7:49 AM
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Killa_Hertz
353 posts
Jan 26, 2016
9:32 AM
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Yea well If you don't wire it up as a rehostat, where are you putting the other leg. A pot has 3 legs. If u use 1 and the wiper its essentially a rstat. If u used 3 where would u wire the other leg to? U have a line and load. No use for the other leg.
Yea the people who don't have a range in their VC used a pot that was too big. The smaller the pot, obviously, the less value change in the turn. So easier to make small changes.
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"Trust Those Who Seek The Truth. Doubt Those Who Say They Have Found It."
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MindTheGap
1043 posts
Jan 26, 2016
9:41 AM
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Oh I see. Well the normal thing is to wire it as conventional potential divider. The signal from the mic goes to legs 1 and 3, and the output goes from the wiper (say leg 2) and leg 1. The output signal voltage varies from 0 to 100%.
I say the normal thing - I mean the normal thing in every other electronics application. Harmonica seems to enjoy special case status in all kinds of ways :)
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Killa_Hertz
356 posts
Jan 26, 2016
9:50 AM
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I'm unclear on what u meant. If you wire the signal to both legs the volume wouldn't go down.
Maybe you mean signal 1 ground 2 wiper 3.
If you ground the second leg to drain that will work.
What your about is the way you would write it for an actual variable resistor. I think.
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"Trust Those Who Seek The Truth. Doubt Those Who Say They Have Found It."
Last Edited by Killa_Hertz on Jan 26, 2016 9:56 AM
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MindTheGap
1044 posts
Jan 26, 2016
10:14 AM
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Going back to your OP - you mention that you can use the pot as a conventional potentional divider (i.e. what it was designed for) or alternatively wire it as a variable resistor in-series.
I'm saying I can see why people might do that, especially for a crystal element as when fully on there is no extra load on the element. But that the taper as you turn it off may be a bit strange.
I was just wondering if people actually do that, that's all.
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SuperBee
3295 posts
Jan 26, 2016
12:53 PM
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I think mine are connected to ground on one side. I only recently took one apart...removed the volume control from a jt30. Thankfully it had been installed so as not to disfigure the shell, which is a very clean and otherwise original 60s job.
I then installed a new crystal in my other jt30 which already had a crystal and 5 meg pot. I think I just hooked it up as it was, probably checked some reputable info source...I was more focussed on heat-sinking the connection of leads to element. Those mc151 elements have very short lugs. Anyway, it was a great improvement on the old element but still not what I wanted. So I'll probably take the volume control out of the circuit and try it.
Thanks for the discussion. I hadn't considered the difference, but I know just enough electronics atm to follow instructions if they're clear. when they're not is when I learn things.
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SuperBee
3296 posts
Jan 26, 2016
12:59 PM
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Oh, I may have mentioned using vc as on/off, but that's not because of the taper, just that I was in the habit of turning right down when not playing, to approach amp etc. So a vc can be treated that way, like a more versatile on off. For me, more useful than a harp shield. I like my inline vc as built by Greg heumann, but the mics I have with pots built in to the mic have not impressed me. It could be coincidence though.
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ME.HarpDoc
104 posts
Jan 26, 2016
5:38 PM
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ummm.....physics and electrical engineering wasn't my strong point!
My JT30 sounds great with one of Greg's volume controls. I had someone who knows mics install the Shure element so I'll still have to see what it might look like inside the mic.
The answer "it's a volume controle"was good for me :-)
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MindTheGap
1047 posts
Jan 27, 2016
12:39 AM
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ME.HarpDoc :)
SuperBee, I wasn't thinking of you there. I have read people say that their VC is either fully on, or if they tweak it a bit it's basically off with not much in between.
I have couple of mics with VC's and one of Greg's XLR ones - I'll have to stick a ohmmeter on them and see what they are doing.
