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Old 22nd October 2004, 08:07 PM   #11
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Quote:
Originally posted by resident
With proper heater practices?What do you mean?
Well, you connect heaters to a 6.3V (or whatever) winding... you might ground one side of that winding. Now it can't move up and down and couple noise to the cathodes (primarily electrostatic, i.e. the capacitance between heater and the cathode around it). That leaves one side wild however, putting an average 3.15VAC between the heater and cathode. So you might ground a center tap of the heater instead (most often done with a pair of resistors, or a variable resistor, known as a "hum balance"). That balances the AC voltage on the heater, nulling the voltage coupled to the cathode.

Better still, however, is elevating the heaters with a DC bias, above all the cathodes (within limits). Say you have a 6C4 with 2V cathode bias and a 6V6 with 15V cathode bias, 30V heater bias would be good. (No more than 70V or so, as that would exceed the 6C4's H-K rating.) This reduces leakage because the heater acts like a filament cathode (I mean, it's hot tungsten, right?) and the inside of the cathode sleeve acts like the plate of a small diode. Putting the heater above the cathode voltage-wise "turns off" the diode.

A combination of these, a voltage divider with capacitor bypassing to ground, is best, and I have had good success with it.

DC heaters are another can of worms, but suffice it to say it isn't necessary in all but the most unruly tubes or high gain stages.

Back on topic, you'd use a mercury rectifier such as #83 or (if you have some real power to rectify) 866. Those are the only types off the top of my head. Note that mercury tubes require RF hash chokes at a minimum, and preferably a good faraday cage to supress all the noise -- they switch orders of magnitude worse than any silicon diode.

Tim
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Old 22nd October 2004, 09:56 PM   #12
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Quote:
Note that mercury tubes require RF hash chokes at a minimum, and preferably a good faraday cage to supress all the noise -- they switch orders of magnitude worse than any silicon diode.
Yup. I've heard this before. Another great example of practice not following theory. Or maybe whatever noise they generate is completely irrelevant to audio? Or due to age related HF hearing loss (14.5kHz) i am simply not bothered? Whatever.

Still, mercury somehow helps making music in a way that no hexfred can ever dream. Tonality, dynamics, bass - done right.

Yes, using them is a bother; they require compulsory preheating; they are not easy to parallel; they really suck current from the heater ps. Not to mention the ghastly purple shine... bit it's all worth it.
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Old 24th October 2004, 08:29 PM   #13
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Hmmm
I thing I値l try those tubes.

Quote:
Note that mercury tubes require RF hash chokes at a minimum

What are these RF hash chokes?

Now I値l use one of the best SS diodes, HFA08TB60.
A lot claim to be the best ones. And I値l try to find a mercury tube to do a test btw those two.
I love tubes and I壇 like to see those SS diodes to sound as crap at the test with the mercury tubes.
These tubes are a little bit rare! I have seen some 83 somewhere but they were expensive.
I値l build this amp with SS diodes, I already have all the components and now I知 placing them together! Hope until Wednesday will be ready.
But mercury will be on my mind!

One question out of our discussion,
Where is the best place to put the stand by switch?
At the CT of the power transformer (SPST)?
Between windings and diodes (DPDT)?
Between diodes and first cap of p-filter?
Or after the p-filter?
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Old 24th October 2004, 10:41 PM   #14
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Quote:
Where is the best place to put the stand by switch?
If you're really mad enough to try the 83 (you know what mad as a hatter means and why, don't you?) make sure you have a spare source of 5v@3A and connect the switch to the CT ground return. The 83 absolutely has to warm up for at least 30 sec before the high voltage hits the plates.

