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Old 26th October 2004, 02:39 AM   #21
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Hi Chris,

Quote:
I guess you could put some small caps across the diodes to slow down the switching speed too.
I agree with the proposed method even though a RC network across each diode is the most common proven and tried method, however it's not the diode's speed that's the problem_ after all you're just handling 50/60Hz which is sloooow_but the way the diode recovers from sudden switch off.

Quote:
Unless you have solid state diodes that put HF hash on the DC which may pass by a regulator.
Unless you purposely bandwidth limit the reg_ easy enough with a classic tubed series reg_ you may, most likely will_ spot traces of HF hash on the scope with almost any untuned solid state series reg IME.

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The mercury must be fully (or nearly so) vapourised to handle all the current at higher loads. Damage occurs when the current flow can not be supported by the ionized gas.
Fair enough, in practice you're not likely to pull the full amount of current from the rectifier with a tubetester but my point was that
from what I read in the manuals this 20' warm-up period is just a break-in cycle.
After that, 30 secs should do or that's what I keep hearing from the guys using these anyway.

Cheers,
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Old 26th October 2004, 02:53 AM   #22
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FYI, series resistance will only kill regulation. In a series circuit, such as the winding, rectifier and cap, additional resistance simply adds to the ON resistance, winding resistance (major) and cap ESR (and ESL being a dynamic situation). However, a capacitor across the diode, or better yet, a snubber of some sort will *kill* it. 0.01uF should be enough; too much and you'll waste VARs, heating the transformer.

Tim
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Old 26th October 2004, 09:38 PM   #23
anatech is offline anatech  Canada
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Hi Tim,
By using a resistance lower than the normal tube rectifier resistance you will have better performance to begin with. The idea is to reduce the surge current and lower the peak charging current. Now, even if you use an equivalent resistance to the tube, your voltage is still higher and regulation is the same. This is without the heater current so the transformer will run cooler. The series ESR of the capacitor will be more pronounced with higher peak currents, but in itself does not get any worse with higher series resistance in the voltage source. When looking at values on the order of 47 - 100 uF this is normally not an issue. Higher capacitance will be very hard on a tube as you most probably will exceed the hot switching current rating.
You are, of course correct when you say that small caps in parallel with the diode kill the switching noise.
-Chris
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Old 26th October 2004, 09:41 PM   #24
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Hi Frank,
I've built several discrete voltage regulators that do not pass the HF noise. The measure and sound very quiet. The biggest problem is the positive tempco of zener diodes. I've had the use thermistors to tame this (in contact with the zener).
All is happy on my bench.
-Chris
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Old 27th October 2004, 04:37 PM   #25
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Quote:
Originally posted by anatech
By using a resistance lower than the normal tube rectifier resistance you will have better performance to begin with. The idea is to reduce the surge current and lower the peak charging current.
This never was, and never is an issue (those SS dudes with their hideous cap input filters are testament); if it ever were, you'd simply use a smaller C1 and more LC filtering after (i.e., pi filter).

Well, with a tube rectifier, peak currents have to be limited. But those are tweaked by capacitance, not resistance, so it doesn't fit the context here.

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Now, even if you use an equivalent resistance to the tube, your voltage is still higher and regulation is the same.


Er... if you add resistance to beef up silicon to the same effective resistance a tube diode presents, the regulation and voltage will be the same as with the tube.

Well, not exactly because you have a linear rather than 3/2 power law characteristic, but close enough, certainly within range of a certain load current.

Tim
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Old 27th October 2004, 05:21 PM   #26
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Default I'm not seeing it.

So, did anyone actually say what the difference between SS and tube rectification is?

I mean if you bandwidth limit the reg., no HF hash, then that would lead you to logically conclude that, the SS reg and the Tube reg now have the same characteristics..... somehow I don't think it's as simple as this.

Perhaps it has something to do with the inherent qualities of SS and tubes reg. I.e. Exponential vs. 3/2 power.
If this is true, then why?