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SuperBee
3303 posts
Jan 28, 2016
5:28 AM
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no no, it just occurred to me that ive said that i use it as an on/off, but i get that some of them only work that way due to poor circuit choices...or good ones i suppose, if thats what you want..
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MindTheGap
1055 posts
Jan 28, 2016
8:05 AM
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OK ta.
On related point, in the main forum they mention the input impedance of Fender amps being as little as 68k. Is that typical in your experience? If so, I would have thought that would have a strong effect on the tone of a crystal element (i.e. not just the overall signal strength).
Those Lone Wolf pedals have a FET-buffered input so I'd expect very high impedance, probably a good reason in itself for putting one in front of an amp.
Edit: that's strange that comment about 68k seems to have gone, maybe someone edited it or the spam filter got it. So, let me revise the question: in your experience, is there a typical value of impedance on the input stage of all those amps you've built and worked on? :)
Last Edited by MindTheGap on Jan 28, 2016 9:52 AM
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SuperBee
3304 posts
Jan 28, 2016
12:18 PM
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I actually have never worked on an amp built by Fender. I have 3, and i just sold 1, but the amps which have been worked on were taken to professional technicians. The deville was modified before i decided i was going to learn to work on amps, and the princeton reverb had issues too baffling for my rudimentary knowledge. The amps ive worked on, i have always used 1meg on the input i think. My 5f1 clone may have had 68k on the schem. Ill have to check
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Killa_Hertz
374 posts
Jan 28, 2016
5:46 PM
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Bee your right the second side is usually drained to ground.
MTG. Sorry i didn't understand you at first
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"Trust Those Who Seek The Truth. Doubt Those Who Say They Have Found It."
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Halffast
32 posts
Mar 30, 2016
7:16 AM
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Ok , I just picked up an Akai DM-13 mic and really like it . No VC so I have to turn it up/down at the mixer . It has the cable attached so I can also control the volume with an inline attenuator/VC (that I bought from a manufacturer out of Australia on ebay ), but that puts the control 6 feet at the end of the cable too , which isn't all that handy either . I have a potentiometer ordered to install in the mic case so I'll have a VC at fingertips . Convenient .
But , after revisiting this thread I'm a bit undecided on how to wire it . Divider or rheostat style ? There are some excellent wiring diagrams on the Lone Wolf site . Two show the pot set up as a divider , one way favors coarse adjustment , the other fine . The rheostat method is said to give the most fine adjustment . I would like to get a consensus on the preferred method from you guys in the know . Of course I can always change it to experiment but I'd like a place to start . If I like it I may not experiment at all . I'm leaning toward rheostat but I really have little knowledge to base that on .
Last Edited by Halffast on Mar 30, 2016 7:18 AM
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MindTheGap
1363 posts
Mar 30, 2016
7:52 AM
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Potential divider for me, for a dynamic element. Now I've picked up on the main forum what socket will fit, this is on my list as a rainy-day project.
If it were a crystal-element mic, I'd try wiring it as a rheostat.
Last Edited by MindTheGap on Mar 30, 2016 8:25 AM
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SuperBee
3548 posts
Mar 30, 2016
2:28 PM
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When Randy is talking about fine and coarse adjustment he is referring to the log taper effect of the pot, and saying if you wire it 'backwards' you'll change the dynamics. It's a pretty minor issue in practice I think, but obviously some think it's worth considering.
On this topic of impedance I notice my crystal mic...which has a 1 meg pot and the ground side of element is wired to the ground side of pot, which is also connected to ground side of connector...which I think you are distinguishing as 'rheostat' above...(whereas I think you're saying 'potential divider' where the ground bypasses the pot and goes directly to connector...)
My crystal mic...I feel like Anne Elk...works much better when it goes though my lone wolf pedal.
And that's nice to discover. In testing I'd just plugged it directly to amp and was a little disappointed but in jam when I put it through the pedals it was bloody good. I started to understand the fuss.
