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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: All pentode el84se no GFB posted by Paul Scearce on March 23, 2011 at 06:25:59:
Siemens 2000 projector amps runs plate to grid negative feedback in their el84 SE amp. Cathode feedback around the output is much better then global and solves the damping factor and trans issues. I wish more trans where available with windings. You should search the 6bm8 club they have done just about every imaginable thing with SE pentodes 10 years ago. While the west where still laughing at little pentodes.They used tubes and transistors to apply all sorts of feedback and direct coupling. I built a el84stc back then and its performance was stunning but with the feedback around the output tube being so severe and the complete lack of trans feedback(tiny trans) it was a little underpowered in the bass at the time I didn't have very efficient speakers. I should build another now that I am building horn speakers. I still use the stop diodes in the power supply and screen grids though.
If I recall correctly it had killer bandwidth only limited by the trans and perfect bass control again only limited by the trans and the most triode sounding amp I ever built unfortunately the most boring too. You can use 6al5 for stop diodes in the screens. Didn't know until yesterday according to the below site that stop diodes affect the op points significantly and the tube behaves more like a triode. I always just thought it gave better regulation. The power supply diode trick increases the damping factor subjectively incredibly and increases the separation in mono supplied stereo amps.The diode goes on the b+ wire of the output. You cant hear the diode I think because it isn't working hard.
My 4 year old daughter and I recently built a little 6aq5 SE amp with a lot of those tricks including the cathode feedback but only cap coupled. It was a sweet little dynamic amp. We named it Astro girl well she named astro girl despite the fact we only had astro boy decals for it. Shes a feminist you know. But a good first amp for her ;) Taking into account global feedback takes real skill to get right plate to grid and cathode feedback is easier to get right. I am no engineer neither is my daughter... Yet!
Edits: 03/23/11
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- RE: All pentode el84se no GFB - tubebuilder 09:10:49 03/23/11 (0)
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