Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Other Topics => Topic started by: SILVERGUN on May 28, 2013, 10:13:51 am
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Just in case you guys missed this episode....
How It's Made - vacuum tubes (http://www.youtube.com/watch?v=8n4WVRKkmww#ws)
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I wonder if they they make pentode tubes
According to KR's website (http://www.kraudiousa.com/tubes.php) they appear to only make triodes.
And also how well the those triode tubes work in class B
As well as any tube, but given their products are low-mu triodes the drive requirements may be severe.
Besides, the people who are willing to pay the money (http://tubedepot.com/kr.html) for those generally want class A triodes.
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Awhile back there was discussion on using triode as PI's in a class B amp. PRR, set me straight that most triodes do not have the power ratings for the PI in a class B amp. I'll have to do a little pencil pushing, to determine if KR tubes would work.
There's a difference between using a triode as a phase inverter to drive a class B (usually class B2, implying grid power is needed) output stage, and using triodes as a class B output stage itself.
The problem is this: Class B requires large plate voltage swings (in general, or very large current peaks). Additionally, class B would theoretically idle just at cutoff; in reality they are biased so cold to have a minimal idle current of a few milliamps.
Because that implies a high negative bias voltage, and with a mu of 2-8 (with mu generally falling at lower plate current), the grid drive voltage needs to be nearly as big as the output triodes' plate voltage swing. Add in that if you're gonna go for class B you might attempt to drive the grids positive for a bit more output, and you have a recipe for significant drive power needed.
Class A2 and class B2 output stages usually are driven by small power tubes with a transformer to derive the push-pull signal. For instance, a KT88 class AB2 output stage might use a KT66 as a driver with a single-ended primary on the KT66 plate side and a push-pull secondary at the KT88 grid side. Hence PRR telling you it makes no sense to use a triode as a phase inverter/driver for a class B output stage.