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MindTheGap
1365 posts
Mar 30, 2016
2:45 PM
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Are we all on the same page re divider vs rheostat? I can't tell from the words in here. Divider...mic element on the left, what goes into the amp on the right.
Re your crystal mic, I would put it down to the higher input impedance of the pedal than the amp, loading the mic less. But then I would say that as I'm clearly obsessed with impedances. Is there a cure?
Here's a good one:
So as the diagram clearly shows, the advantage of the rheostat is that when it is fully 'on', the mic is only loaded with the impedance of the amp. The pot is effectively out of the picture. The disadvantage is that you can't turn it completely off, and also the taper of the thing depends on the amp impedance - so it could work ok for one amp and not very well for another.
Whereas with the divider, the mic is loaded by the pot resistance in parallel with the amp input resistance. So a lower load resistance, which means a lower signal voltage going into the amp. And possibly nasty effects on tone as the crystal mic's source impedance is dependent on frequency.
I still don't know what people do in practice, I must get the ohm meter out. But I said that already.
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Last Edited by MindTheGap on Mar 30, 2016 3:12 PM
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SuperBee
3551 posts
Mar 30, 2016
2:51 PM
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No I don't think so. I can't follow that diagram but I have to run to work now.
I especially don't know what you mean mtg. I think I am on same page as kHz
Oh...I think you edited. I'll look again later.
Last Edited by SuperBee on Mar 30, 2016 2:52 PM
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MindTheGap
1366 posts
Mar 30, 2016
3:07 PM
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Yes I edited it further. Must stop.
...but I did learn a new thing about Anne Elk and Elk Theories. I didn't know that. It's a good concept:
The sketch inspired the concept of "Elk Theories" to describe scientific observations that are not theories but merely minimal accounts
Last Edited by MindTheGap on Mar 30, 2016 3:13 PM
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Killa_Hertz
884 posts
Mar 30, 2016
4:11 PM
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A rstat is a pot actually ... so ....
Nevermind ive been confusing everybody .. as well as myself ....
Short answer is MTG is right. What he has above. Is the way you wanna go.
1. Line. 2.Load (wiper) 3. Ground
Just make sure you have a linear pot and you size it right.
Electronics is not my area of expertise so I sometimes say things that are confusing. Or just incorrect. Sorry.
As me about Transformers, Programmable Logic Controllers, Ladder Logic, Conduit, Motor Controls, Relays, Switch Gear, etc. Im good.
The little stuff I have to do some remembering. I had to go look it up n refresh my memory.
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"THE BLUES IS STRONG WITH THIS ONE! "
Last Edited by Killa_Hertz on Mar 30, 2016 4:19 PM
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Killa_Hertz
886 posts
Mar 30, 2016
4:50 PM
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The only difference is the ground, so i want too far off. But funny enough i just went over to my breadboard to see how i have these vol. Gain. Tone pots set up. They are all grounded. Lol. So apparently voltage divider is the way to go ... lmao.
Pay no attention to the man behind the curtain. And dont pet the monkey (He Bites!)
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"THE BLUES IS STRONG WITH THIS ONE! "
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SuperBee
3552 posts
Mar 30, 2016
5:26 PM
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if i understand those diagrams...the "Z Load" part represents the amp (for instance)?
in which case all the controls ive wired are as per the 'potentiometer' example.
the 3 diagrams halffast refers to above are on the lone wolf site...
the first shows a log taper pot wired as per the potentiometer example in MTGs latest post.
the second shows the same control wired backwards to get the taper working the other way around
the third shows the ground lead bypassing the pot, so only 2 of the 3 legs of the component are connected..the ground leg is not connected...signal goes in on one leg and comes out on the wiper, thus the control is just a variable resistor.
