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Author Topic: Noisy pots  (Read 2729 times)

mckayprod

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Noisy pots
« on: May 24, 2017, 12:54:41 AM »
I've been working on an old Stromberg-Carlson PA amp hoping to get some tube mojo to drive with my (also in-progress) Elliot preamp.  When I get the PCB done, I think it'll be fine, but I've noticed that my guitar's volume controls are awfully noisy when I plug directly into the mic inputs. According to the schematic, the input preamp is a 12AX7, running grounded grid (nothin' fancy here) with a 4.7megohm grid resistor.  Somewhere in the back of my mind I remember seeing where too HIGH of an input impedance can give you that "scratchy pot" problem.  The problem is worse with my humbucker pickup guitar, better with my Tele-clone.  Am I mis-remembering something, or is there a problem with the amp I'm missing?  And, why would the input z make a difference?  I thought input impedance had more to do with frequency response by loading of the pickup.

Enzo

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Re: Noisy pots
« Reply #1 on: May 24, 2017, 03:23:58 AM »
Might need a blocking cap, is ther any small DC voltage on that input, like from the tube grid?

mckayprod

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Re: Noisy pots
« Reply #2 on: May 24, 2017, 09:31:09 AM »
Blocking cap is a good idea.  The amp is down & out now, so no testing is possible, but I'll measure for DC when the new board gets fabricated.  My main thrust is, can an input impedance be too high?  I thought higher is better, to be blunt.  4.7meg isn't very high, compared to acoustic (piezo pickup) input stages that run 10meg or so.  As you might expect, when I run through a pedal the scratchy pots are just fine.

phatt

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Re: Noisy pots
« Reply #3 on: May 24, 2017, 10:17:00 AM »
Blocking cap is a good idea.  The amp is down & out now, so no testing is possible, but I'll measure for DC when the new board gets fabricated.  My main thrust is, can an input impedance be too high?  I thought higher is better, to be blunt.  4.7meg isn't very high, compared to acoustic (piezo pickup) input stages that run 10meg or so. As you might expect, when I run through a pedal the scratchy pots are just fine.
[/b]

That's because it is isolated (decoupled) via a capacitor in the pedal.
The guitar is DC (Direct Coupled) hence the scratch.

Re Input Z; It's horses for cources, yes a piezo needs very hi Z otherwise you loose bass.
For Mag PU the Input Z does not have to be insane high. Anywhere from 200k up to 1 meg is all that is needed. A Carvin Legacy guitar amplifier has ~220k input Z and it's a very hi gain rig with more top end than one could ever need. Going to **1 meg** on a circuit like that would just deliver more fizz, hum and buzz as it would be too sensitive.  :loco
Phil.

Ed; Opps meant to say 10 Meg :-[
« Last Edit: May 25, 2017, 01:56:10 AM by phatt »

mckayprod

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Re: Noisy pots
« Reply #4 on: May 25, 2017, 12:31:13 AM »
Curiouser and curiouser!  I've not heard of this issue from the other PA-head aficionados, but there you are.  If this rig were used as intended, with a (hi-z) microphone as the input, there would be no problem, since those devices don't have pots.  This shouldn't be a problem if I use an external preamp through the phono (line) input or just a pedal ahead of one of the mic inputs.  Thanks for the analysis.

phatt

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Re: Noisy pots
« Reply #5 on: May 25, 2017, 07:10:25 AM »
You mentioned Phono and Line as being one and the same input? ???
Just be aware Phono and Line are very different inputs.

Phono is for those old vinyl records and the freq response is dramatically altered to match either a Ceramic cartridge or a Magnetic cartridge.
IIRC
Ceramic Phono PU is very high Z, while magnetic input is around 50k impedance at maybe 50mV sensitivity.

Line level is maybe 10k impedance expecting a sensitivity of 700mV or maybe even 1V. Sometimes older equipment came with a switch to switch input from line to phono sensitivity.

If you have a preamp then try the line input (if that is what it says)
If you want a bit more glass magic then change the 4.7Meg grid resistor down to 470k,, or any value from 220k up to 1Meg. Try a few different values until you are happy with the result.  8|
As I don't have the schematic,,You may have to bypass a lot of the line/phono networks and such and wire direct to V1.
HTH, Phil.
« Last Edit: May 25, 2017, 07:12:17 AM by phatt »

mckayprod

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Re: Noisy pots
« Reply #6 on: May 25, 2017, 12:42:50 PM »
This amp has nothing so fancy as a "real" phono input...it just provides an RCA jack feeding the second amplifier stage, no eq network, just a 500k pot and a 470k isolation resistor.  "Phono" probably refers to a ceramic phonograph cartridge with a big output level.  In broadcast use, I might have connected a radio tuner to provide audio at a remote location, mixed with the local mic.  Should be pretty forgiving of level and impedance.  I could post a schematic if there's interest, but this thing is pretty primitive.

