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Old 28th October 2004, 12:36 AM   #31
anatech is offline anatech  Canada
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Hi Tim,
There are many tube rectifier circuits with series resistance to limit the peak current.
Anyhow, all this is within the context here. I see the biggest problem in mis-application of capacitance. How many tweaks do you see that are mainly an increase in supply capacitance? This puts the peak currents well above recommended levels. In properly designed supplies I don't see much difference in sound quality unless you "crank it". There will be more voltage sag with the tube units over SS with resistance and snubbers. There will be a difference in sound that may be audible. Unpleasant? Up to you.
If you have time, remove the large dropping resistor in a push pull amp. Something around 30 - 50W is good. Replace it with a SS regulator in a classic zener on emitter, error voltage to base, collector to pass transistor setup. Take the base drive for the pass transistor from an LED constant current source and filter the rest in good practice. Listen to the noise floor drop and the sound remain clear at high power levels. The voltage amp stages are now getting clean DC unaffected by the main supply. If you had a tube rectifier, try now setting up a SS compensated rectifier (use a standby switch). I'm not saying there won't be any difference, but most of the nasties should be gone (if not all).
-Chris
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Old 28th October 2004, 12:54 AM   #32
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Hi,

Quote:
Gas discharge tubes are too noisy (neons as well).
I don't think so.
In fact I'd prefer a glow discharge over a zener, or string of zeners, for any application requiring highish voltages.
The noise from a glowdischarge tube is far more broadband_less peaky_ and much less obtrusive than that from a zener.
Moreover these tubes don't suffer the tempco problems most zeners do either.

As for the SS diodes: yes, putting the correct snubber network across them does help reducing switching transients and RF noise considerably. A series resistor before and after the diodes may help kill some more misbehaviour but most will agree that the best sounding rectifiers are damping diodes and the now trendy mercury vapour types.
The damper diodes offer much lower voltage drop due to their low internal resistance and don't nearly pollute the circuit with RF spuriae as much as their SS counterparts.
As you put it yourself, no need to filter out noise if you don't introduce it in the first place.

For sensitive circuitry such as phonostages and mic preamps you don't need any current to speak of, so why not opt for what does the better job in the first place instead of applying bandaid upon bandaid?
You could use Schottky diodes for the LT circuits such as bias and heaters, tubes for anything HT.

A decent PS choke does a very good job at isolating the AC part of the PS, add a well design tube regulator where stability is required and you're pretty close to simulating a battery PS without its practical disadvantages.

Using an isolation xformer is certainly a very good idea, the endresult will surely depend on the quality of the xformer(s) themselves...Quite another topic that's being overlooked. Regretably so.

Designing high performance tube circuits while sticking to engineering theories based on published specsheet figures will rarely ever yield an outstandingly good sounding design.
Engineering for audio can only be learned through years of trial and error and a will to listen to the results, not just by looking at an o-scope IMHO.

Cheers,
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Old 28th October 2004, 01:07 AM   #33
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Hi,

Quote:
There are many tube rectifier circuits with series resistance to limit the peak current.
Do you mean the resistors in series with the plate circuit?
If so, these are only there to limit inrush current and won't affect current capability of the PS proper.

Quote:
There will be more voltage sag with the tube units over SS with resistance and snubbers.
Perhaps so for class AB1 or class B designs but that's only a sign of bad PS design and often done on purpose in guitar amps...
With a class A amp such as the currently fashionable SE miniwatt amps this issue is rather moot.
Provided the PS is well designed, peak current demand should be met by the reservoir capacitance, not being drawn straight from the mains socket so to speak.

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Old 28th October 2004, 01:10 AM   #34
anatech is offline anatech  Canada
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Hi Frank,
I listen to the supplies too. I use a blocking cap to the input of whatever amp is a bench amp at the time. I'm looking for obvious noise. Very revealing.
I generally agree with you, but get there my way. No bandages in my stuff. A great deal depends on circuit layout and ground paths as well. Any topology depends on the execution of the design.

