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Old 12th August 2012, 06:13 AM   #1
yero is offline yero  United States
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Smile when do filament supplies need to be floated

I want to build franks 6sn7 preamp. I have read that the 6.3 filament supply should be floated. the circuit shows The filament winding has two resistors in series across the 6.3 winding with ground between the two resistors. I have built a Pas preamp with out floating the filaments. why is floading recommended. If the filament is DC or used with tube rectfiction have any thing to to with floating or not floating the supply? I believe the reason for the floating filament has to do with 60HZ hum? The pas preamp I built used a 12v regulator for the filaments without floating the filaments with no hum. When is floating needed or recommended? Thanks in advance, Roy

Last edited by yero; 12th August 2012 at 06:14 AM. Reason: typo
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Old 12th August 2012, 06:26 AM   #2
AJT is offline AJT  Philippines
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if you look at datasheets for tubes there is a heater cathode voltage rating for indirect heated cathodes...

cascodes, mu-followers, srpp tubes circuits or even dc coupling grids can have these ratings exceeded.....that is why filaments need to be floated....else to avoid hum dc biasing filaments are resorted to....
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Old 12th August 2012, 07:18 AM   #3
yero is offline yero  United States
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Thanks Tony, So if I use franks interstage capacitor coupled version which is one stage amplification and a A cathode follower stage. would I still need to float the heaters. I'me not sure how to float a filament supply. I can follow the circuit diagram, but not really understand totally. Was floating done on early designs? I thought I have seen the filament and B+ referenced from the same point. Which is circuit ground? Thanks all, Roy
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Old 12th August 2012, 09:34 AM   #4
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yes......when one side or even the center tap of the filament winding is connected to ground then the chances of exceeding the heater cathode voltage rating is a real possibility....

biasing the filament to 1/4 of B+ is recommended for Aikido preamps....and many others...
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Old 12th August 2012, 01:20 PM   #5
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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.
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Old 12th August 2012, 03:38 PM   #6
DF96 is offline DF96  England
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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.
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Old 12th August 2012, 05:12 PM   #7
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I have heard that, as a rule of thumb, you should never exceed 100V from heater to cathode.
When I originally built my 6SN7 mu-follower, I tried a single heater supply for both the "Upper" current-source tube and "Lower" signal tube referenced to a voltage divider on the B+. It was set at 1/3rd of a 220V Plate supply.
As my tubes got about a few years old they would start to intermittently pop and crackle. Nothing else seemed to be wrong. I could only figure that I was near the maximum heater cathode voltage for the "Lower" tube.
On my last redesign, I added an additional regulated heater supply referenced to ground for the "Lower" signal tube. and left the heater supply for the "Upper" CCS tube referenced to 1/3rd the B+ voltage. This allowed me to finally reduce the resistance in the CRC B+ supply and raise it over 300V DC, where the Octals are much more happy.
It has been several months now, and I have experienced no crackling or pops.
Octals Rule!
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Old 12th August 2012, 05:16 PM   #8
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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. the circuit shows The filament winding has two resistors in series across the 6.3 winding with ground between the two resistors. I have built a Pas preamp with out floating the filaments. why is floading recommended. If the filament is DC or used with tube rectfiction have any thing to to with floating or not floating the supply? I believe the reason for the floating filament has to do with 60HZ hum? The pas preamp I built used a 12v regulator for the filaments without floating the filaments with no hum. When is floating needed or recommended? Thanks in advance, Roy
"Floating" is a very ambiguous and context-sensitive word. I use it in the sense I think you mean it.

Resistor values are generally around 100 ohms, chosen to minimise "floating", and wasted current, whilst still providing an effective centre ground connection. The reason for a centre ground is so that the heaters mean potential remains static: one end of the heater rises as the other end falls. Sometimes a pot is used for the purpose, so it can be adjusted for minimum hum. It's then imaginatively called a "hum pot". A grounded centre-tap transformer is arguably better because no resistor is necessary, so there's no "floating" at all.

Doesn't apply to regulated DC, obviously.

Ground potential may not be the best choice, BTW. Apart from the maximum allowable value for heater-cathode voltage difference, there is arguably an ideal value in each case. I can't remember why, or what the value is...possibly such that the heater is always positive wrt the cathode, which would avoid thermionic emission from h to k...anybody?
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Old 12th August 2012, 06:00 PM   #9
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"Floating" is almost universally used in electronics to define a high impedance or undefined state

It gets ambiguous when others misuse common terms. Which is my intention to avoid in correcting the initial poster.
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Old 12th August 2012, 07:07 PM   #10
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Quote:
Originally Posted by Michael Koster View Post
"Floating" is almost universally used in electronics to define a high impedance or undefined state

It gets ambiguous when others misuse common terms. Which is my intention to avoid in correcting the initial poster.
As in "floating paraphase" perhaps?

Taking your more generally correct meaning, there are degrees of floating, depending on the meaning of "high impedance", from loosely tethered to cast adrift. 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.

I assumed that's the kind of direction the OP was coming from. Better to be positive about understanding than precious about ignorance, I think.

Others appear to have been misled by the usage, so in this company you do have a point. "Misuse" is a bit strong though. Intelligible, certainly in common usage, and in the specialist sense too, I thought, with not much effort.
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