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Old 15th August 2012, 04:36 AM   #21
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PlasticIsGood, measured heater cathode resistance increases as the voltage differential increases away from zero. Different preamp tubes show different levels of 'zero voltage differential' resistance, and different increasing resistance characteristics as the voltage difference is increased (and with both symetric and asymmetric behaviour for positive and negative polarities).

For the small number of 12AX7's that I have measured, there has been no obviously generic characteristic or magnitude of resistance other than a minima around zero voltage differential. But I also note that the measured resistance levels observed at 0V have been very high (much much higher than some spec sheet 'minimum' levels), and so in context I couldn't see how dc elevation would provide any improvement at all to hum - and so I would suggest that dc elevation is not a good first in design use unless there is specific evidence that it is the root cause of any discernable hum.

In those situations where cathode-heater resistance contributes to a discernable hum, then introducing a voltage difference will per se increase the effective resistance, and so can help lower hum. Given that the pk heater AC voltage may be say 9V, then applying a dc elevation higher than 9V will start to move all parts of the heater-cathode voltage differential waveform away from 0V. General recommendations I've seen are to use at least 20-30V elevation at least. I've got a few old amps that use the bypassed cathode bias of the output stage as an elevation voltage.
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Old 15th August 2012, 05:01 AM   #22
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Quote:
Originally Posted by PlasticIsGood View Post

How can I calculate the best reference voltage for a heater?
1. it starts with the tube(s) you want to use, and the heater cathode voltage ratings for that tube, this is found in datasheets....

2. next is the topology you want to use the tube(s) in....

knowing these 2 things, you can then proceed to calculate voltage dividers for heater reference....this voltage reference need not draw large currents but may also serve as bleeders to discharge psu caps at shutdown...

in my tube builds, i have resisted using dc supplies to the heaters, having a dc reference helped to combat hum....
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Old 15th August 2012, 05:06 AM   #23
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Quote:
Originally Posted by Michael Koster View Post
What most people call "floating" is when there is no connection, directly or indirectly, to a fixed voltage reference.

"Filament" is the common name for a directly heated cathode.

If it's an indirectly heated cathode, we call the resistor thingy inside a "heater".

Your heaters are not floating, they have a fixed voltage reference.

Heaters should never be left floating. Always respect the heater-cathode voltage rating including the max signal swing and startup swings.
thanks for clarifying and reminding....
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Old 15th August 2012, 08:49 AM   #24
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Quote:
Originally Posted by PlasticIsGood
How can I calculate the best reference voltage for a heater?
I think the quick answer is that you can't. However, in his book "Getting the Most Out of Vacuum Tubes" the author says that life tests have shown that there is a broad curve with best behaviour at +40V wrt cathode. The more typical 'slightly negative wrt cathode' is almost as good, because it is a broad curve. Most datasheets say +-90V or something like that.
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Old 15th August 2012, 09:35 AM   #25
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Originally Posted by DF96 View Post
Yes, floating filaments means an open-circuit cathode in a directly heated valve. Indirectly heated valve heaters should never float. They need a DC reference to keep the valve happy, and some degree of AC grounding to control noise, buzz etc.
This isn't quite true. I did a project where the heaters float due to the use of a SS/HS hybrid power supply. The SS negative rail comes up much faster than heaters can warm, and would bust the Vhk spec of the 6FQ7 I used as a grid driver.

Floating these heaters didn't make for excessive noise at all. "Buzz" is more likely a grounding issue.
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Old 15th August 2012, 11:16 AM   #26
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I had low level buzz due to rectifier spikes on the heater wiring, when DC elevated for an LTP. No amount of decoupling would help. Grounding the secondary CT solved the problem, even though it left the heater sitting about 80V below the cathode (ECC83).
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Old 15th August 2012, 01:58 PM   #27
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i use DC filament supply for both the input - AX7 and invertor - AU7 stages but AC for the output stage - 6550. so do i need to ground or reference the DC filament supply cct? its floating right now as it has its own line transfo and the whole thing does not tie to any part of the main HT supply cct.

cheers
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Old 15th August 2012, 02:38 PM   #28
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Valve makers tell us that the heater circuit should not float. I believe them.
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Old 16th August 2012, 09:18 AM   #29
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Originally Posted by DF96 View Post
Valve makers tell us that the heater circuit should not float. I believe them.
they maybe referring to AC filaments heaters. if its so also for DC, do i just tie the negative leg of the heater to the star ground?
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Old 16th August 2012, 09:30 AM   #30
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Well, valve manufacturers would never have thought of DC heaters for indirectly heated valves but the reason for not letting them float is still the same. Yes, you can tie one side of the heater supply to your star ground.
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