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6BQ5 triode driver stage operating point
6BQ5 triode driver stage operating point
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Old 20th November 2016, 01:31 PM   #1
Schmitz77 is offline Schmitz77  Germany
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Default 6BQ5 triode driver stage operating point

Folks,
want to build a power amp driver stage with 6BQ5 trioded tube but I wonder what operating point to choose for good sound to drive 6550 tubes.
Could not afford 20mA on a 9K load as this many voltages are not available.
Maybe some good "starved operation" modes are guessed for good sound.
Many thanks in advance.
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Old 20th November 2016, 11:43 PM   #2
TonyTecson is offline TonyTecson  Philippines
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6BQ5 triode driver stage operating point
here is one guy who is fond of that tube as drivers....education+diy
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Old 21st November 2016, 04:16 PM   #3
Lingwendil is offline Lingwendil  United States
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6BQ5 triode driver stage operating point
What are the other design points you will use, IE, B+ and push pull, SE, etc? This will make a difference for sure...
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Old 22nd November 2016, 04:59 PM   #4
Bangla H is offline Bangla H  Germany
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Default KT-88/6550

tested both on Micheals SE with Visaton VIB 200 GF
KT88SE High End Röhrenverstärker
my very first experience in Tube Amplifier.
here another one:
KT88-PSE-Amp, von Christoph Lassok
on Christophs page is an SE with 6SL7.
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Old 23rd November 2016, 05:47 PM   #5
Schmitz77 is offline Schmitz77  Germany
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Hello and many thanks for those replies and informations provided.
The amp should be 6550 SET design, but I suspect that any operating point for 6BQ5 driver would result in too much gain. I first thought to compensate for the gain with a current neg. feedback loop but has come to the conclusion that a 6AQ5 based trioded cathode follower would do the job of driving the load more efficiently, although my first intention was to avoid CF in my design.
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Old 24th November 2016, 05:51 PM   #6
Lingwendil is offline Lingwendil  United States
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6BQ5 triode driver stage operating point
Quote:
Originally Posted by Schmitz77 View Post
Hello and many thanks for those replies and informations provided.
The amp should be 6550 SET design, but I suspect that any operating point for 6BQ5 driver would result in too much gain. I first thought to compensate for the gain with a current neg. feedback loop but has come to the conclusion that a 6AQ5 based trioded cathode follower would do the job of driving the load more efficiently, although my first intention was to avoid CF in my design.
Why are you avoiding cathode followers? They don't add much complexity, and will be so low in distortion that they will have absolutely zero impact on audible sound quality.

If you want to go for something a bit easier, you could use a Mosfet as a source follower, this will allow you to drive the 6550 with any triode you want, without fear of loading down your gain stage. run a negative rail about twice the bias voltage (if running fixed bias you will need a negative rail anyway) and have the Mosfet direct coupled to the grids of the output tubes. this will feed the output tubes all the current they want with absolutely zero blocking distortion, and let you run a larger resistor at the input to the output, since it will just be at the grid of the Mosfet. Adjust the output tube bias as you normally would, just at the gate of the Mosfet.

Last edited by Lingwendil; 24th November 2016 at 05:56 PM.
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Old 25th November 2016, 09:03 AM   #7
Bangla H is offline Bangla H  Germany
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Default Mosfet source Follower

Quote:
If you want to go for something a bit easier, you could use a Mosfet as a source follower, this will allow you to drive the 6550 with any triode you want, without fear of loading down your gain stage. run a negative rail about twice the bias voltage (if running fixed bias you will need a negative rail anyway) and have the Mosfet direct coupled to the grids of the output tubes. this will feed the output tubes all the current they want with absolutely zero blocking distortion, and let you run a larger resistor at the input to the output, since it will just be at the grid of the Mosfet. Adjust the output tube bias as you normally would, just at the gate of the Mosfet.
That sounds interesting- i always be learning, no expert knowledge.
Have you a schema?, what it explains a bit by viewing.
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Old 25th November 2016, 09:27 AM   #8
Schmitz77 is offline Schmitz77  Germany
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Bangla,
the Mosfet solution sounds interesting to me, too.
Could you please advise which Mosfet type to use for this application?
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Old 25th November 2016, 09:51 AM   #9
Schmitz77 is offline Schmitz77  Germany
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Heres a fine link to the schematic to use
http://www.geofex.com/Article_Folder...osfetfolly.htm
Bangla, do you agree that the Zetex ZVN0545A Mosfet is a good choice for driving a power tube like 6550 with an EF86 voltage amp or is there some better Mosfet available for this application?
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Old 1st December 2016, 11:41 PM   #10
Lingwendil is offline Lingwendil  United States
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6BQ5 triode driver stage operating point
Quote:
Originally Posted by Schmitz77 View Post
Bangla,
the Mosfet solution sounds interesting to me, too.
Could you please advise which Mosfet type to use for this application?
I like IRF820, or something similar, but there are a ton that people successfully use.
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