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Old 27th January 2005, 10:54 PM   #1
Fuling is offline Fuling  Sweden
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Default Pentode spud...?


Ive become a bit fascinated by the idea of building a tiny spud (single tube) amp.
The idea of a single active element per channel is interesting and the absense of coupling caps/transformers is also attractive.

Ive seen quite a few schematics showing amps built with WE417, 6C45 and some triode strapped pentodes like E55L and PL802.
What about using a pentode in pentode mode?
I know that the output impedance would be very high unless NFB is applied (and we dont want to do that, right?), but in my particular case this might not be a bad thing.

Nelson Pass has written an article about his experiments with fullrange drivers in BL horns paired with high impedance amplifiers and Im quite inspired by his ideas and results.
My main speakers are BL horns with FE107E drivers and my own tests indicates that they like a bit of output impedance rather than close to 0 ohms.

Ive been looking at quite a few different tubes for this project but high gm pentodes that are linear enough to work without NFB are rare animals.
Today I stumbled across a E280F datasheet and this little critter seems just right for the task.
At 150V plate voltage and 5,6kohm load they will give just over half a watt, pretty much the same as the amp Im using now.
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Old 28th January 2005, 11:30 PM   #2
RE 604 is offline RE 604  Germany
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Hello ,
what about C3m , C3g , D3a , E282F , E810F and EL/PL802 ?
Regards , Alex .
No reason to get excited : Jimi Hendrix .
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Old 29th January 2005, 06:36 AM   #3
Fuling is offline Fuling  Sweden
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Join Date: Aug 2001
Location: Sweden (Mora)

Probably good tubes all of them, but few of them seems to be locally available at reasonable prices.
Ive already made a deal on three pairs of E280F.
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Old 29th January 2005, 07:26 AM   #4
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Location: Victoria, BC, NA, Sol III
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I built a 50EH5 amp (1-1.4W)

Click the image to open in full size.

I like it enuff i have a project in the works with much more substantial OPTs... a reader of my site, built this amp up, and having them lying around, substituted a pr of Lundahl 2A3 OPTs into the amp making it a "$405 buck" amp. he said it was quite impressive.

My buddy built this ELL80 amp (the 2nd tube is an EZ81 rectifier), which puts out about 3W (but needs some serious drive (~4V) -- his pre puts out way more than that.

Click the image to open in full size.

community sites,, ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
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