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Old 26th March 2004, 08:19 PM   #1
Bryan is offline Bryan  United States
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Default Heater To Cathode Potential

Hey Guys,

Some general questions.....

With respect to Heater to Cathode Potential, what are the general rules of thumb.

Specifically, when does one need to raise the reference voltage of the heater above ground?

DHT vs IDHT?
AC vs DC?

What are the physical and sonic differences?

Thanks,

Bryan
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Old 26th March 2004, 09:42 PM   #2
SHiFTY is offline SHiFTY  New Zealand
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A DHT heater is the cathode, so they are at the same potential. In an autobias ( cathode bias) DHT amp, the cathode resistor lifts the heater /cathode above ground. DC is recommended for a heater supply for hum reduction. Do not ground the heater supply!

For IDHT like 6SL7, 6SN7, there are heater to cathode maximums that must be observed when using circuits like SRPP, cathode follower. The numbers can be found in the data sheets for each. This can be achieved by using a reference voltage from the B+ and attaching this to the floating heater winding.
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Old 26th March 2004, 10:03 PM   #3
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> Do not ground the heater supply!

I did not understand this. With a floating cathode, how could you measure plate potential ? I will be appreciated if you explain it.

MB
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Old 26th March 2004, 10:26 PM   #4
EC8010 is offline EC8010  United Kingdom
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As ShiFty implies, if you were to ground the heater of a directly heated valve using cathode bias, you would be short-circuiting the cathode bias resistor, which would cause unrestricted current flow, upset the operating point, and possibly destroy the valve. Not a good idea.
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Old 26th March 2004, 10:33 PM   #5
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OK. Sorry, I just misunderstood. Thanks...

MB
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Old 29th March 2004, 11:27 PM   #6
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Hi,

Quote:
For IDHT like 6SL7, 6SN7, there are heater to cathode maximums that must be observed when using circuits like SRPP, cathode follower. The numbers can be found in the data sheets for each. This can be achieved by using a reference voltage from the B+ and attaching this to the floating heater winding.
With totem pole type circuits it's easier to use twin triodes where top and bottom share the same enveloppe.
It makes it easier to bias the heater close to cathode potential that way.

Ideally heater and cathode should be at the same potential to avoid secondary emission from either one of them.

Cheers,
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Old 29th March 2004, 11:37 PM   #7
Bryan is offline Bryan  United States
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Frank,

This being said, would I get any benefit from raising the heater potential in my 12B4A linestage. Currently it is sitting at a humble "0" volts.

Thanks,

Bryan
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Old 29th March 2004, 11:55 PM   #8
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Hi,

Quote:
With totem pole type circuits it's easier to use twin triodes where top and bottom share the same enveloppe.
Thinking right but writing wrong....
I mean the top triodes being the same tube and the bottom a different one.
I realise the quotation can be confusing.

Bryan,

Quote:
Currently it is sitting at a humble "0" volts.
How's that? Is the cathode grounded?
If it is it can't emit anyway so you're safe.

Cheers,
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Old 30th March 2004, 12:03 AM   #9
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Hi, Frank -

Do you know how bad secondary emission can get in what would be considered a 'good' miniature tube with a heater/cathode potential up to 100VDC over its lifespan?

This may impact how I configure the driver stage in my hybrid amp. If as a voltage/current gain stage, its cathode will be sitting nearly two hundred volts below ground, so I'll probably split the cathode potential halfway between that and ground, but I'm afraid that secondary emission might then deteriorate my dc offset stability.

Thanks in advance for any input.

-thoriated
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Old 30th March 2004, 12:16 AM   #10
Bryan is offline Bryan  United States
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Frank,

Let me clarify.

The cathode is at approximately +9volts. The heater is referenced to ground. The anode is at about 100V.

Sorry about the confusion....
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