220uF for a rectifier tube. Whats so hard to drive?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

ChrioN

Well-known member
Joined
Aug 31, 2005
Messages
1,235
Location
Gothenburg, Earth
I'm still learning, or shall I say, we are still learning, but on different levels... When a rectifying tube is speced to be able to handle "a 60uF load". What does that mean? Back to the basics I guess. "How does a capacitor work?". Is it the time when the capacitor needs to get "juiced up" thats so hard for the tube to handle? What exactly happends? Is there a close-to-a-short for a while when the cap is getting "loaded"? Maybe I haven't grabbed the basics at all.
 
> When a rectifying tube is speced to be able to handle "a 60uF load".

Where do you find that as a MAXimum Spec on any original tube data sheet?

Yes, the cap is a near-dead short. In an extreme case, hot-start current is near-infinite, tapering as cap charges.

Any tube can pull a 10,000 uFd cap IF sufficient resistance is in circuit. Resistance is cheap, free. Low-low resistance transformers cost much more. Rectifier resistance is signiticant. Caps have resistance though in this case not enough to matter.

There may be a "Typical" "Max cap" on old sheets to guide designers to a quick-easy happy solution. If you wanna get crazy, you simply run through a long tedious nonlinear analysis to determine peak current and energy/time curve. A Differential Engine or Analog Computer helps; today we have Duncan PSD.
 
PRR, thanks for the comment, and also, I would really like your input on the Duncan PSD, as I just discovered this app yesterday. Is it accurate in simple designs such as passive RC filters and such?
 
I simmed the situation. I did not have a 5U4 sim, so I diode-strapped a large triode, which works the same. I was too lazy to do full-wave; half-wave changes details but not the overall concept. 400V peak AC. I marked 40 Watts as a generous plate dissipation rating. I probed diode voltage drop, diode current, plotted the instantaneous Power V*I, also rms() of power to smooth the spikes.

Three plots.

200 ohms, 40uFd-- typical PT and first cap. The peak power is over 100 Watts, the short-term "rms" touches 90 Watts, but the RMS has dropped below 40 Watts within 1/10th second. Tubes will take this abuse fine.

1 ohm, 10,000uFd, first 1/10th Second-- peak Power is 375 Watts, RMS touches 240 Watts first peak and 180W subsequent early peaks. This is heavy abuse.

1 ohm, 10,000uFd, long run-- RMS Power falls as cap charges-up. 150 Watts after 1 second, 85W at 10 seconds, 70W at 20 seconds. After a minute it is still 50W, in excess of the assumed 40W rating. We do not get down to 40W for three minutes! Tubes will take this, but it shortens their life.

Unless something else happens. Unlike an amplifier, a rectifier has Reverse Voltage on it half the time. We expect the hot cathode to emit electrons, and the "cool" plate to not emit electrons, block the reverse voltage. But at 5 times the rated dissipation, that plate will be HOT! Hot as a cathode. Hot enough to emit electrons. Now the tube conducts BOTH ways. The cap never charges. The high peak current and power are doubled and "never stop". An "arc-back" in a big power supply is a pretty dramatic thing, and leads to some major damage.
 

Attachments

  • Rect-Abuse.gif
    Rect-Abuse.gif
    28.3 KB · Views: 29
this is a good read on choke input
and how it affects rectifier loading...

"Rectifier Load
The rectifier is a switch, which is a non-linear component. The rectifier only conducts when there is a positive voltage difference between the transformer secondary and the filter input. It is physically impossible for a single rectifier diode to conduct throughout the entire secondary sinewave cycle. It is also impossible for current to flow backwards out of the filter into the transformer. ( edit: well, most of the time) The diode will always turn off at some point. The current drawn from the rectifier, and the voltage waveform that appears on the filter input terminal, depend strongly on the electrical characteristics seen by the rectifier looking into the filter and on the DC load current drawn from the supply. The DC voltage at the output of the filter, in turn, depends on the shape and the amplitude (that is to say, the average value) of the filter input voltage waveform. If the average DC input voltage changes substantially with load current, the supply will have poor regulation.

In this sense, the transformer, rectifier, and first filter input component are like a kind of complex, ratcheting electrical machine which functions to turn the AC voltage coming out of the transformer into a raw form of DC that can be purified. The complexity of the interaction between these three components is what makes the filter topology and especially the value of the first filter component so important to the performance of the supply.

The main distinction between a choke-input filter and a cap-input filter is that the choke-input filter loads the rectifier so that current flows into the filter almost continuously. If the choke were infinitely large, the current would be constant DC. With a practical choke, the current into the filter consists of a DC component equal to the DC load current with a superimposed AC current approximately equal to the ripple voltage divided by the choke impedance, summed across all the harmonics of the AC supply frequency.

This helps explain the definition of “critical inductance”. Ignore the higher order harmonics and only consider the dominant 120Hz (or 100Hz) component of ripple voltage. The first capacitor following the choke, if it is large enough, looks pretty much like a short at this frequency. So the entire ripple voltage appears across the choke and the resulting current is proportional to the ripple voltage and inversely proportional to the choke inductance at the ripple frequency. The higher the voltage or the smaller the choke, the greater the ripple current. To keep the choke conducting, the DC current bias has to be large enough that the negative peaks of the ripple current never reach zero. It is possible to derive a simple, approximate formula for the the minimum inductance to give continuous rectifier conduction as a function of DC load current. The familiar formula is:

Lcrit = Vdc / Ima

where Lcrit is the minimum (critical) choke inductance, Vdc is the supply DC output voltage, and Ima is the load current in milliamperes.

There are three main reasons continuous current conduction is important. The first is that a continuous current with a lower peak value makes more efficient use of the power transformer. This is because power lost to heat is a function of the square of the instantaneous current. You generate less heat conducting a lower current for a longer time than a higher current for a proportionally shorter time.

The second reason is that the smooth, relatively low peak supply current creates less switching noise and electromagnetic interference to pollute the signal circuitry.

The third reason is that continuous current conduction gives much better DC voltage regulation. This is because the duty cycle of the filter charging current doesn’t vary as long as the load current exceeds the minimum threshold as calculated by the critical inductance formula. And therefore, the filter input voltage waveform is constant, and the average DC input voltage is constant, too. Contrast this with a cap-input filter where the charging duty cycle is a function of load current. This is a bit of a subtle point and may deserve some follow-up discussion. In the old days, when large filter capacitors were not available, improved regulation was a major advantage of the choke-input filter. This distinction is less important nowadays.

In practice, the diode conduction current is not continuous. This is because diodes do not conduct until their minimum forward voltage drop is exceeded. So even with a critical choke-input filter there is a dead zone in the conduction waveform and the potential for generating switching noise. Solid-state diodes can also conduct backwards momentarily as minority carriers get swept away, creating another source of switching noise. But the noise in general is less than that produced by a cap-input filter, and there are techniques for “snubbing” the noise (e.g., as documented by Jones in “Valve Amplifiers”).

To summarize, we can say overall, from the point of view of efficiency, noise generation, and regulation, the choke-input filter provides a superior interface to the rectifiers, and this may be a factor in the subjective perception that choke-input filters give better sonic performance."

from

http://ken-gilbert.com/choke-input-power-supplies-part-1-henry-pasternack
 

Latest posts

Back
Top