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Old 25th December 2010, 03:00 AM   #1
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Default Correct Path: Diodes Or Rectifier Tube?

I've built a pair of highly efficient FR speakers. My next goal is to build or purchase an SET 300B Tube Amp.

I am determined to choose an amp that uses tube rectification. I believe this is the correct path to that classic tube sound.

Am I on the right path? Does tube rectification sound "better"?
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Old 25th December 2010, 03:53 AM   #2
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Both paths are correct.
Build a SET with both properly designed recitification methods and switch between them. The end is decided by your ear.

The other question is are you a purist? In that case stay with tube. Personally, my own journey has led me to build with SS rectification on full range power amps and Vacuum rectifiers in my line stages.
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Old 25th December 2010, 04:00 AM   #3
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Am I on the right path? Does tube rectification sound "better"?
Not necessarily. For quite some time, SS rectification has lead to better bass. Unfortunately, all PN junction diodes generate switching noise. That noise is objectionable. Vacuum rectifiers don't produce that noise. Fairly recently, high PIV Silicon carbide (SiC) Schottky diodes were introduced. Schottky diodes are "noiseless", while retaining the other virtues of SS rectification.

My advice is that you rectify the B+ with high PIV Schottky diodes.
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Old 25th December 2010, 06:01 PM   #4
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Yeesh, build an amp correctly, it's not hard to neutralize switching noise.

BTW, schottky diodes are equally capable of producing noise in switching circuits. They may not have charge storage, but the dramatically higher junction capacitance near forward bias looks the same. I once made a pulse generator with a schottky:

Click the image to open in full size.

Unfortunately, the contrast on my scope at the time was poor, but you can just barely see a very narrow downward blip at 6.8 div from the left. This occured when the junction capacitance of this particular diode went from about 3nF near forward bias to about 100pF in reverse bias. You get the same charge delivery effect which makes inductors go "splat", which includes stray wiring inductance.

The solution is simple, add an R+C damper, typically 0.01uF + 100 ohms, right across the diode, and keep your rectifier wiring away from signal current paths.

Tim
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Old 25th December 2010, 07:34 PM   #5
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So what is the down side with simply using a tube foe rectification? Is it softer bass? Higher distortion? Inquiring minds want to know.
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Old 25th December 2010, 07:49 PM   #6
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Hi!

Quote:
Originally Posted by JimOfOakCreek View Post
So what is the down side with simply using a tube foe rectification? Is it softer bass? Higher distortion? Inquiring minds want to know.
There is no simple answer to this. It totally depends on the curcuit which is supplied. A SE amp will react differently than a PP. Class A different than Class AB. Even within the same class of amp not each circuit will react the same. Try it and listen for yourself!

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Old 25th December 2010, 08:06 PM   #7
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by JimOfOakCreek View Post
So what is the down side with simply using a tube foe rectification? Is it softer bass? Higher distortion? Inquiring minds want to know.


Just for interest,

Tube rectification.

Volt drop across the tube is higher than with diodes.
However different diodes "sound different".
It all comes down to personal taste!

You can snub diodes again this alters the perceived sound.
The type of music you listen to and the expectation of what you think "Tube sound" sounds like!

The harmonic content of the amplifier will also make a difference!

I know this is going to create more questions than answers, however there is no direct answer to what you would like / dislike.

Some people don't think that HIFI sounds like a tube amp should sound!
(Old valve radios sound is nice, However it is not HIFI!)


Regards
M. Gregg
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Old 25th December 2010, 09:26 PM   #8
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On my guitar amps I routinely bypass the diodes with .01uF ceramic caps.
I use the Ultra Fast UF5408 diodes.

This is to snub the transient noise.

I haven't looked on a scope to see this or if it actually works.
It's just something I have always done.

But I was reading a book last night that talked about diode ringing and that the caps help prevent that resonant from forming as well.


As was mentioned, tube purists can't even stand the mere suggestion of using any Silicon based products in their amps while many others tolerate that bit of modernization in the power supply.
Trade off can be a better regulated power supply and tighter bass due to increased filtering.
I have also heard that you can hurt the tone with too much filtering so there is probably an equation somewhere that addresses that.
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Old 25th December 2010, 09:48 PM   #9
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Quote:
Originally Posted by JimOfOakCreek View Post
So what is the down side with simply using a tube foe rectification? Is it softer bass? Higher distortion? Inquiring minds want to know.

Are inquiring minds also lazy bums? Nothing comes close to first hand experience.

My subjective impressions are indeed softer bass and less distortion/more resolution elsewhere. I always use high performance regulators, at least to power the small signal stages, but the bass remains softer than SS rectifires. SIC Schottkys didn't work for me. A sonically acceptable compromise are mercury rectifiers (great bass) if you can live with all their idiosyncrasies. Turns out i can't.
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Old 25th December 2010, 10:08 PM   #10
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Tube rectification is much softer than SS.

I have had problems switching high voltages with diodes with switching spikes.
Had to add capacitors across the diodes to stop the glitches getting on the audio.
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