Pentodes.

[bigwill]

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

[richwalters]

case for purchasing a Morgan Jones book. Valve amps 3rd edition. No wonder sonny boy likes his Glenfiddich.

richy

[Yvesm]

Welcome in "real" word

Yves.

[SY]

Originally posted by richwalters
case for purchasing a Morgan Jones book. Valve amps 3rd edition. No wonder sonny boy likes his Glenfiddich.

richy

Only if you want a clear explanation of what the problems of feedback are and ways to avoid them.

Screen drive with the right tubes is a great option. And sweep tubes are cheeeeeeeeap.

Welcome to the Dark Side.

[bigwill]

Originally posted by richwalters
case for purchasing a Morgan Jones book. Valve amps 3rd edition. No wonder sonny boy likes his Glenfiddich.

richy

Have it already

[Cassiel]

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.

[Wavebourn]

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.

[richwalters]

Originally posted by bigwill
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

[Wavebourn]

Originally posted by richwalters



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.

Please see my previous post.

[Yvesm]

An "All Pentodes" 300B Killer



Even at clip limit H2 is still predominant:




Yves.