im glad of this discussion as i'm in the process of replacing a tone pot in one of my amps. its originally a Kriesler mantle radio from 1964, which has been gutted and rewired as a single-ended guitar amp, using original transformers, many old caps and the old fittings from the radio...tone and volume controls...it has an illuminated tuning dial...looks pretty cool and is rather low gain, kind of like an old champ, but with kalamazoo tubeset...6X4 rectifier, 12AX7, EL84. the original speaker is an oval 6" x 4" (funny coincidence with the rectifier tube designation!)but has provision for an external speaker.
the tone pot is cactus...and its also the on/off switch. kind of hard to obtain a spare part...its a long shaft, 25K pot with switch rated at 2amp at 250VAC...and the way the guy has wired it i found rather hard to follow...but looking at the 'rheostat' drawing above i think i may be getting some insight.
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Killa_Hertz
887 posts
Mar 30, 2016
5:31 PM
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However after looking at your rstat diagram again i believe it is wrong.
The rstat should be
1. Line 2.load (wiper) 3. Unused.
I think thats right.
Edit : apparently both are correct my way and your way.
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"THE BLUES IS STRONG WITH THIS ONE! "
Last Edited by Killa_Hertz on Mar 30, 2016 5:32 PM
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MindTheGap
1367 posts
Mar 30, 2016
11:13 PM
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kHz - re the rheostat diagram, yes I had a double-take when I saw that. In practice I'd wire it the way you say. Maybe it's a drawing convention, or maybe it's a way they wire power-rheostats to ensure they never go open-circuit if the wiper loses contact. Just speculating.
I found the page you mention on the LW site: lonewolfblues.com/techmicpot.html
I can see the point about backwards wiring. Let's not miss that kHz mentions a 4th option - to use a LINEAR taper pot, which would give you something half way between the two. That's a very good idea.
Also you could apply this to the rheostat wiring to have three options: log taper forward/reverse or linear taper. The resulting taper would be different in each case.
I have to stress I've not actually wired any of these, other than in my mind :) It's cheaper like that, and in my mind my soldering is very neat.
For completeness I'll say that the LW page says that in divider mode "Impedance to the element will equal the impedance of the control." which as soon as you connect it to an amp is an approximation that's probably fine for a dynamic mic, but may not be fine for a crystal mic, depending on the particular values of pot and amp. That's the point I'm trying to get across.
IMO a much better VC for a crystal mic would be a little hi-impedance preamp. And that's what the LW pedals give you. They are on the floor though, so there is that.
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Last Edited by MindTheGap on Mar 31, 2016 12:00 AM
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Killa_Hertz
890 posts
Mar 31, 2016
1:07 AM
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Yes, the lw has it the other way.
That's what i was saying origionally.
That's why is kinda confusing it depends on the kind of pot, and the element aswell.
I like the way LW did it.
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"THE BLUES IS STRONG WITH THIS ONE! "
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MindTheGap
1370 posts
Mar 31, 2016
1:41 AM
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Yes. I didn't draw those pictures btw, they were plucked from the air.
Thinking about it theoretically is all well and good (and it is good) but when I'm there with the smoking soldering iron, I also want a diagram that says plainly 'connect the red wire to terminal 1'.
...actually there's a good analogy with music theory vs real playing in there somewhere. I can't quite put my finger on it.
Yet another example of why engaging with MBH is good, in spite of everything. I'd have never have considered using a rheostat as a volume control. Tramlined thinking again.
Last Edited by MindTheGap on Mar 31, 2016 1:53 AM
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Killa_Hertz
891 posts
Mar 31, 2016
4:21 AM
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In spite of everything like what?
I mean i can imagine. Necoran (Nate) has told me the horror stories of the"good ol days" but nothing on here really bothers me. Accept people can be a bit uptight and don't really know how to joke around to much. Seriousness. Always Seriousness. ... Lol. But its whatever. I like to think that maybe a few of the comments ive made, might have taught somebody something. Or at the very least pointed them in the right direction. Cus god knows the shape ide be in with out the good advice of others on here. So .... it is what it is.