David ... who really wishes someone one use a high powered oscilliscope to make some data known.
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Old 27th October 2004, 06:50 PM   #27
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Tubes have inherent resistance (somewhat variable due to 3/2 power) whereas diodes, due to their exponential "turn-on" characteristic, drop no more than 1.5V peak. The majority of resistance in a tube rectifier is in the tube itself, whereas for SS, the diode resistance is so diminishingly small that the resistance of the winding and to a lesser extent, the capacitor, is greater. For example, you might read 10Vrms drop on a winding of say 200 ohms (100mA DC output) and 300VAC output for both tube and SS rectifiers. However, the output voltage for the tube might be 320V for a 5Y3, whereas the SS might read 400V. The 10Vrms drop corresponds to the RMS voltage generated across the winding due to charging current (V = IR). Since you can assume a 1V drop for the diode when turned on, the peak of the sine wave input is clipped to 401V, instead of the real 424.26V peak. (If anyone does measure or simulate these figures, remember I'm just SWAG'ing ballpark figures here.)

Since the SS has a full-load voltage much closer to actual peak voltage, regulation is *very* good. 20V good in this case. Whereas the tube drops 100V from peak, which if left unloaded, it will attain. (I.e., any load causes a DC output voltage less than sqrt(2) * VAC.)

Tim
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Old 27th October 2004, 10:01 PM   #28
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Default still missing the point

Soo, in this particular aspect, the 3/2 vs. the exponential resistences are what define the rectifier behavior then.

The majority of the posts are hinting at or stating outright that tubes have this quality about them that, while hard to explain in words, is very noticable. While I'm not disputing this, I just want to know what causes this proclaimed effect.

If you did answer my questioin, I'm sorry I'm being dense. The only thing I could come up with, from your explanation, is that the higher resistence and corresponding voltage drop creates a type of DC feedback which would cut down on the hash, ripples, or whatnot.

Would a tube rectifier be Voltage, Current, or Power constant?

The thing I'm struggling with is that I find it hard to believe that it is impossible to get the same effect from SS regulation.

David

P.S. Hell I'd go through the trouble just for that sexy purple glow
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Old 27th October 2004, 11:21 PM   #29
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Quote:
I mean if you bandwidth limit the reg., no HF hash, then that would lead you to logically conclude that, the SS reg and the Tube reg now have the same characteristics..... somehow I don't think it's as simple as this.
I think the main difference is, (as have been mentioned earlier) that a tube rectifier doesn't have any reverse conduction as ordinary SS diodes have and therefore there will be less RF hash produced. My experience is that it is possible to remove the RF hash by using snubbers over each SS diode, but
that these snubbers are necessary, (it also depends on the bandwidth of your amp, with a wide band OTL amp the problem is easier to notice)

Another difference is of course the higher resistance in tube rectifiers which limits the current peaks which is also helpful in order to eliminate noise, using a rersistor in series with each SS diode limits the current pulses.

Regarding measurements on RF hash from SS diodes, I found this http://www.hagtech.com/pdf/snubber.pdf , the problem with making measurements is that you need a current probe which is not what most DIYers have at home, however to see that there is some RF hash is easy with an ordinary oscilloscope.

Regards Hans
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Old 28th October 2004, 12:20 AM   #30
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Hi all,
Philips PM3070 good enough? Had a Tek with current probe too. After much looking & tinkering I find that the biggest fault with SS rectifiers are the "pips" on the DC waveform and distorted AC waveform (more noise). When designing a low noise supply, you must look at the application and all circuits together. A low current supply with many 1,000 uF will allways sound bad. To reduce noise, try not to make noise to begin with.
That means that you increase the resistance in the leads of the diodes to cut the peak current and di / dt. You place snubbers across each diode to damp transients. You use a low noise regulator to isolate the raw DC from the circuit. Three terminal regulators don't "cut it". And use realistically sized capacitors for the application. To much capacitance is a bad thing, just as too little capacitance is ineffective.
High current supplies are a different animal.
At this point, improvement in all types of high voltage supplies have been found with good solid state regulators. Gas discharge tubes are too noisy (neons as well). That means filtered zeners. Transistors can be quieter than tubes, so why not? After a good regulator, tube or SS rectifiers can not be heard. Not unless you have other problems which need to be solved as well.
-Chris
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