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Voltage Ratings on Coupling Caps - Printable Version

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Voltage Ratings on Coupling Caps - makinrose - 03-30-2023

I have what probably a dumb question:  It's common for coupling caps in tube amps to be rated at 400 VDC.  In in TUT3 most of the caps are 400 VDC and some are rated at 630 VDC.  When the amp is working with the tubes in the voltages are well below the voltage rating but when testing without the tubes the voltages are close to the V+ and often exceed the voltage rating.  Is this any reason for concern?  

Thanks for the help!


RE: Voltage Ratings on Coupling Caps - K O'Connor - 03-31-2023

Hi Guys

Plate coupling caps in tube circuits are usually rated to withstand the supply voltage for the circuit they are in. As Makinrose points out, at a cold startup, the plate supply is unloaded and there is the possibility for every plate cap to see the maximum voltage from the rectifier. However, there are many series resistances between the rectifier and each coupling cap and this limits the current that might flow through the caps.

Note that at DC, a capacitor should look like an open-circuit, where it looks like a varying resistance (impedance) at different signal frequencies. During the startup, there is "voltage in motion" and therefore charging currents through the caps.

Once the tubes warm up and conduct current, the voltage each plate coupling cap sees is much reduced.

As an example, many Fender amps have 470Va for the output stage once they are in operating condition. Down the line, the splitter may be at 450V, as will be the reverb driver, and preamp stages may have 250-300V on the plate. The splitter caps see the highest voltage spanning from the tube plate to the power tube grid. These voltage might be +350V and -50V, respectively, so 400V across the cap. Generally, you see a 630V cap here. Everywhere else, you might only see 400V caps. A 500V cap would be fine here but we are often limited to what is available. There is no real need to use a massively over-rated plastic cap in any circuit.

Plastic dielectric caps tend to have large safety margins. If you read some data sheets, this can be up to 2.5 times the rated voltage. This reflects that there can be variation in the quality and thickness of the dielectric, both of which one would hope to be quite well maintained.

Note that electrolytics are rated at their maximum working voltage (WVDC), which plainly means you can operate the cap at that voltage all of its life. Again, there is an allowance for surge voltages higher than the rating, which used to be printed on the cap along with WVDC, but might only be found in the data sheet these days.

Have fun


RE: Voltage Ratings on Coupling Caps - makinrose - 04-03-2023

(03-31-2023, 01:18 AM)K O'Connor Wrote: Hi Guys

Plate coupling caps in tube circuits are usually rated to withstand the supply voltage for the circuit they are in. As Makinrose points out, at a cold startup, the plate supply is unloaded and there is the possibility for every plate cap to see the maximum voltage from the rectifier. However, there are many series resistances between the rectifier and each coupling cap and this limits the current that might flow through the caps.

Note that at DC, a capacitor should look like an open-circuit, where it looks like a varying resistance (impedance) at different signal frequencies. During the startup, there is "voltage in motion" and therefore charging currents through the caps.

Once the tubes warm up and conduct current, the voltage each plate coupling cap sees is much reduced.

As an example, many Fender amps have 470Va for the output stage once they are in operating condition. Down the line, the splitter may be at 450V, as will be the reverb driver, and preamp stages may have 250-300V on the plate. The splitter caps see the highest voltage spanning from the tube plate to the power tube grid. These voltage might be +350V and -50V, respectively, so 400V across the cap. Generally, you see a 630V cap here. Everywhere else, you might only see 400V caps. A 500V cap would be fine here but we are often limited to what is available. There is no real need to use a massively over-rated plastic cap in any circuit.

Plastic dielectric caps tend to have large safety margins. If you read some data sheets, this can be up to 2.5 times the rated voltage. This reflects that there can be variation in the quality and thickness of the dielectric, both of which one would hope to be quite well maintained.

Note that electrolytics are rated at their maximum working voltage (WVDC), which plainly means you can operate the cap at that voltage all of its life. Again, there is an allowance for surge voltages higher than the rating, which used to be printed on the cap along with WVDC, but might only be found in the data sheet these days.

Have fun

That explanation makes a lot of sense.  I service lots of amps with 400VDC caps and often test without the tubes without issue, but I really wanted to understand why it was okay.  Thanks for the explanation.