With regard to coupling caps, specifically in front of input tubes (12AX7, etc), I often see values of .01uF, .1uF, etc, and voltage specifications of 400~630VDC or even higher.
I certainly understand the need for HV capacity in power stage smoothing caps, and also where HV will be present, such as B+. But I don't understand why the need for high voltage specifications for input coupling caps.
Can someone explain? Thanks!
I certainly understand the need for HV capacity in power stage smoothing caps, and also where HV will be present, such as B+. But I don't understand why the need for high voltage specifications for input coupling caps.
Can someone explain? Thanks!
There is no need for it. They are the caps that are on hand usually.
Then there is the discussion about cap quality; it could be that higher voltage cap of the same capacity is a bit higher quality in a low voltage position. Or not.
Builders buy caps in bulk, so they are at hand. Why bother buying a low voltage part.
Then there is the discussion about cap quality; it could be that higher voltage cap of the same capacity is a bit higher quality in a low voltage position. Or not.
Builders buy caps in bulk, so they are at hand. Why bother buying a low voltage part.
In a typical common cathode stage, the coupling cap only needs to be rated at a value of 1.2 x the worst case peak input voltage.
That said, many types of capacitors are not readily available in low voltage ratings.
They are available in the voltage ratings you see.
You use what is available.
That said, many types of capacitors are not readily available in low voltage ratings.
They are available in the voltage ratings you see.
You use what is available.
Strictly speaking, an input cap is not necessary if there is no DC on the grid, but it protects from DC from the source, and controls the bandwidth
that the circuit has to deal with. Some think that higher voltage caps of the same value sound better. Also, a circuit is more failsafe if all caps
(except say, cathode bypass) have voltage ratings greater than the power supply.
If you have a fixed grid bias on the input stage grid, and if that bias is derived from the same winding as the B+, in theory a fault could happen where there is a high voltage on the grid. It would then destroy the input tube, but if the grid capacitor is rated for high voltage, it would not destroy the device preceding the amp input.
OK, thanks all. I recently received an order from Allied and I noted to my dismay that in my desire to save money, I had purchased bulk quantities of .01, .1, and .22 coupling caps that were specified for 63V. I was afraid I had wasted my money. Sounds like I can still use them, so long as I am careful about the places where I use them. Is that correct? Thanks again!
Ah, that makes sense, thank you! I have not yet attempted to build a fixed-bias design amp. If I do, I will keep that in mind.
No sweat, I believe my question has been answered. I also deliberately didn't mention anything about quality; I hate getting into those kinds of discussions, as they usually/always end in tears. Let everyone believe as they wish concerning that kind of thing, I say. I am old and have lousy hearing anyway.
You can still use them for cathode bypass, input coupling, heater elevation, or of course in lower voltage circuits.
Don't use them anywhere that the voltage across them could surge high even temporarily, like during warm up or turn off.
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I thought cathode bypass were generally electrolytic? Does it not matter?
In any case, yes, my plan was to use them only for input coupling; I'm glad to see I'll still be able to use them. They were quite inexpensive, so I'm glad of that.
Sometimes smaller caps are used on the cathode for tuning the hf response. Since the cathode resistor is usually on the order of 1k,
you usually need 10-100uF for a bypass there, which requires an electrolytic.
In the case of input coupling caps, they need to safely tolerate an internal electrode short between grid and anode, rare as it may be. Otherwise, under fault conditions, there's risk of the input terminal having a path to B+ potential should the cap breakdown because of insufficient rating.
A large voltage on the input capacitor of valve amplifiers is for safety reasons.
Should a fault develop and the anode voltage gets onto the grid you can get lethal voltages on the input.
Should the fault arise a low voltage capacitor would quickly short out passing the lethal voltage back to a guitar or mic.
In the 60's quiet a few guitarists got killed by faults on valve amplifiers.
Should a fault develop and the anode voltage gets onto the grid you can get lethal voltages on the input.
Should the fault arise a low voltage capacitor would quickly short out passing the lethal voltage back to a guitar or mic.
In the 60's quiet a few guitarists got killed by faults on valve amplifiers.
Nigel, with all due respect, I agree with you on most matters, however I disagree on this one.
Most of those faults were related to two pin plug, hot chassis issues and not shorted anode to grid faults. The infamous killer cap on the power input to ground, and it's eakage current was at fault.
HOT chassis issues were common in radios as well as guitar amps.
Theoretically, a grid to anode short in conjunction with the anode resistor (100K) in series with the grid stop resistor (68K?) and divider grid to ground resistor resulted in less than 3mA leakage current.
I don't believe grid to anode short was responsible for deaths nor do I believe it is necessar4 to use a high voltage rated input capacitor.
It might be a good idea, but not necessary.
Most of those faults were related to two pin plug, hot chassis issues and not shorted anode to grid faults. The infamous killer cap on the power input to ground, and it's eakage current was at fault.
HOT chassis issues were common in radios as well as guitar amps.
Theoretically, a grid to anode short in conjunction with the anode resistor (100K) in series with the grid stop resistor (68K?) and divider grid to ground resistor resulted in less than 3mA leakage current.
I don't believe grid to anode short was responsible for deaths nor do I believe it is necessar4 to use a high voltage rated input capacitor.
It might be a good idea, but not necessary.
agree with this one too...sometimes even with 2 pin plugs, you find caps of about 0.1ufd connected from hot line to chassis, and this is the cause of many accidents....if only they earthed their equipment...
I knew people had been killed in the 60's due to valve amps.
I wasn't aware it was due to the mains plug and capacitor.
I obviously got things twisted from my little knowledge of the matter.
I apologise.
However I personally would still use a high voltage capacitor on the input.
If a valve gets bashed and the anode hits the grid then there could be a problem.
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