To fully enjoy the benefits of low impedance rectifiers you may wish to choose transformers and chokes with as low dc resistance as possible. Luckily Lundahl make some PS chokes with extremely low resistance but the transformers have to be custom wound. I can attest to the benefits of low DC res chokes but still have to try some truly low resistance transformers. 3-4 ohms resistance on the secondary seems to be the goal.
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Old 25th October 2004, 02:23 AM   #15
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Hi,
I have equipment that uses 83's. The manual recommends a much longer warm up time to allow the mercury to evapourate fully to support the current flow. I belive the figure was around 20 min. before applying plate voltage and a load. This is test equipment.
Yes, RF chokes are needed. As is a shield around the tube (well ventillated. I am sure the tubes are in use without these precautions, but they will not last as long or you will pick up the hash. The forward voltage drop will be around 15V and reasonably constant with load.
I use silicon diodes in anything new. The voltage drop is less and more consistant. The noise is much less and no lost power to the heater. Include a series resistance to greatly reduce switching noise.
-Chris
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Old 25th October 2004, 07:00 AM   #16
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@ analog sa
Quote:
connect the switch to the CT ground return.
If I'll use SS diodes,connect the st/by switch at CT, too?

@anatech
Quote:
Include a series resistance to greatly reduce switching noise.
Where to you put this series resistance?Between the diodes and p-filter?Value?
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Old 25th October 2004, 07:13 AM   #17
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YEs, switch in CT loop is fine for SS as well.

Neither myself, not anyone else i know have ever experienced problems with 83 noise. I don't claim they don't generate noise; it just appears to be a non-issue. Small chokes connected in series with the plates appear to be harmless but resistors will destroy the advantages of low forward resistance.

Warm up: the 20 or so minutes is recommended as a one off conditioning after the tubes have been in storage. In normal operation 30 sec is sufficient. There are quite a few tube testers powered by an 83 with no delay between heaters and plate voltage and they seem to last.
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Old 26th October 2004, 12:08 AM   #18
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Hi analog_sa,
If you use a 'scope on the rectified output, you will see "hash" on the waveform. This may interfere with a tuner. Resistance in series with silicon diodes removes the "spike" from the output. Given the resistance and voltage drop of tube rectifiers, this is not an issue at all. You are still way ahead with the "sand".
No, I don't replace existing tube rectifiers with solid state units unless I have a good reason to do so. But if I am designing something new .... silicon diodes. The higher B+ allows me to use a voltage regulator for really low noise (SS - sorry) B+ lines for preamp stages.
My tube tester recommends a 20 min warm up every time. I'll follow this recommendation, it may only need 5 - 10 min.
-Chris
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Old 26th October 2004, 12:38 AM   #19
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Hi,

Quote:
Given the resistance and voltage drop of tube rectifiers, this is not an issue at all. You are still way ahead with the "sand".
What you're doing with the series resistance and diodes is exactly what's done with solid state replacements for tube rectifiers such as the 5AR4. Except that the series resistance is calculated to give the same drop as the real tube rectifier.

However, from my experience I noticed the following:

A series resistor won't do away with the fact that the diodes will still spit back a nasty spike and considerable amounts of RF hash into the mains.
It is not very effective in doing away with all the hash going into the circuit either and doesn't give you the nice slow start most tube rectifier do.

Last but not least: no matter how much snubbing applied, it still doesn't make it sound the way a true tube rectifier does....
Not even the better SiC Cree diodes. Much to my dismay, BTW.

The best compromise I ever tried so far is the hybrid rectifier scheme which seems to combine the best of both worlds.

Note: I never tried mercury vapours as every engineer I ever had working for me always warned against the RF noise problem and of course the longish pre-heating protocol.

Quote:
My tube tester recommends a 20 min warm up every time.
I still have half a dozen of tubetesters employing 83s among other rectifiers and they all have been working reliably for about twenty years with just a few minutes warm-up.

Cheers,
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Old 26th October 2004, 01:45 AM   #20
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Hi Frank,
Quite correct in that there is no soft start. I put the series R in the anodes. I guess you could put some small caps across the diodes to slow down the switching speed too. That should reduce the amount of hash into the mains. Basically, you only need to slow down dv/dt to kill the hash. (snubbing).
For preamps, all you need to do is ramp up the reg output. You can not hear the rectifier type at all doing this. Unless you have solid state diodes that put HF hash on the DC which may pass by a regulator.
The reason your 83's may last is that you aren't drawing near the max current in this situation. In a power supply for an amp, this isn't the case. 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.
-Chris
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