Ahh yes ... one piece of advice. Buy a decent little bread board. Great for experimenting with electronics.
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"THE BLUES IS STRONG WITH THIS ONE! "
Last Edited by Killa_Hertz on Mar 31, 2016 4:22 AM
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MindTheGap
1371 posts
Mar 31, 2016
4:37 AM
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In fact I had in mind the slightly addictive nature of the thing. Like my somewhat obsessive crusade over impedance matching :) On the other hand I quite like that aspect too.
But yes, there has been plenty of argy-bargy in the past. I don't like it, but I don't mind it that much other than it gets in the way.
I think your contributions and enthusiasm are a very positive force and I'm sure have helped a number of people. Certainly it's helped me.
Re breadboard. Good idea. Coincidence, I was looking at a YT vid the other day (something about valve amps) and there was a breadboard, and I thought that was a good idea, but as usual, did nothing about it.
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Last Edited by MindTheGap on Mar 31, 2016 4:45 AM
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Killa_Hertz
893 posts
Mar 31, 2016
5:28 AM
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Thanks man i really appreciate that. I could only hope as much. Enthusiasm Keeps it driving u kno. If not is just work.
My breadboard isn't anything fancy. Just a $15 - 20 radio shack board. Only thing is dont get a really tiny one because you ruin it off space quick. Mine is aprox. 8" x 3" ... something like that. Is got "enough" space, but not ample by far. But they make ones smaller and idk how you de get anything done on them.
For jumpers .. take a piece of cat5 phone cable , cut into 6" pieces, viola free jumpers. They sell jumper packs, but why buy em? I have fun with mine. Biggest upside is, if your building something like an amp, you can change the values of different components (caps., resistors, etc. ) by plucking it out n plugging in a different one. Just to experiment with what effect it will have on your sound or range of adjustmant, etc.
Really handy to have. I think you ll like it. And relatively cheap.
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"THE BLUES IS STRONG WITH THIS ONE! "
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Halffast
36 posts
Apr 08, 2016
2:21 PM
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Just finished wiring my other Akai DM13 ( bought it as a "spare" and sacrificial/experimental mic , in case I screwed up installing the VC ) . Thanks to discussion in this thread , I wired up a tiny , 250K ohm pot. as a voltage divider in the " reverse " configuration . It works great ! I'm very pleased as it has quite a range of volume control , doesn't affect tone , and will shut the mic down completely if I want . No need to experiment any further as it does everything I had hoped . Thanks , guys , as I had originally planned on wiring it up as a rheostat . Now I'm going to do my original mic when I get a chance too .
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MindTheGap
1435 posts
Apr 09, 2016
1:28 AM
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Very good. Can you show us the pot (or link to a photo) - how small does it need to be?
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Last Edited by MindTheGap on Apr 09, 2016 1:29 AM
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Halffast
37 posts
Apr 09, 2016
6:38 AM
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http://i.ebayimg.com/images/g/SrcAAOSweW5VWBvX/s-l500.jpg
Got the pots off ebay ( hope the link is above ; I don't know how to post a pic ) . They are very small and I believe they were actually designed to be circuit-board mounted . I bent the connecting terminals straight forward , then bent the board mounting tabs down flat ( well , actually crimped one onto a wire that goes to the right terminal and soldered it to act as ground ). With all wiring in place it was a bit snug but I had used a lot of shrink tube over the connections which made things a little stiff ; probably wasn't necessary . I also had drilled the mounting hole pretty far back on the mic body so as not to be in the way . That made it harder to get the pot into the mounting hole but not too bad . Totally worth the effort . Used it to sit in with a blues band last night and it worked great .
Last Edited by Halffast on Apr 09, 2016 6:41 AM
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MindTheGap
1439 posts
Apr 09, 2016
9:18 AM
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Got it, thanks. When I finally get round to this, I think I'm going to try put the pot as high up towards the hands as I can manage.
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