The one thing I like about VR tubes is the gentle drift, rather than the positive tempco. The thermistors go a 10V drift at 275VDC down to 2V. This I can live with. I've tried 100V zeners and 33V zener stacks. The temperature drift is the same. The zener is bypassed, as is the positive voltage sense resistor (RC).

I design both ways but I've had better success with SS regulation, both specs and sound. The way I see it, I'm attempting to apply the best devices to a particular job, whatever the devices may be. Keeping an open mind has paid off well for me.
-Chris
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Old 28th October 2004, 01:43 AM   #35
anatech is offline anatech  Canada
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Hi Frank,
Just got your last post. I'll count myself out of the single ended discussion. Class AB and class A (not classic) are my areas of interest. The exception is a voltage gain stage with tubes.

Voltage sag in a power AB or B design is a fact of life to some degree. The current variation forces this to occur unless you install a power regulator as well. I'd rather have the extra voltage and isolate the voltage amp stages to minimise the effects of this.
I shutter to think how much power I'd require dissipated to run PSB Stratus Gold's, never mind more power hungry speakers that exist, in a class A tube amp design. A little 35W/ch tube amp does okay here. Class AB.

Some tube rectifiers I have seen use 500 ohm per plate. That matters. I guess I'm just pointing out some bad designs patched to hang together. (This particular unit is a preamp, uses two cetrons per channel that occassionally arc - designer on drugs I think) Anyway, I was referring to the hot switching current limiting, as you pointed out.
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Old 28th October 2004, 02:05 AM   #36
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Hi Chris,

Quote:
I shutter to think how much power I'd require dissipated to run PSB Stratus Gold's, never mind more power hungry speakers that exist, in a class A tube amp design. A little 35W/ch tube amp does okay here. Class AB.
Sure, that's one of the paradoxes of audio: linear speakers (not that they really exist) versus high effieciency at the expense of BW extension and perhaps some other oddities.

However, if you can live with a 35W class AB amp_I assume it's a PP design_ you could certainly do away with the crossover distortion these AB amps invariably run into and enjoy the effortless power of a true class A amp.
The difference in mere musicality between the two can be amazing as I'm sure you're aware of.

Naturally, there are good and not so good designs of either conviction but all else kept equal, the class A amps, more often than not, are the ones that are the more respectful of that ultimate illusion called music IME.

Quote:
Some tube rectifiers I have seen use 500 ohm per plate. That matters.
Yep...470R is a commonly used value.

Quote:
This particular unit is a preamp, uses two cetrons per channel that occassionally arc
Hmmm...What type of tube are those Cetrons and what's the current draw of the cct?

Cheers,
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Old 28th October 2004, 05:07 PM   #37
yagas is offline yagas  Greece
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Talking Although a little bit late...!

follow this link where my friend Aristidis describes his amp (using stand by - soft start)

http://aca.gr/paper31_build.htm
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Old 28th October 2004, 08:38 PM   #38
anatech is offline anatech  Canada
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Hi Frank,
A Cetron is a make or brand of tube. The rectifiers in this case are 5U4GB's on steriods. 5U4's explode in this silly preamp.
The preamp in question is a Michael Elliot / Counterpoint thing. It has two outboard heavy power supplies (one per channel). These ultimately run 3 6DJ8's per channel. He uses 5881's as shunt regulators, 'cause these are faster. ( ) Every heater is shunt regulated as well. This thing gives off more heat than you can imagine. I forget the model, but the description will be enough.

Another one of Counterpoints monsters use the 500R plate resistors. 20W jobs that mount on the chassis with grease. When this preamp goes, so do those resistors sometimes. They run very warm normally.
These are my examples of very poor supply design.

Class A designs can be good. I like low feedback AB tube or transistor designs. They don't sound "squashed" either, very musical.

Thanks Frank -Chris
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