I used to subscribe to the "Triodes ONLY" school of thought for output stages, but I've recently come around to Pentodes / Tetrodes for output devices, as they have a lot of advantages, and with clever local feedback, can achieve the same or lower output impedance than triodes, with similar distortion. They can be driven in weird and wacky ways like screen drive or enhanced triode mode (not sure if anyone actually uses this though, sounds a bit tough / fatal to the screen grid)
They are much more efficient, and can make a lot more power than most pure triodes, and are easy to find
Pentodes rock! I think they get too much bad press
I'm even coming around to global negative feedback, maybe I'm going insane
They are much more efficient, and can make a lot more power than most pure triodes, and are easy to find
Pentodes rock! I think they get too much bad press
I'm even coming around to global negative feedback, maybe I'm going insane
The secret with pentodes is the feedback, you do that right and they can sound as good as triodes.
Now i have a pp 6v6 triode amp with global negative feedback and sounds better than any se triode amp or pentode pp amp i've ever heard and i have heard many. But i don't need the power of pentodes.
Right now i am working with a pp pentode amp, the mission is to equal or better my pp triode amp. Just because i want to hear if that is possible.
People mod their Marantz and Dynaco pentode amplifiers to triode and they don't go back.
Two years ago i felt global feedback killed the soul of the recordings....how wrong i was. But feedback loops are tricky and i'm not an engineer.
Respect to people who know their trade, most dyers don't, but is still fun as a hobby.
Now i have a pp 6v6 triode amp with global negative feedback and sounds better than any se triode amp or pentode pp amp i've ever heard and i have heard many. But i don't need the power of pentodes.
Right now i am working with a pp pentode amp, the mission is to equal or better my pp triode amp. Just because i want to hear if that is possible.
People mod their Marantz and Dynaco pentode amplifiers to triode and they don't go back.
Two years ago i felt global feedback killed the soul of the recordings....how wrong i was. But feedback loops are tricky and i'm not an engineer.
Respect to people who know their trade, most dyers don't, but is still fun as a hobby.
If you go with pentodes, you can go farther and use them in a driver.
This driver was developed to drive GU-50 pentodes, with local parallel feedback by voltage over each output tube (240K resistors from their plates to plates of drivers). After I finish my current hybrid project I am going to redesign this one a bit, and make PCBs available for DIY community, if such demand exists. Not necessary to use it with GU-50, there may be KT-88, or other high power tubes that need a voltage swing.
This driver was developed to drive GU-50 pentodes, with local parallel feedback by voltage over each output tube (240K resistors from their plates to plates of drivers). After I finish my current hybrid project I am going to redesign this one a bit, and make PCBs available for DIY community, if such demand exists. Not necessary to use it with GU-50, there may be KT-88, or other high power tubes that need a voltage swing.
bigwill said:I used to subscribe to the "Triodes ONLY" school of thought for output stages, but I've recently come around to Pentodes / Tetrodes for output devices, as they have a lot of advantages, and with clever local feedback,
I'm even coming around to global negative feedback, maybe I'm going insane
No you aren't going insane. I've been using true tetrode/pentode output stages with GNFb for over 40 yrs. Once youv'e mastered the quirks then you can really stab power triodes or triode config pentodes in the back. However there is competition, don't forget the lower working impedance of triodes; it makes the output tranny much easier to design and that's a big bonus.
Sy mentions screen grid drive; I've always found tubes don't last long with this when compared to conventional driven p-p: there are circuits around but reliability ?
Perhaps someone can correct me on a tried and tested design.
richy
bigwill said:
Yesterday I used my amp prototype on a folk ensemble concert. When one dombrist saw GU-50 tubes, he said, "Wow! No wonder it sounded so nice!"
He said, he used an equipment containing them, and he had to return them when replaced because they contain 20 gram of silver each. I did not know that, but I knew they have nickel alloy plates that withstand very high temperatures, and their external shape is rough for bigger square of heat radiation. After getting orange red they continue working still fine. Also, they were made with precisely aligned grids. Initially they were developed by Telefunken, then adopted by winners after the WW-II for own military purposes.
I am getting conservative 80W from a pair (10K P-P, 800V b+), while usually they give 120W per pair. Speaking of sound, they don't sound at all. With such a driver as shown, what really sounds, is the 1'st pentode, and on half power almost 2'nd harmonic below 80 dB is visible.
In order to obtain such results I built all 3 versions of tube amps on rock solid SS foundation (filament, bias, screen grid, preamp voltage regulators). That's why they are called Pyramid.
Michael Koster said:
Theoretically? Yes, or simply to shunt R6 by a cap: more DC stable and less frequency dependent on highs, but practically it already has a little bit too high amplification factor to be used as an output amp with 0 dB input sensitivity. However, if you want it to be driven by a vinyl corrector directly, you may go for it.
It is the main reason why I want to redesign it, for a different input tube. Probably it will be ECF200.
SY said:
Funny, that's the working title for a pentode + beam tetrode project
I'm working toward. The name comes from not being able to see the
filaments on the output tubes, but it applies...
This is inspired by the O.H. Schade idea of plate-grid feedback on the
output tetrodes to emulate a triode transfer characteristic.
Michael
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Yvesm said:
Blowing like Dark Wador ! !
What kind of Mosfets (they look like 5W class depletion mode) do you use ?
Yves
2CFM per tube at idle, 3CFM per tube at full power (150W diss per tube)
back pressure is 0.2" at idle, 0.3" full flow. I'll use 2 small blowers in
series with a control... So yeah, Darth Vader, LOL, would be another name
but not nearly so loud with the blowers deep inside the chassis. One
blower can be turned off at low power levels and inlet air <25C
I generally use DN2540s, but also IXYS 10M45 when I need higher
-Vgs and 10M90 if I need high voltage.
Cheers,
Michael
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