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Old 29th March 2004, 11:23 PM   #11
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Hi,

Quote:
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?
You'll probably never notice secondary emission effects in an audible way, I think.

However, if we were to cross the limits stated by the manufacturer, or you have a somewhat dodgy tube where the heater is at some point too close to the cathode sleeve, we risk a sticky heater and that can give us a very nasty bang.

Inbetween, before it actually sticks, you may experience HF oscillations capable of killing your amps, tweeters and drive all pets from the room.
Whether your hearing is good or not so good, this one won't pass unnoticed, believe me.

Quote:
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.
Below ground potential meaning you run it from a bi-polar supply?
The same would apply as if it was above ground, i.e. positive, so the same precautions should be taken.
As an example and regardless of max. heater to cathode voltage diffs. if the cathode has a potential diff. of 100V with respect to the heater I'd bias the heater upwards so the final diff. equal 0 volts.
This is the theoretically ideal situation according to the Philips engineers.

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Old 29th March 2004, 11:33 PM   #12
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Hi,

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The cathode is at approximately +9volts. The heater is referenced to ground. The anode is at about 100V.
C = +9V
F = +12.6V (as an example)
A = Anode, don't play.

All voltages measured with respect to true 0 volt; ground.

Heater is grounded at one end, the negative leg.
So the heater is 3.6V high with respect to C. No problem whatsoever.

Don't go paranoid over this secondary emission, a few volts aren't going to start an electron war inside the tube.

I was just crossing the Ts and dotting the I's, this is much more important with CRTs and labtests than I'd imagine it to be for audio purposes anyway.

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Old 30th March 2004, 05:23 AM   #13
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Quote:
Below ground potential meaning you run it from a bi-polar supply?
That's correct, at +/- 175VDC, and the input stage of the 6K11 will have its cathodes at ground potential. I have some concern that secondary emission may degrade long term dc offset stability in some cases with the filament at, say, -85VDC if this topology is used.
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Old 30th March 2004, 03:41 PM   #14
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Hi,

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I have some concern that secondary emission may degrade long term dc offset stability in some cases with the filament at, say, -85VDC if this topology is used.
You're quite likely pretty safe.
Emission starts to occur at 30V potential difference, do you happen to have a diagram of the cct for us to look at?

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Old 31st March 2004, 05:33 PM   #15
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Hi, Frank -

This is probably more or less how it would look (I am using the driver as an inverted gain stage instead of a follower here) except that I'll be running the supply voltages somewhat lower, say +/- 170VDC instead of 200VDC. However, that still comes rather close to the maximum cathode heater ratings for the 6K11 with the heater potential centered between ground and the negative -170 V supply. Do you think secondary emission might adversely affect bias points over time?

TIA. OTLs forever!
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Old 31st March 2004, 09:47 PM   #16
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Hi,

Since I can only guess the voltages on the cathodes, I suggest you build it as you want, measure all cathode to heater voltages on all sections of the 6K11 to start with.

The problem you'll be facing is that the potentials are likely to vary rather wildly from one section to the next so we'll need to find some compromise.

Sorry, I can't be of more help to you right now.

Maybe someone with more insight into semi-conductor than i do can help you out on this one.

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Old 31st March 2004, 09:53 PM   #17
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Thoriated: Looking at your diagram, you seemed to be in semiconductor mode when assigning values to your global feedback loop. You can cheerfully scale your resistors by a factor of at least ten, allowing you to use a much smaller capacitor than that 10uF you currently have.

You definitely need separate heater supplies for the input pair and the voltage amplifier. Expect DC problems if you don't.
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Old 31st March 2004, 09:56 PM   #18
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Hi,

Quote:
You definitely need separate heater supplies for the input pair and the voltage amplifier.
Thing is, it's a single 3 section compactron...

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Old 31st March 2004, 09:59 PM   #19
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Oh dear. That puts a rather large spanner in the works. I'll bet it's cheap, too.
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Old 31st March 2004, 10:05 PM   #20
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Hi,

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Oh dear. That puts a rather large spanner in the works.
My thoughts exactly...Unless, perhaps Thoriated wants to change to a 12AX7A + a not sure here, EC90??
1/2 a ECC82 like the EC92 is half a ECC81.

Checking Duncan Amps.......yes, a 6C4.

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