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Old 20th February 2015, 03:32 AM   #1
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Default Capacitance of rectifier tubes/valves near V=0??

Hi everyone,

Someone asked me how to estimate the anode-to-cathode capacitance of a valve rectifier near Vak=0, and the truth is that I have no clue. N.O.C.L.U.E.

So I turn to you folks, domain experts, to ask if there exist reasonable approximate values that apply to two general classes of valve circuitry,
  • Class #1: rectifier valves that supply power to valve preamps (phono + linestage)
  • Case #2: rectifier valves that supply power to valve power amps (?? 50 WPC RMS ??)
I'm hoping that the various different choices for valve rectifiers, give similar ranges of values in the two different classes. (??)

Thank you from someone whose valve-virginity is intact,

-- Mark Johnson
(link to original query)
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Old 20th February 2015, 08:02 AM   #2
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Once again thanks for your efforts Mark

I was considering starting working with tube rectifiers for my projects, but I need some more know-how about them, since they seem to be quite different animals than SS diodes, with much higher voltage drops and a rather resistive character

I have found that as a start, but I think I have to build something and just check it.

http://frank.pocnet.net/sheets/049/6/6CL3.pdf
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Old 20th February 2015, 08:20 AM   #3
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Valve rectifiers have desirable characteristics regarding capacitance. It is lower than many semiconductor alternatives, and it does not increase with decreasing reverse-voltage.

Valve data sheets for rectifiers do not often give a value, but smaller Damper diodes (efficiency diodes) have about 9pF, larger dampers about 14pF.

Many of us are turning to these damper diodes now, as they give good performance and are still plentifully available.
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Old 20th February 2015, 10:59 AM   #4
Keit is offline Keit  Australia
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When used for their intended purpose, rectification of AC power mains, directly of off a power transformer secondary, the anode-cathode capacitance of power rectifiers is entirely negligible.

Perhaps if you can tell us why you need to know the capacitance, as you'll only need to know it if you have some non-standard application in mind, we can give better advice. The capacitance of any vacuum tube diode has two parts:-
a) the static capacitance, which can be calculated from the tube structure dimensions using standard formulae;
b) the additional capacitance due to teh sapce charge. This capacitance depends on the applied anode voltage.

Of course, you can, with care, measure the static A-K capacitance. The Q-meter method is the best way. If neccessary this can be done with nothing more than an oscillator and a radio reciever (used as a detector) and a hand made coil.

Measuring the dynamic capacitance under working conditions takes a bit more ingenuity.
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Old 20th February 2015, 01:19 PM   #5
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Quote:
Originally Posted by Keit View Post
... the anode-cathode capacitance of power rectifiers is entirely negligible.

Perhaps if you can tell us why you need to know the capacitance, as you'll only need to know it if you have some non-standard application in mind, we can give better advice. The capacitance of any vacuum tube diode has two parts:-
a) the static capacitance, which can be calculated from the tube structure dimensions using standard formulae;
b) the additional capacitance due to teh sapce charge. This capacitance depends on the applied anode voltage.

Of course, you can, with care, measure the static A-K capacitance. The Q-meter method is the best way. If neccessary this can be done with nothing more than an oscillator and a radio reciever (used as a detector) and a hand made coil.
Thank you Keit! The original requestor is diyAutio member dimkasta, who has joined this thread. He is considering whether or not to connect a dissipative snubber across the secondary of his power transformer, which feeds valve rectifier(s). Why? I don't really know but I suspect the answer might be, why the hell NOT arrange the secondary to be critically-damped (Zeta=1.0); it might help and it cannot possibly hurt. Even if valve rectifiers exhibit no reverse recovery spikes whatsoever, certainly it's harmless to add a couple capacitors and a resistor that set the damping ratio to a number of the designer's choosing? (rather than letting random parasitics + Murphy's Law (evil, vindictive, mean-spirited Murphy's Law) set it for you).

To choose a starting point for a critical-damping snubber, it helps to have a guess of the rectifier capacitance and to have a guess of the transformer self-capacitance. That's why dimkasta has asked the odd-sounding question. In fact, for the "CRC" snubber he contemplates, all he really needs is an upper bound on rectifier capacitance. It sounds like Crect <= (Ntubes x 30pF) will take him where he wants to go.

Last edited by Mark Johnson; 20th February 2015 at 01:26 PM. Reason: less-than-or-equal-to
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Old 20th February 2015, 02:39 PM   #6
Keit is offline Keit  Australia
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Snubbers are usefull with semiconductor diodes because the diode charge storage time is appreciable - microseconds. The diode keeps pasing current in the reverse direction for an appreciable time.

