Clarification of floating heater supply

duderduderini · 25th October 2007, 12:44 PM

Hi Everyone
When you have a scenario where the heater cathode voltage exceeds the limit in one half of a twin triode yet the other half doesnt as in the case of the aikido and srpp I note the advice is to elevate the heater supply which i understand.
What confuses me is that no reference is made to the cathode heater half which isnt over the limit.
It seems there is no advice on say using 2 triodes per channel, one dual triode sharing the "Upper valve" duties (and having the elevated heater supply) and the other sharing the lower half duty.
Can I assume this is the convention or can you just float at say 50V or some arbitrary figure so that both are elevated but within limits (if one triode is being used as upper and lower halves).
What is the correct method?
Thanks

Tom Bavis · 25th October 2007, 12:48 PM

You've got it - set the heater to a voltage that's within H-K limits for both sections. Maybe better yet is to split the two triodes across channels - upper halves in one bottle, lower in another. But you'll need separate heater windings to bias them optimally.

Akita · 13th August 2008, 02:24 AM

I still can't get the idea. Can anyone here explain more detail on how to float the heater?

Poindexter · 13th August 2008, 04:18 AM

Here's a schem:

The reference voltage is taken from B+ with a resistive divider.  Total resistance of the divider should be about as many kilohms as the volts of your supply; you want to draw about 1mA with the divider, to get a stable voltage.  If you have a center tap on the heater transformer/winding, you attach it to the junction of the divider.  If not, a 'fork' of resistors, 330Ω or so, goes to the two sides of the winding.

Questions?

Poinz

Stixx · 13th August 2008, 05:35 PM

Floating AC heaters I have done...
but how do you float a regulated DC heater supply...?
Also by applying the fork of two equal resistors, or by connecting the divider to the return side of the supply?

Thanks for any input!

Tom Bavis · 13th August 2008, 07:00 PM

I've always preferred the term "biasing" to "floating"... the latter seems to imply that it isn't connected to anything. With DC heaters, no need for any resistors - connect the heater bias to either side of the DC supply.

gingertube · 14th August 2008, 12:08 AM

I likewise have often "biased" the AC heater supply to a positive DC voltage.

In some schematics, I have also seen the heater left truely "floating" with respect to DC and just a capacitor used from one side to tie the heater to 0V AC wise (at higher frequencies anyway).
When might you use this rather than bias the heater to a fixed DC voltage? Is there any benefit or is the opposite true (not as good)?

Cheers,
Ian

aardvarkash10 · 14th August 2008, 12:52 AM

I could never get that whole part of the circuit clear in my head... I only just realised what the resistor fork is doing!

Akita · 14th August 2008, 01:31 AM

Hi Poinz,

Thank for your explaination. So in your schematic, the heater have been floated to 47v.. right?

Poindexter · 14th August 2008, 03:27 AM

It is not floated.  Floating means that it is not referenced to the main circuit (the heaters have nothing directly to do with the audio amp; they just keep the cathodes hot) at all.  You are correct, it is referenced to a DC voltage 47Vdc above audio circuit ground - roughly halfway between the two cathodes.

You can't float your heater supply.  It'll reference itself all over the place, and you will have weird fizzy humming issues.

How do I know this, hm?

Aloha

Poinz

25th October 2007, 12:44 PM	#1
duderduderini diyAudio Member Join Date: Mar 2004 Location: Albury NSW Australia	Clarification of floating heater supply Hi Everyone When you have a scenario where the heater cathode voltage exceeds the limit in one half of a twin triode yet the other half doesnt as in the case of the aikido and srpp I note the advice is to elevate the heater supply which i understand. What confuses me is that no reference is made to the cathode heater half which isnt over the limit. It seems there is no advice on say using 2 triodes per channel, one dual triode sharing the "Upper valve" duties (and having the elevated heater supply) and the other sharing the lower half duty. Can I assume this is the convention or can you just float at say 50V or some arbitrary figure so that both are elevated but within limits (if one triode is being used as upper and lower halves). What is the correct method? Thanks __________________ "Better to say nothing and keep them guessing than to speak and remove all doubt."

14th August 2008, 12:52 AM	#8
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	thanks poinz! I could never get that whole part of the circuit clear in my head... I only just realised what the resistor fork is doing! __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012

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25th October 2007, 12:48 PM	#2
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	You've got it - set the heater to a voltage that's within H-K limits for both sections. Maybe better yet is to split the two triodes across channels - upper halves in one bottle, lower in another. But you'll need separate heater windings to bias them optimally.

13th August 2008, 02:24 AM	#3
Akita diyAudio Member Join Date: Dec 2006 Location: Kuala Lumpur	I still can't get the idea. Can anyone here explain more detail on how to float the heater?

13th August 2008, 04:18 AM	#4
Poindexter diyAudio Member Join Date: Oct 2004 Location: Maui, Hawai'i, USA	Here's a schem: The reference voltage is taken from B+ with a resistive divider.  Total resistance of the divider should be about as many kilohms as the volts of your supply; you want to draw about 1mA with the divider, to get a stable voltage.  If you have a center tap on the heater transformer/winding, you attach it to the junction of the divider.  If not, a 'fork' of resistors, 330Ω or so, goes to the two sides of the winding. Questions? Poinz

13th August 2008, 05:35 PM	#5
Stixx diyAudio Member Join Date: Sep 2003 Location: Southern Germany	Floating AC heaters I have done... but how do you float a regulated DC heater supply...? Also by applying the fork of two equal resistors, or by connecting the divider to the return side of the supply? Thanks for any input!

13th August 2008, 07:00 PM	#6
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	I've always preferred the term "biasing" to "floating"... the latter seems to imply that it isn't connected to anything. With DC heaters, no need for any resistors - connect the heater bias to either side of the DC supply.

14th August 2008, 12:08 AM	#7
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	I likewise have often "biased" the AC heater supply to a positive DC voltage. In some schematics, I have also seen the heater left truely "floating" with respect to DC and just a capacitor used from one side to tie the heater to 0V AC wise (at higher frequencies anyway). When might you use this rather than bias the heater to a fixed DC voltage? Is there any benefit or is the opposite true (not as good)? Cheers, Ian

14th August 2008, 01:31 AM	#9
Akita diyAudio Member Join Date: Dec 2006 Location: Kuala Lumpur	Hi Poinz, Thank for your explaination. So in your schematic, the heater have been floated to 47v.. right?

14th August 2008, 03:27 AM	#10
Poindexter diyAudio Member Join Date: Oct 2004 Location: Maui, Hawai'i, USA	It is not floated.  Floating means that it is not referenced to the main circuit (the heaters have nothing directly to do with the audio amp; they just keep the cathodes hot) at all.  You are correct, it is referenced to a DC voltage 47Vdc above audio circuit ground - roughly halfway between the two cathodes. You can't float your heater supply.  It'll reference itself all over the place, and you will have weird fizzy humming issues. How do I know this, hm? Aloha Poinz

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