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Old 22nd April 2003, 01:55 AM   #21
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fddegrove,

So then this notching occurs mostly with indirectly heated tubes, and not directly heated triodes, pentodes, etc. I have heard about this, but not in those terms. I have heard it referred to as breakdown voltage, maximum heater to cathode voltage, heater to cathode shorting and the like. The nice thick coating of whatever material they use is supposed to prevent this. Hence another reason one would want the heaters to be at equal potential to the cathode, so there will be no attraction of electrons from the heaters to the cathode. There is more than one way to skin this cat. But... how on earth would it be a problem with light bulbs??????

EC8010,

Interesting. Yes, our 300bs are precious and expensive... but I don't know about this being a significant enough problem. This is just my ignorance, to be sure.

Thanks for all your replies!

Gabe
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Old 22nd April 2003, 10:24 AM   #22
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Default FILAMENTS.

Hi,

Quote:
So then this notching occurs mostly with indirectly heated tubes, and not directly heated triodes, pentodes, etc. I have heard about this, but not in those terms.
Not just DHTs or IDHTs or whatever but anything filamentary, hence the lightbulbs.
Nowadays manufacturers must have found ways to solve this since you rarely see this anymore bar for the cheapest lightbulbs.

Quote:
I have heard it referred to as breakdown voltage, maximum heater to cathode voltage, heater to cathode shorting and the like.
Heater to cathode insulation is another issue entirely and polarising the heater with respect to the cathode solves it nicely except for the occasional tube that has its heater placed too close to the cathode, granted this is a construction error that should not happen.

It seems to me you're assuming the heater element is a purely resistive element, it often is not symmetrical though exhibiting different resistance at different places along its entire length.
Therefore it is best to adjust it for balance with a pot or rheostat, this is why this technique is used to cancel out AC related hum.
And by doing so the cathode is more evenly heated too.

Cheers,
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Old 22nd April 2003, 11:11 AM   #23
SHiFTY is offline SHiFTY  New Zealand
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One question I have pondered:

I have read that power tubes with a cathode should be heated before B+ is applied, to avoid cathode stripping. You can see that the cathode "tube" around the heater does indeed take a few seconds to start glowing.

BUT- given that the cathode in a directly heated triode is the filament, which heats up virtually instantaneously, what is the need in a DHT amp for delayed B+ turn-on? The triodes would heat up and begin conducting as fast as the rectifier, and you would only be waiting for the preamp tubes.

So SET amps actually don't need a warm-up switch????

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Old 22nd April 2003, 11:30 AM   #24
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Default STRIPPER.

Hi,

When the B+ is coming from a tube rectifier you'll never need to worry about cathode stripping, be that for a DHT or a IDHT.

The B+ ramps up at roughly the same speed required for the heaters to reach operating temperature.

Cheers,

For info:Svetlana Techtalk
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