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Old 25th September 2006, 09:25 AM   #1
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Default Tube vs. Semiconducter rectification

Hi all..
Got to wondering ..why do most amps still use tube rectification in their designs...all the literature I'm reading, somewhere it says " as capacitors at that time were not available with that voltage rating"...seems to me that us designers would embrace the benefits of the new stuff. I know of the RF, harmonics & phase shifting of the coil & cap arrangment & the warm-up issue but I think a pure high mikes of caps and attention to the warm-up issue & startup surge are easy fixes.
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Old 25th September 2006, 11:11 AM   #2
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This topic can invoke strong views. My thoughts on this:

Tube rectifiers are quieter than most SS rectifiers, some of which tend to produce 'hash' or switching noises. Tube rectifiers also warm up and start conducting slowly, especially the indirectly heated variety, which many people prefer because they consider it to be kinder on the tubes in the amp and the smoothing caps. These are the main reasons why tube rectifiers have retained their popularity in 'HiFi' amps.

However, tube rectifiers have significant foward voltage drop, which increases with increasing load current, like a resistor but less linear. This causes sag, which guitar players may like but 'HiFi' afficianados do not. With amps in classes other than A, this can be a problem. To an extent, it can be countered by using choke input filtering and big reservoir capacitors but you need a higher initial voltage for this to work. The use of TV damper vacuum diodes as rectifiers can reduce the voltage drop still further but will not eliminate it completely. The most effective cure for the sag of tube rectifiers, in my opinion, is voltage regulation. However, like choke input filtering, this solution requires a higher voltage to allow the regulator to do its job, since it can only work by reducing the output voltage.

With modern SS rectifiers such as HEXFRED, Ultrafast recovery and Schottky diodes, the hash problem can be overcome but the fast warm up problem is still there. Another cure for hash, which doesn't give too much voltage drop and does give a slow warm up, is a hybrid bridge using SS diodes in one half and TV damper diodes in the other.
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Old 25th September 2006, 11:47 AM   #3
SY is offline SY  United States
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Quote:
why do most amps still use tube rectification in their designs.
In decending order of importance:

1. Fashion. But that's true of far too many design choices in audio. The extra glowy bottles are attractive, and fashion will trump engineering 9 times out of 10.

2. Noise. What Ray said, except the noise thing is vastly overblown. Yes, you can measure all sorts of differences within the rectification/filtering loop, but if the grounding is done properly, most of those differences go away when you measure the output of the supply (note the excellent noise measurements on my preamp, which has only silicon in the DC path). However, the tube rectifier, with its high internal impedance, tends to help out when the builder is sloppy about grounding. You can get the same benefit (perhaps more) by merely adding a series resistor to the SS diodes.

3. Noise again: because tube rectifiers are quite weedy as regards their tolerable pulse currents, they enforce design choices that minimize ripple currents. Of course, the designer can do exactly the same thing with SS, but it's so tempting to use that big ol' filter cap....

4. Warmup and surge. There actually is a real advantage here in 1kV and higher supplies, where cathode stripping is a possibility. At the voltages used in more common circuits, the tubes are run well below their Vao ratings, and I have yet to see any evidence of increased lifetime with gradual or delayed B+ turn-on (this excludes special oddballs like 6528). Surge suppression is a valid point, but can be done without compromising supply impedance by the use of an NTC resistor on the primary side of the power transformer.

As you might gather, I don't like to use tube rectifiers except in very special circumstances. Where I have heard differences in properly designed and constructed circuits (that's an important caveat!), the tube versions were distinctly inferior as one would expect with the higher source impedance and degraded regulation.
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Old 25th September 2006, 02:04 PM   #4
Sherman is offline Sherman  United States
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Quote:
Originally posted by SY


...

1. Fashion. But that's true of far too many design choices in audio. The extra glowy bottles are attractive, and fashion will trump engineering 9 times out of 10.

...
SY,

Guilty as charged. I once actually selected a tube rectifier based on the shape of the bottle. (The specs were more than good enough so I went for looks. )

However I've also now built a few amps with SS rectification and think they sound fine as well.

It is nice to see that I'm not in danger of cathode stripping since all my amps are decidedly well under 1kV. Do you recommend a B+ delay circuit for "lower voltage", under 1000V or do you think they are not needed?
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Old 25th September 2006, 02:12 PM   #5
SY is offline SY  United States
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I've used them (time delay relays or the Amperite thermal modules) and just used NTC. If there's a clear advantage one way or the other, I haven't seen it.

Quote:
The specs were more than good enough so I went for looks.
Nothing wrong with that!
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Old 25th September 2006, 03:56 PM   #6
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Hi all,

I'm mostly doing restauration job, on old amps; and then have the habit to work reusing original circuity:
If I found a tube rectifier, I will reuse it with some "elegant" ameloration if possible (like NTC and others).
If I found a SS rectifier, I stay in the same topology replacing old SS diodes by generic TV FRED (i Bought a lot of cheap BYV96E giving me satifaction) adding caps in // for peaks, NTC on primary ... usualy this give a good upgrade to sixties SS supply.

Quote:
Ray said:

Another cure for hash, which doesn't give too much voltage drop and does give a slow warm up, is a hybrid bridge using SS diodes in one half and TV damper diodes in the other.
I'm very curious about that !
could you give me please some examples of that topology (generic schematic)?
wich kinds of tubes are usable ? (caracteristics/models?) I have on hand 2X2, PYxx, EYxx, DYxx, UYxx

I have to add that:
- I personnaly have nothing against slow warm up (In the case it adds only "psychologic peace of mind" it won't do something bad either (especialy cathode stripping)).
- I often like sag, sure I play guitar, but I like that """kind"" of compression" even on old HiFi tube gear (I own old Thorens / dynacord stuff that use tube rectifiers and sounds very pleasing to my ears (and are able to reproduce square waves like nothing else I saw for now).
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Old 25th September 2006, 05:19 PM   #7
rdf is offline rdf  Canada
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The only caution is that low rectifier impedance - at least at the PSU Designer II and Spice simulation level - doesn't appear to be a universal panacea. So much depends on the type of supply and the quality of components used. For example, a strong potential for supply ringing exists when solid state rectifiers (or low loss tubes such as an 83 (<<<not recommended, major toxicity alert!!)) are combined with low DCR inductors and transformer secondaries. The trick apppears to be tuning, tuning and tuning.
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Old 25th September 2006, 07:36 PM   #8
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I tend to use snubbers on the secondaries of my power transformers(about 0.1uf to 0.47uf at at least 3x the secondary voltage). I don't see any ringing on the scope but my scope ain't that great.
I got into this habit in my gainclone days and remember a slight softening of the sound.

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Old 25th September 2006, 07:54 PM   #9
anatech is offline anatech  Canada
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Hi SY,
Thank you for your first post there. I am in full agreement with you on all points.

I will restore a unit with the original power supply unless there is a compelling reason to do something else. There should be a term for "an unreasonable fear of solid state devices" that afflicts some audiophiles.

Hi rdf,
I have a bunch of 83's. I do my best not to drop them. In case any runs out, I have a 1 oz bottle of mercury around here somewhere for refills. (can't figure out how to pour the air out, more keeps coming back in) It's in a small plastic bottle. Have you seen it?

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Old 25th September 2006, 08:26 PM   #10
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Quote:
There should be a term for "an unreasonable fear of solid state devices"
Sandophobia!
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