Charge storage occurs in vacuum tube rectifiers as well (it's the space charge) but its nanoseconds. In rectification at power mains frequency it is entirely negligible.

As I have said, you can completely ignore the tube capacitance. It is of the order 10 pF or so, whereas the self capacitance of a typical tube power transformer secondary is of the order of nanofarads.

The diode capacitance (picofards) is essentially in parallel with the transformer self capacitance (nanafarads) (the return path is via the first filter cap - effectively a faction of an ohm ESR). So, the tube capacitance adds something like 0.1% and anybody who thinks it is a starting point in any calculation lives in fantasy land.



Snubbers are routine with semiconductor rectifiers in major part because low ripple capacitor input filters are used, making the diode conduction time very short, so the peak diode currents are considerable. With vacuum tube rectifiers, tube ratings mandate that the first filter capacitor must be of low value such that the ripple and diode conduction time is a substantial fraction of the total half cycle time. So the diode peak currents are relatively low.


I cannot see the logic in adding components "because it cannot do any harm". Any good engineer soon learns that the best way to excellence is to find the simplest way to meet the specification.

The more parts you add, the more there is to go wrong. And any such snubber capacitors (& resistors if used) are going to be seriously stressed. Take a typical example: Secodary 350V per side; DC voltage on fisrt filter cap 410 V. The rectifer, and any snubber capacitor must withstand a peak voltage of 760 V. And there should be an allowance for mains voltage variation and mains switching surges. This is NOT a standard capacitor. Any series resistor used in snubbing must also withstand a peak voltage of 760 V. This is most certainly not a standard resistor from the point of view of voltage rating. And, to comply with Ul and house contents insurance requirments it would have to be an intrinsicly fire-proof part, installed so as to guarantee the fire proofing.

Don't do it, unless you know what you are doing. An if you knew what you were doing, you woudn't do it.


I could not access the Morgan Jones article. But going on Morgan Jones' other writings, it is likely to be a mix of fact, complete nonsense, and misunderstood principles.

Last edited by Keit; 20th February 2015 at 02:50 PM.
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Old 20th February 2015, 04:02 PM   #7
DF96 is offline DF96  England
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The preview suggests that the Morgan Jones article is about semiconductor rectifiers. As Keit says, stored charge in vacuum rectifiers is negligible. Also, they have a more gradual slope than solid state so less likely to excite transformer ringing.
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Old 20th February 2015, 04:32 PM   #8
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Thanks, Keit! I am sure diyAudio member dimkasta appreciates your comments.

The we-like-snubbers crowd tend to select 650VAC rated (1600VDC rated) film capacitors like the MKP10 from WIMA (here's one at Mouser). You are right, they ain't cheap. At 50Hz the magnitude of impedance of this 150nF cap is about 20Kohms. This limits 50Hz current through the snubber, a series RC circuit, to something less than (350V/20KR) = 18 mA. For the snubber resistances typically encountered {5R <= Rsnub <= 500R}, snubber dissipation is less than 200 milliwatts: IsquaredR. So the inexpensive Stackpole CF1 1-watt resistors, rated for 500VAC, are plenty adequate. Thanks to the voltage divider effect of the snubber capacitor in series, the snubber resistor sees less than 10% of the secondary voltage. Conservative people could of course put three resistors in series if they wanted an even greater margin-of-safety.

The snubber design starting point is (Ctrafo + Crectifier) or an upper bound thereof. In cases where Crectifier totally dominates Ctrafo {often seen in solid state poweramps that use 35 ampere bridge rectifier assemblies with toroidal transformers}, Ctrafo is so small it is negligible. Irrelevant. In other cases, such as the valve ones you describe, where Ctrafo totally dominates Crectifier, Crectifier is irrelevant. It is good that member dimkasta asked the question, and even better that you kindly provided an answer!
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Old 20th February 2015, 09:16 PM   #9
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There is also substantial bulk capacitance between PT windings typically used in a valve rectified amp. For a 240V mains to 385-0-385 secondary, I measured 3.5nF between each of the 385V half secondaries (when CT tap was separated, and with windings shorted), and from one 385V half secondary to primary was 0.4nF, and 0.2-0.4nF to heaters, and 1.3nF to earth screen.
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Old 20th February 2015, 11:42 PM   #10
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Sounds like that screen was ineffective, if primary-to-secondary capacitance was 400 pF! You'd expect the screen to cut it down below 10pF.
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