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.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.
I have noticed that as you turn the volume controls down on most if not all my guitars while plugged in, the amp hum gets louder. Whats up with that?
Here are some of the amps it happens on, 78 Fender Pro, 63 Mangatone M15, Peavey classic 50 and classic 30, amongst othersWithout 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.
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.I have noticed that as you turn the volume controls down on most if not all my guitars while plugged in, the amp hum gets louder. Whats up with that?
the conversion for 'puffs' to 'deebs' is what?shunted by about 200 puffs of capacitance
After looking at the schematics, yes, its caused by the grid stop resistor not being there.Here are some of the amps it happens on, 78 Fender Pro, 63 Mangatone M15, Peavey classic 50 and classic 30, amongst others
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 conversion for 'puffs' to 'deebs' is what?
You need a special tool to measure the capacitance..not a DVOM..not my DVOM.
No? Ever tried to make an RF amplifier with a Triode*? I guess you are pulling leg?I dont subscribe to that 'miller effect' nonsense. ..haha
and 'deebs' is decibels ..im hip. I know. Im not 'new'.
No but I accidentally turned my record player into an AM radio..haNo? Ever tried to make an RF amplifier with a Triode*?
Well, I am not about to add them. Was just wondering what was causing it.After looking at the schematics, yes, its caused by the grid stop resistor not being there.
Check out this link for many vintage electronic books. Free downloads.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.
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.Well, I am not about to add them. Was just wondering what was causing it.
I just thought you might find those books interesting.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.
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.pdfI just thought you might find those books interesting.