Guitar pot question
- Thread starter dogooder
- Start date
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
the transformer windings are shorting internally...no seriously it is only a guess. recieving no signal it shorts, but some signal it still tries to amplify.
dogooder
Well-known member
I think it happens on all, most of my amps? I have to check again but once the pots are all the way down the hum returns to normal? I am too old for this shit, I just want to play and me or something is always a pain in the ass.
Without giving amp examples, it would be because the grid for the tube afterwards don't have what is called the grid stop resistor. So what happens is the grid element, which is a wire loop wrapped around a support frame inside turns into an electromagnet that attracts the heater emissions from the heater instead of the cathode (ac hum). Some manufacturers will leave out this resistor because you have to use certain type of resistors (cost more) or the performance would not be the same as no resistor. The grid stop resistor is just one of the two common design flaws around the volume control. The other is bias stabilization when the volume control degrades and becomes scratchy.
If you want a detailed explanation with certain amps, list them. But the above is the common occurrence.
dogooder
Well-known member
Here are some of the amps it happens on, 78 Fender Pro, 63 Mangatone M15, Peavey classic 50 and classic 30, amongst others
Pretty simple really, the impedance of a guitar pickup is normally about 10k Ohms. The input circuit of yer common valve amp is almost always 1M Ohm (1.000,000 Ohms) shunted by about 200 puffs of capacitance so 10k 'looks' like almost a short from the grid to earth/ground. That means any internal (heater e.g.) and external mains hum is virtually shorted out.
Guitar VC pots are usually between 220k and 470k so at their mid point the resistance across the grid is much higher and hum can intrude.
This is also why guitar amp input jacks are almost always 'switched' types so that they short the grid to ground when the plug is removed.
Dave.
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
the conversion for 'puffs' to 'deebs' is what?
You need a special tool to measure the capacitance..not a DVOM..not my DVOM.
After looking at the schematics, yes, its caused by the grid stop resistor not being there.
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
what would cause a grid resistor failure on all his amps?
is this a case of improper power conditioning?
is this a case of improper power conditioning?
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
-not being there from the start- got it..
Most decent, say above about $30 DMMs will measure capacitance to below 200p but in the case of the valve grid you can't anyway! The capacitance the guitar 'sees' is mostly "Miller effect" capacitance (but you can 'infer' it if you know how on a live amp) This is a property of valves, especially triodes and is greatest for the high 'mu' types such as my sobriquet.
The Miller cap actually performs a valuable service., in conjunction with the grid stopper resistor it forms an effective RFI block.
"They" forgot this when they started using transistors especially Silicon planar types and early kit picked all the ***t in creation! I spent much of my time fixing such problems in the 60s and beyond.
Oh and "puffs" is old radio guys speak for "picofarads" 1x 10 to the -12 Farads.
Dave.
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
I dont subscribe to that 'miller effect' nonsense. ..haha
and 'deebs' is decibels ..im hip. I know. Im not 'new'.
and 'deebs' is decibels ..im hip. I know. Im not 'new'.
No? Ever tried to make an RF amplifier with a Triode*? I guess you are pulling leg?
*Well you can as grounded grid, Z in about 600 Ohms or a Cascode but you need two sections.
Dave.
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
No but I accidentally turned my record player into an AM radio..ha
dogooder
Well-known member
Well, I am not about to add them. Was just wondering what was causing it.
dogooder
Well-known member
Check out this link for many vintage electronic books. Free downloads.
No problem. Curiosity is a flavor of life. Without going through the formula for calculating that resistor, that is most likely not in those books in the link above, It would be a small value on a triode to begin with. So they save about $5 a unit not adding a 300-500 ohm resistor there. If you really wanted to know.
dogooder
Well-known member
I just thought you might find those books interesting.
I find them nostalgic. But I do find it interesting that the text books I learned tube theory in college long ago are even harder to find. But this was one of those classroom text books: http://www.tubebooks.org/books/peters_theory.pdf
Of course people would freak out if I broke out in a discussion about negative grid impedance condition when its grounded, or in your case, volume control turned all the way down and causing the current flow from heater to grid.
Last edited:
As Popeye was oft'want to say "I's had all I can stand! I can't stands N'MORE!
The variation in hum pickup in a guitar amp front end is FA to do with whether or not the valve has a grid stopper!!
And there is nothing special about the TYPE of resistor used in that position. For most of their existence valve amps used boggy standard Carbon composition Rs. "We" use Metal Film because they are lower noise and very, very reliable.
Dave.
The variation in hum pickup in a guitar amp front end is FA to do with whether or not the valve has a grid stopper!!
And there is nothing special about the TYPE of resistor used in that position. For most of their existence valve amps used boggy standard Carbon composition Rs. "We" use Metal Film because they are lower noise and very, very reliable.
Dave.