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Old 12th August 2012, 08:29 PM   #11
20to20 is offline 20to20  United States
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Yero,

Is your question about "lifting" the filaments, as in applying a DC to them to bring them closer to cathode voltage?

Or is it about seperating the filament circuit from a hard ground point, ( so that it can be "lifted") at the winding (centertapped or end grounded)?
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Old 12th August 2012, 10:05 PM   #12
DF96 is offline DF96  England
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Quote:
Originally Posted by PlasticIsGood
A relative matter, I guess, and 50 ohms or so is quite high relative to the resistance of several heaters in parallel. I would say that's quite loose.
The correct comparison is with typical valve impedances of k ohm. The aim is to stop the heater from picking up any voltage significantly different from valve electrodes, especially the cathode. A truly floating electrode could pick up low velocity electrons and gradually become negative, or it could suffer from secondary emission caused by high velocity electrons and become more and more positive. Many datasheets therefore specify that the DC resistance between heater and cathode should not exceed a few 10's of k, except for the special case of a CF, LTP or cathodyne.

So from this point of view, 50 ohms is almost the same as a short circuit.
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Old 13th August 2012, 12:00 PM   #13
roline is offline roline  United States
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I'l raise the heaters center tap wire to 50v, by-passed zener reference to keep them a little quieter, also reduces the heater to cathode voltage on the splitter.
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Old 13th August 2012, 06:37 PM   #14
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by yero View Post
I want to build franks 6sn7 preamp. I have read that the 6.3 filament supply should be floated.
You have the terms reversed. "Floating" means the heaters are not referenced to any DC voltage. They should NEVER be left floating.

The simplest thing to do is tie the heater circuit to ground by grounding the center tap of the 6.3V secondary. Lacking a CT you can connect a couple of 100R resisters and tie the mid point of those to ground. Next step up in sophistication is to tie them to an elevated voltage. Next step is to use a pot in place of the two resistors you you can adjust for best balance.

But never leave them floating.
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Old 13th August 2012, 06:46 PM   #15
ChrisA is offline ChrisA  United States
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Originally Posted by DF96 View Post
The correct comparison is with typical valve impedances of k ohm. The aim is to stop the heater from picking up any voltage significantly different from valve electrodes, especially the cathode. A truly floating electrode could pick up low velocity electrons and gradually become negative, or it could suffer from secondary emission caused by high velocity electrons and become more and more positive. Many datasheets therefore specify that the DC resistance between heater and cathode should not exceed a few 10's of k, except for the special case of a CF, LTP or cathodyne.

So from this point of view, 50 ohms is almost the same as a short circuit.
I agree with this analysis. The cathode to heater current is _tiny_ and so by Ohm's law almost any resistance will work.

I have one amp where I did in fact use a dead short. I let the heater circuit float except I use a piece of wire on a power tube sock to short the cathode to one heater pin. This biased to the entire heater loop to the same voltage as the cathode. Worked great for killing the hum and the parts found was a small as it gets.

The key to minimizing 60Hz hum from heaters is to experiment. There are many known tricks and which one will work for you on your amp? No one can know in advance. Even DC heaters may not work some times
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Old 14th August 2012, 03:21 PM   #16
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Quote:
Originally Posted by DF96 View Post
The correct comparison is with typical valve impedances of k ohm. The aim is to stop the heater from picking up any voltage significantly different from valve electrodes, especially the cathode. A truly floating electrode could pick up low velocity electrons and gradually become negative, or it could suffer from secondary emission caused by high velocity electrons and become more and more positive. Many datasheets therefore specify that the DC resistance between heater and cathode should not exceed a few 10's of k, except for the special case of a CF, LTP or cathodyne.

So from this point of view, 50 ohms is almost the same as a short circuit.
You're right to point out that common mode impedance is important, but you may be wrong to suggest that differential mode can be ignored: there can be more than one "correct comparison".

I'm sceptical about your hypothesis for several reasons:

The heater is different from the valve's electrodes in that it is insulated so the effective resistance is several orders of magnitude higher than "typical valve impedances of k ohm".

The heater is enclosed within the cathode which forms a Faraday cage, which isolates it from other electrodes, so only the cathode is significant.

The hypothesis doesn't explain why resistors of such low values are used to establish the heater centre ground. Why not use the max allowable value, which would presumably be several k ohm at least?

It ignores the contribution of heater/cathode capacitance, which is the principle conveyor of heater hum and other noise to the cathode.

It doesn't explain why any of the effects it claims are bad, except for the assertion that a totally floating heater circuit may continue to "pick up voltage" indefinitely...presumably until the maximum heater/cathode voltage is exceeded. This seems implausible because I believe an equilibrium voltage would soon be established.

It ignores the fact that there are usually several heaters in parallel, belonging to different valves.

It ignores other considerations originating in the heater circuit itself, such as common mode hum and other noise.

It abandons concern about differential mode impedance.

I would be pleased if you could demonstrate your case. Perhaps you could derive from your hypothesis a method for calculating the ideal value of resistors to use? Then the relative significance of all the parameters would be plain.

