post on amp design by a newbie - Page 3

SY · 18th June 2008, 10:59 PM

They absolutely can use negative feedback. They can also be run without it using much simpler circuits than their solid state equivalents.

ultraelite · 18th June 2008, 11:05 PM

That's what I was thinking but then I read

Quote:

Tube amplifiers could not, and did not need to, use as much negative feedback (NFB) as transistor amplifiers due to the large phase shifts caused by the output transformers and their lower stage gains. While the absence of NFB slightly increases harmonic distortion, it avoids instability, as well as slew rate and bandwidth limitations imposed by dominant-pole compensation in transistor amplifiers.

http://en.wikipedia.org/wiki/Tube_sound

I should have known better than to trust wikipedia blindly without checking references (wikipedia is only a secondary source of information). Thanks for the quick reply.

SY · 18th June 2008, 11:18 PM

"Not as much" does not mean "zero." 20dB is a typical number for a conventional push-pull transformer-coupled amp.

ultraelite · 18th June 2008, 11:21 PM

I see, I misread the "as much part", in my defence english is my second language.

kenpeter · 19th June 2008, 03:22 AM

In the case of Triodes, local negative feedback from Plate to Grid
is already built-in. They are extremely linear in voltage output,
and fairly linear in current output, all without much help. Break
that internal feedback loop, and you have a Tetrode.

Same goes for Pentodes, Heptodes whatever....
Tetrodes are very solid-statish in behavior. Not terribly linear
without help from an UltraLinear tap on the transformer. Or
external plate to grid feedback scheme such as O.H.Schade
They handle more power, but if you want to retain linearity
(as-if a Triode) there will be a feedback somewhere...

The same schemes can be applied to sand but is rarely done
correctly. Search the Solid board for "FETron", as I think that
is one of the best. And may help you to grasp what a triode
really does inside to make it so linear. All without the help of
a dozen external parts to fake it.

In the case of P-Triode, faking it might be worth the effort.

18th June 2008, 10:59 PM	#21
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	They absolutely can use negative feedback. They can also be run without it using much simpler circuits than their solid state equivalents. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

18th June 2008, 11:18 PM	#23
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	"Not as much" does not mean "zero." 20dB is a typical number for a conventional push-pull transformer-coupled amp. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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18th June 2008, 11:21 PM	#24
ultraelite diyAudio Member Join Date: Aug 2005	I see, I misread the "as much part", in my defence english is my second language.

19th June 2008, 03:22 AM	#25
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	In the case of Triodes, local negative feedback from Plate to Grid is already built-in. They are extremely linear in voltage output, and fairly linear in current output, all without much help. Break that internal feedback loop, and you have a Tetrode. Same goes for Pentodes, Heptodes whatever.... Tetrodes are very solid-statish in behavior. Not terribly linear without help from an UltraLinear tap on the transformer. Or external plate to grid feedback scheme such as O.H.Schade They handle more power, but if you want to retain linearity (as-if a Triode) there will be a feedback somewhere... The same schemes can be applied to sand but is rarely done correctly. Search the Solid board for "FETron", as I think that is one of the best. And may help you to grasp what a triode really does inside to make it so linear. All without the help of a dozen external parts to fake it. In the case of P-Triode, faking it might be worth the effort.

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