Tube-based CCS- 0.2uF cap OK?

awedio · 10th September 2007, 09:47 PM

Everywhere I've seen a tube- (triode-) based constant current source schematic, the value of the grid bypass cap is ALWAYS 0.1 uF. I have never seen this value explained or calculated...

So, can someone explain what happens if the value is doubled? (Yes I have a bunch of 0.22uF caps!!!) I have a feeling that I could try it out to see, but probably not hear or measure a difference. So does anyone know how to explain how this value is determined?

Thanks.

Geek · 11th September 2007, 07:48 AM

Hi,

Go ahead and use the 0.22uF ... it's an arbitrary value just to keep the thing from oscillating.

Too low C, audible to supersonic oscillation and/or noisy. Too high C, risk of motorboating (depending on tube).

But you should know that triodes make bad CCS', because their Rp is so low compared to a pentode. But if you have a lonely triode section looking for something to do, it's better than a plain R, I guess

Cheers!

10th September 2007, 09:47 PM	#1
awedio diyAudio Member Join Date: Jul 2005 Location: PA	Tube-based CCS- 0.2uF cap OK? Everywhere I've seen a tube- (triode-) based constant current source schematic, the value of the grid bypass cap is ALWAYS 0.1 uF. I have never seen this value explained or calculated... So, can someone explain what happens if the value is doubled? (Yes I have a bunch of 0.22uF caps!!!) I have a feeling that I could try it out to see, but probably not hear or measure a difference. So does anyone know how to explain how this value is determined? Thanks. __________________ "Where is my mind? Where is my mind? Wheeere is myyy mind? Waaay out, in the water, see it swimmin'..." Black Francis

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11th September 2007, 07:48 AM	#2
Geek diyAudio Member Join Date: Sep 2004	Hi, Go ahead and use the 0.22uF ... it's an arbitrary value just to keep the thing from oscillating. Too low C, audible to supersonic oscillation and/or noisy. Too high C, risk of motorboating (depending on tube). But you should know that triodes make bad CCS', because their Rp is so low compared to a pentode. But if you have a lonely triode section looking for something to do, it's better than a plain R, I guess Cheers!

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