J M Fahey

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Re: Noisy pots
« Reply #7 on: May 26, 2017, 11:27:09 AM »
1) starting from the end: please do post the schematic, we always love to see weird ways to do things
2) that 4M7 grid resistor most certainly means the first tube is grid leak biased.
Grid leak meaning exactly that: some electrons are catched by the grid, turning it negative and providing "free"  bias  :o
Now if you let the grid free float it does not provide a voltage reference relative to cathode (which is what matters) and plate current wanders all over the place because actual bias floats all over the place.
And if grid to ground resistor is way too low it sucks those electrons down and bias disappears; I bet some brave soul found hatbby using a high but defined resistor there bias voltage is quite stable; you will find 5M to 10M resistors there, maybe the odd 3M3, hardly any lower.
Of course, this is a real and measurable DC voltage; if it interacts with a guitar pot it *will*  scratch  :o
If the amp is old enough, (say 40`s or 50`s) normal cheap microphone was a high impedance crystal one, and normal phono pickup was again a high impedance, very high output crystal powered too.
Those might provide up to 1V RMS output, a few 1.5V  :o

Those were basically flat, and old records were only lightly compensated, nothing like bass heavy treble shy RIAA which came later, and at the beginning was a "Hi Fi guys"  only technology.

I started working on this in 1968 and some early jobs (which seasoned Pros would reject as worthless of repair) were by then old (do the Math) small town dancing Clubs, old School/Church PA systems, etc.
Think your Bogen and similar PA amps, and invariably consisted of a squarish chassis, with 2 EL34 (we are quite European minded and 6L6 were unused and practically unknown) , driven by a couple octals in the older ones, or miniature 8 pin (not a typo) Philips "A technique" tubes, or the odd EF86.
Front panel showed a Mic Volume pot, for the announcer and/or singer (orchestras were unamplified), a Phono input for "selected recordings" played by the Stone Age DJ ancestor, sometimes a single knob Tone control, .... and nothing else.


People hated those amps because all were poorly maintained, hummed/buzzed (you could always tell whether the PA was ON or OFF  ::) ) and when they came for repair they not only distorted a lot (that was almost acceptable) but chopped audio to the point of being fully unusable.

Typical problem was that leaky paper in oil caps leaked horribly (both electrically and actually dripping oil) and misbiased the next stage.
A 12AX7 triode (6AV6 was very popuar and it´s "1/2 of a 12AX7") with, say, +5V on its grid becomes saturated all the time and chops audio going through it, only allowing highest peaks through.

mckayprod

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Re: Noisy pots
« Reply #8 on: May 28, 2017, 10:41:38 PM »
Thanks for the review of grid leak bias; the DC present at the tube input probably accounts for the scratchy pots I noticed.  Converting old PA amps for guitar of harp duty is kind of a "thing" around here.  A local guy (Skip Simmons) has made a name for himself doing that, in addition to the hot-rod mods he does on old Fender stuff.  He declined to work on this amp due to its pcb construction, and when you try to desolder components from that 60-year-old board, you'll get his point!  Here's a schematic:

Thanks for your interest.  I always feel like one of the big kids when Sr. Fahey replies to my posts.

Enzo

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Re: Noisy pots
« Reply #9 on: May 29, 2017, 01:34:07 AM »
I have been soldering on 60 year old pc boards since, well, 60 years ago.  I;d say the boards in a 60 year old PA amp are probably more sturdy than the boards in a cheap practice amp today.  I have had no trouble soldering on them.  The difficulty arises when one wants to change the circuit.

mckayprod

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Re: Noisy pots
« Reply #10 on: May 29, 2017, 11:37:08 PM »
Yes, soldering is no problem, but the desoldering was another story...the pads & traces came up like cheap paint on waxed paper.  A new board is in the works.

Enzo

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Re: Noisy pots
« Reply #11 on: May 29, 2017, 11:48:44 PM »
Sorry, I meant soldering in all its forms, including removal of parts.

J M Fahey

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Re: Noisy pots
« Reply #12 on: May 30, 2017, 09:07:57 AM »
Thanks for posting the schematic.
Nice amp, and has the classic configuration expected.
Those mic inputs are grid leak biased after all, so connecting anything there which provides a DC path to ground, either guitar pots or the typical resistor to ground at the output of pedals will affect it, clearly they expected a capacitive only source (crystal mics or old style phono capsules) which you will NOT use.
Unless you want to throw a full **vintage** party that is, complete with a Big Band, a tuxedo clad oily hair "Jazz crooner" (think Frank Sinatra or Dean Martin) , and old scratchy ´78 rpm records


For any modern use either add an input coupling cap (say, .01uF polyester) or mod the stage to classic "Fender" values: 1k5 in parallel with 10uF cathode, 100k or 220k plate, and in that case grid resistor can be standard 1M and will not be affected by guitar or pedal electronics.

jfetter

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Re: Noisy pots
« Reply #13 on: July 10, 2017, 05:26:21 PM »
maybe leave plate resistor alone and put a single Germaniun diode frm tube cathode to ground, no capacitor.. then solder a 1M resistor across the 4.7M 'grid leak' resistor.

that amp's control panel graphics are a beautiful period example.


 

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