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Something I don't Understand...

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Posted on November 11, 2014 at 15:13:18
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 ...OK, there's a lot of that. But here's one I've struggled with and can't figure out by thinking about it.

There's always at least a little heater-to-cathode leakage because the insulating material conducts "some" at the high temperatures at which the heater operates.

To combat hum (when operating heaters on AC) it's a common practice to raise the heater voltage above that of the cathode.

That's what I don't understand. The cathode is good at emitting electrons, so I would think that operating the heater positive with respect to the cathode would make the problem worse. It seems to me that operating the heater negative with respect to the cathode would be a better idea, as the heater is not a particularly good electron emitter (tungsten at red heat is a poor electron emitter).

Clearly my reasoning is flawed, or heater negative is the way it would be done. So where have I gone wrong in my reasoning? What's the real story?

 

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RE: Something I don't Understand..., posted on November 11, 2014 at 15:19:45
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Even though the heater is not coated with emitting material, some thermionic emission occurs and its level oscillates at the power mains frequency. By making the heater positive with respect to the cathode, the "oddball" thermionic electrons don't create a current between heater and cathode.


Eli D.

 

Read this, posted on November 11, 2014 at 16:29:03
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 Awesome site.

 

RE: Something I don't Understand..., posted on November 11, 2014 at 18:27:41
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 Chip's link covers it well.

I like to think of it this way,

Do you want a little music on your heater or a little heater (hum) on your cathode (music)?

With the heater positive WRT the cathode the cathode will conduct to the heater.

With the cathode positive WRT the heater the heater will conduct to the cathode.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

"This effect works in both 'directions'", posted on November 11, 2014 at 21:20:39
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 That's what the article at Valve Wizard says. Anyone here tried a negative heater?

 

RE: Something I don't Understand..., posted on November 11, 2014 at 21:54:56
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 The inside of the cathode sleeve is NOT coated, and is a poor emitter. But either electrode will still emit enough electrons to the other positive electrode.

Because the emission is poor, this thermionic diode is easily saturated. At typically 30v or more difference either way, the leakage current is saturated and does not vary with the heater instantaneous voltage - the voltage fluctuations can't cause a hum because the current does not fluctuate.

Of course there is still capacitive coupling between the electrodes ...

 

RE: "This effect works in both 'directions'", posted on November 11, 2014 at 21:59:56
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 You will see both a positive and a negative heater in 6922 cascodes. The lower triode's cathode is negative with respect to the heater and the upper triode's cathode is positive with respect to the heater. More than anything else, protecting against the heater to cathode potential limits being exceeded is what's going on.

BTW, check out the asymmetry in 6922 heater to cathode potential limits. Philips intended, from the outset, that the type be used in cascodes.


Eli D.

 

RE: Something I don't Understand..., posted on November 12, 2014 at 00:24:06
Alex Kitic
Audiophile

Posts: 182
Location: Serbia
Joined: June 25, 2014
 I tend to agree that there is not need whatsoever to elevate the potential of heater filaments in respect to the indirectly heated cathode - other than guarding the maximum allowed cathode-heater potential difference.

A good measure against hum is grounding the heaters effectively.

One must keep in mind that in most of the cases it is not the heaters that cause HUM issues, rather the wiring that is used to connect the heaters to the secondary of the transformer. If the wires are not twisted, and not carefully "routed" inside the amp, they can cause HUM.

When you find a good way to interest people to address the AC heaters HUM related issue (like the nice but not actually true story about the heater-cathode complex acting like a diode) -- they will pay more attention to the grounding of the heaters, and the wiring, thus achieving better results in avoiding HUM.
******

http://rh-amps.blogspot.com/

 

Riddle me this... , posted on November 12, 2014 at 08:04:29
SteveBrown
Audiophile

Posts: 2454
Location: Portland, OR
Joined: November 14, 2002
 I've found it interesting that not all amps of the Golden Age used twisted heaters, and yet didn't have a hum problem for example, my Leak Stereo 20 does not, and the heater wires are bundled together with other wires. But I've also heard that running the heater wires side by side is also efficent in canceling hum, is this what's at work in amps like the Leak, or is it related to the PP output?

 

RE: Something I don't Understand..., posted on November 12, 2014 at 12:44:06
Stuben
Audiophile

Posts: 669
Location: Guber Ohio
Joined: December 30, 2005
 I have always belong to this school of Filament hum...

120 HZ hum is induced by the heating and cooling of the filament heated with a 60 HZ supply...Twisting your feeds will help prevent induced currents in your chassis and components local to the AC feeds.

I'm sure others will chime in on this one...

Stuben

 

RE: Something I don't Understand..., posted on November 12, 2014 at 12:52:59
Stuben
Audiophile

Posts: 669
Location: Guber Ohio
Joined: December 30, 2005
http://www.dmitrynizh.com/hum-moondog.htm

Another site....

Stuben

 

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