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Film Capacitors In Tube Power Supply Circuits?

tubesandtransistors 0

Recently in a discussion on a Facebook tube radio group, it was debated whether the electrolytic multi-section “can” capacitors in old tube electronic gear could be safely replaced by anything but electrolytic capacitors. Some posited that electrolytics were the only type of capacitor that could “handle” HV power supply filtering chores.

I wondered how this could be true. If a capacitor had the correct capacitance value and voltage rating – shouldn’t it be up to the task? Certainly, if one could replace electrolytic capacitors with film in a vintage circuit, the chances of capacitor failure go down significantly over time, thus “future-proofing” these circuits to a much higher degree.

The challenge with using film caps in as filter caps in power supply circuits is:

  1. They are usually physically larger than new radial electrolytics and in a rectangular form factor with short leads meant for a PCB.
  2. The large value film caps (>10µF) tend to be more expensive than a similar value electrolytic.
  3. They do not seem to be available at capacitance values over 20 µF at voltage ratings > 400 VDC.

The Contenders: 10 µF and 20µF Film capacitors

While some tube circuits require filter capacitors of 33µF and higher, many early tube circuits used 10, 12, or 20 µF filter caps. While these lower values may not be sufficient filters in some circuits, they would work for many. One could always connect them in parallel to double their capacitance value.

Here are a few film capacitors I found on Mouser that could do the job:

KEMET 20µF Polypropylene Capacitor

The specifications of this capacitor look very good – and it is the most inexpensive of the four we will look at.

Capacitance 20µF @ 5% tolerance
Voltage Rating 650 VDC
ESR 5 mOhms (0.005 Ohms)
Price $4.14 (Mouser)

WIMA 20µF Polypropylene Capacitor

WIMA caps are highly regarded – and this one is the second-best of the bunch in terms of price.

Capacitance 20µF @ 10% tolerance
Voltage Rating 600 VDC
Price $6.06 (Mouser)

EPCOS/TDK 20µF Polypropylene Capacitor

Strange how the specs on this cap are lower than the first two but the price is higher.

Capacitance 20µF @ 10% tolerance
Voltage Rating 500 VDC / 250 VAC
Price $8.03 (Mouser)

 

For fun, I’ve ordered one of each of these three capacitors to test in a tube circuit – and we’ll compare them to my current favorite electrolytic to use as a filter cap: The Nichicon UCY.

Nichicon UCY 22µF Electrolytic Capacitor

Capacitance 22µF @ 20% tolerance
Voltage Rating 500 VDC
Ripple Current 280 mA
Lifespan 10,000 hours
Price $2.67 (Mouser)

Once they come in, we’ll run them through their paces and I’ll report back in a future post!

 

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