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Old 30th December 2007, 07:33 PM   #11
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No problem, I though that's what you meant.
Enlighten me a little more on the diode, is it to limit current or voltage on warm up? Sy recommended I add one on the cathode side of a 6SN7 tube on my preamp I built.
Glenn
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Old 30th December 2007, 07:38 PM   #12
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In my implementations I have the 100 ohm resistor in the grid lead . It is in the circuit all of the time, and functions as a "stopper" resistor. It can help prevent oscillation.

The diode in the screen grid lead has been discussed before, and many people are of the opinion that it could cause harm with some tubes, or it can cause distortion. Others claim that it prevents damage and cures distortion.

My testing showed that it didn't make much difference in normal HiFi applications, but did cause some weird sounding effects in a guitar amp operating well into clipping. It could cause some damage in an RF tetrode operating at high power since the screen grid may actually source current (caused by secondary emission) but is likely harmless in an audio application.
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Old 30th December 2007, 07:45 PM   #13
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That makes sense, I've seen this in guitar amps before.
I guess the 100R is low enough a value that it doesn't change the voltage much. Probably worth putting it in the circuit all the time if it helps with oscillations. 5W wirewound okay? I just happen to have those around.

I think I'll leave the diode out for now.

Thank you for the explanation.
Glenn
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Old 30th December 2007, 07:55 PM   #14
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A wirewound resistor may cause the oscillation that you are trying to cure since it is by definition also an inductor.

Many people tell you (and the math confirms) that this resistor will dissipate low power so you can use a 1/2 watt resistor. I can tell you from experience that the 1/2 watt metal film resistors sold by Mouser or DigiKey will fry if the amp is overdriven. I have toasted 3. I now use 1 or 2 watt carbon film or carbon composition resistors and have not killed any even when using my SimpleSE as a guitar amp.
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Old 30th December 2007, 08:19 PM   #15
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Thanks, I was wondering about that after I posted.
I remember people trying to use wire wound resistors to load guitar amps for testing instead of using a speaker (like THD's "Hotplate"). "non-inductive" resistors were always recommended for this application.
Well, time to order some resistors
Thanks again.
Glenn
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Old 30th December 2007, 08:58 PM   #16
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This was interesting reading on using the diode:
http://www.webace.com.au/~electron/tubes/oes.html

The diode is in series with the grid stopper, cathode towards the grid, but is the order important? I assume the diode goes between the grid stopper resistor and the plate because the object is to keep the grid stopper soldered close to the pin of the tube, right?

[EDIT]
I just realized that you probably also leave the grid stopper resistor in the circuit because it's soldered right on the pin of the grid.
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Old 30th December 2007, 11:25 PM   #17
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Ouch ...
I may not be popular for posting this, but in fairness to the less experienced, a note of warning must respectfully be sounded with regard to the "Optimised Electron Stream" article given in the internet link above. While a lot of very valid basics are admirably covered, the author is in error with some of his reasoning a.o. the physical electrode distances, function of G3 and the effect of the G2 diode for dc and ac (basic electronics, not an opinion).

It is probably unfair to simply say this without a more complete discussion, but the latter will take a whole lot of space, and has been covered before as said ... but I apologise (can't seem to find that thread now).

As an aside, a wire-wound resistor as a loudspeaker replacement load is nearer a loudspeaker equivalent impedance than a totally non-inductive load, in as much as any inductance does play a role.

I also agree with Tubelab regarding the wattage of the G2 stoppers. But then again, I had tubes saved by the burning out of G2 stoppers when the anode circuit failed, so it worked both ways. But Tubelab's argument probably occurs more frequently, so.
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Old 31st December 2007, 02:41 AM   #18
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Johan,

I think you could be referring to this thread.
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Old 31st December 2007, 11:23 AM   #19
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Thank you Johan for that information, and Ray_moth for that link.
I've only been in this hobby for four years, so I'm still learning. That's one of the reasons I read this forum so much.
Sounds to me like I should leave the diode out.
Plus it's nice not having any silicon in my amp
Glenn
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Old 31st December 2007, 02:22 PM   #20
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Quote:
But Tubelab's argument probably occurs more frequently, so.
Not everyone is crazy enough to plug a guitar preamp into their HiFi amp and overdrive it by 10 db either. I do this to all of my designs to find their weak points. In this case I zapped a few screen resistors, so I started using bigger ones.

Quote:
As an aside, a wire-wound resistor as a loudspeaker replacement load is nearer a loudspeaker equivalent impedance than a totally non-inductive load, in as much as any inductance does play a role.
This is true, and for simple amp testing and burn in I use a pair of 8 ohm 500 watt wire wound resistors that I got from Ebay. A forum member much wiser than me pointed out that these should not be used to load an amp for critical measurements like frequency response and power output. I built a non inductive load box and it did give different results.

I do not put much faith in the "Optimised Electron Stream" theory either, but that was covered in the thread mentioned.
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