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Calling Daren - EF86 Content

3/29/2005 4:28 PM
MarkB Calling Daren - EF86 Content
Regarding this post:  
With any of these circuits, i found they don't like being loaded down with too weak an impedance - a Crybaby or Vox wah, Fuzz Face, or just your plain pickups will sometimes tend to oscillate, sounding like microphonics. I have absolutely no idea what was happening, less idea what was causing it, but empirically solved it by buffering the input a'la Matchless Clubman. Maybe if the Matchless guy reads this he can enlighten the rest of us? :-)  
The 10k plate/2k2 cathode unbypassed before the pentode gives you a nice low gain and nice lowish output impedance, and adds to the 'boutiquey' sustainy sound of any pentode I tried. Cool trick. I use the other half of the dual triode as a cathode follower as well, for after the pentode.  
I have a single channel AC15 build with an EF86 preamp. I get buzz that varies as I turn it up, which turns into a squeal past about 5-6. I've tried some bandaids - cap across the PI plates, grid resistor going into the PI - and they don't help. Is that what you were referring to above? I already have a tube socket in place from a previous build so I could add a triode stage without too much trouble.
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3/29/2005 7:28 PM
MarkB Make that "Darren: (sp)
3/29/2005 9:25 PM

Hey! :-) I haven't been here in ages, fluked me!  
Yes, thats just what I was meaning. You could also try and reduce microphonics by mounting the EF86 on rubber grommets or putting RTV instant gasket silicone goop blobs on the outside of the tube if you don't care about how it looks.  
Let us know how you go!
3/29/2005 10:33 PM
Thanks for responding - I took a chance. I really like the sound of this amp, so I want to get it right. The Rt666 tone stack gives a useful range of tones.
3/30/2005 3:06 AM

I dunno why none of the 'major' amp manufacturers haven't cottoned on to this whole 'preamp pentodes sound f(* cool' thing! :-)  
probably lack of supply of the tubes actually, and the microphonics issue.
3/30/2005 10:08 AM
I'm having trouble downloading the pdf file - voltage amp data - that was posted below with component values for pentode tubes. I downloaded it at the time, but its' dissappeared off my hard drive. Is that still available?
3/30/2005 4:18 PM

Yeah, I forgot to renew my domain name :-[  
ummmm try here :-  

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