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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Pentode spud...?

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Hi

I´ve 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.

I´ve 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 don´t 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 I´m 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.

I´ve 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 I´m using now.
 
frugal-phile™
Joined 2001
Paid Member
I built a 50EH5 amp (1-1.4W)

http://www.t-linespeakers.org/tubes/SEP_50EH5.html

An externally hosted image should be here but it was not working when we last tested it.


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.

An externally hosted image should be here but it was not working when we last tested it.


dave
 
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