My view is that, for common-mode AC at least, it is sufficient simply to consider heater/cathode capacitance. Calculations using this simple expedient appear plausible.

Note that by this account the common mode impedance of the heaters depends on frequency. A good test of the efficacy of a method would be to consider a gain stage with unbypassed cathode resistor, followed by a cathode follower, with both heaters sharing the same heater circuit. How much hum would result if the heater ground were one-sided? How much of a 20kHz signal would be fed back to the gain stage cathode from the CF, via the heater circuit? How much do these answers depend on the resistance of the heater ground connection?

Can you use your hypothesis to calculate answers to these questions?
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Old 14th August 2012, 03:44 PM   #17
DF96 is offline DF96  England
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Quote:
Originally Posted by PlasticIsGood
I'm sceptical about your hypothesis for several reasons:
Your scepticism is noted. Read a few valve datasheets, or even textbooks. Isolated electrodes in valves can and do pick up voltages, as I described. The heater is not entirely enclosed in the cathode.

The reason people often use lowish value resistors across the heater circuit with a grounded junction is to help short out capacitive noise from the mains transformer, either incoming noise or rectifier spikes. A secondary CT is better, but can cost more. For the purposes of establishing a reference level anything below a few 10's of k is fine. For shorting noise the lower the better, but too low starts eating up heater power so a few hundred ohms or so is a compromise.

So there are two issues in play: establishing a reference, and grounding noise. In the context of the former, a few hundred ohms is a short circuit not a 'loose' connection as I believe you said. Forgive me if I have misunderstood you.
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Old 14th August 2012, 09:11 PM   #18
Merlinb is offline Merlinb  United Kingdom
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Small values eliminate the possibility of picking up RF which could otherwise be demodulated by a nonlinear heater-cathode junction.
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Old 15th August 2012, 03:31 AM   #19
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And the coupling of heater voltage to control grid can become noticeable, due to amp wiring, and internal tube and valve holder construction. And the impact can be exacerbated by the heater containing additional frequencies, such as from HT rectification or flat-topped mains in your house. A humdinger pot can certainly help to null out the contribution from each heater lead transfering to the control grid - Merlin has presented a dramatic example of this impact.

Note that dc elevation of heater would only alleviate any hum contribution from the dc resistance leakage between cathode and heater, and would not influence other coupling mechanisms such as capacitance between heater and cathode, or control grid coupling. DC elevation also requires the applied voltage not to have significant AC components on it (eg. some bypassing of a HT divider is needed).
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Old 15th August 2012, 04:02 AM   #20
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Quote:
Originally Posted by DF96 View Post
Your scepticism is noted. Read a few valve datasheets, or even textbooks. Isolated electrodes in valves can and do pick up voltages, as I described. The heater is not entirely enclosed in the cathode.

The reason people often use lowish value resistors across the heater circuit with a grounded junction is to help short out capacitive noise from the mains transformer, either incoming noise or rectifier spikes. A secondary CT is better, but can cost more. For the purposes of establishing a reference level anything below a few 10's of k is fine. For shorting noise the lower the better, but too low starts eating up heater power so a few hundred ohms or so is a compromise.

So there are two issues in play: establishing a reference, and grounding noise. In the context of the former, a few hundred ohms is a short circuit not a 'loose' connection as I believe you said. Forgive me if I have misunderstood you.
I haven't argued that there is no heater cathode leakage, there is and it is specified in datasheets, and measurable. It is very small. There are more than two issues at play, but two's a worthwhile advance from one.

The issue for me here from the start has been the OP's use of the word "floating". I took this as an encouraging observation for a novice but warned against ambiguity. Others have taken a strict line, claiming that there is a true unambiguous meaning that should be used exclusively for fear of misunderstanding. I understand the importance of jargon, but feel that plain English should also be acceptable in a DIY forum as long as it is intelligible, albeit with a little effort, and that electronics engineers should ease up a tad, and not play dumb in the face of common language.

My point was that the circuit is, in some sense and to some degree, floating on the ground resistance. I had in mind wrt to differential mode, low-impedance signals, such that typical centre-grounding resistance is large compared to resistance around the heater circuit. Whether or not this is floating or not in the strict sense the strict people wish to apply is of no consequence: I understood what the OP meant. I think I can see why he used the word "floating". I introduced "loose" to distinguish between "cast adrift" and "loosely tethered". It is arguably a specious point, because differential mode signals are cancelled by the cathodes as long as the centre ground holds firm, maybe, ish. But a novice might not know that.

My only question in the thread remains unanswered, btw.

I've read some books, thanks, and checked with datasheets and a real valve before I wrote what I wrote. I don't believe the answer to my question is in any of them.

How can I calculate the best reference voltage for a heater?

Thanks for engaging with the technical issue: it's been useful to refresh my mind, and learn a little, too. Please reconsider, in your own mind, the importance of heater/cathode capacitance, and downgrade emissions to "rarely significant".

On the social issue that originally attracted me, I feel resigned to ploughing a lonely furrow, paradoxically. I'm quite good at helping novices, but my method stops working in the presence of sticklers.
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