865 tetrode in triode mode...WOW ! - diyAudio
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Old 15th January 2013, 03:30 AM   #1
aldovan is offline aldovan  Brazil
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Location: Curitiba
Default 865 tetrode in triode mode...WOW !

Friends

Some old direct heated transmitters tetrodes work very fine in triode mode.
I made some experiences with the 865 in triode mode and it sound wonderful.
See the videos:
https://vimeo.com/57420060
https://vimeo.com/57420059


Aldovan
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Old 27th January 2013, 09:11 PM   #2
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I was able to get 865 plate curves up to 420 plate volts using a bench power supply.
See the linked doc for a crude drawing
https://docs.google.com/file/d/0Bzuo...JVSkxiaVE/edit

Measurements were taken with 7.5 volts DC on the filament.
Control grid volts were relative to the negative filament supply.
Plate resistance is about 4300 ohms.

The 865 should handle single ended triode operation at voltages up to about 400 vols on the plate. Load resistance would need to be 8-10K.

Screen grid current was 26 ma with control grid at +7.5 volts and 320 plate volts.

Send me an email address if you want the raw data.
I also may try to get data for plate voltage higher than 420 volts.
I mostly want to know where the tube gets less linear.

It looks like the 865 wants positive control voltage if you want enough plate current.

Control grid current at +7.5 volts was only .6ma.
Control grid current at +15 volts was only 3ma.
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Old 17th June 2013, 02:02 PM   #3
aldovan is offline aldovan  Brazil
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Quote:
Originally Posted by daqvin_carter View Post
I was able to get 865 plate curves up to 420 plate volts using a bench power supply.
See the linked doc for a crude drawing
https://docs.google.com/file/d/0Bzuo...JVSkxiaVE/edit

Measurements were taken with 7.5 volts DC on the filament.
Control grid volts were relative to the negative filament supply.
Plate resistance is about 4300 ohms.

The 865 should handle single ended triode operation at voltages up to about 400 vols on the plate. Load resistance would need to be 8-10K.

Screen grid current was 26 ma with control grid at +7.5 volts and 320 plate volts.

Send me an email address if you want the raw data.
I also may try to get data for plate voltage higher than 420 volts.
I mostly want to know where the tube gets less linear.

It looks like the 865 wants positive control voltage if you want enough plate current.

Control grid current at +7.5 volts was only .6ma.
Control grid current at +15 volts was only 3ma.
Can you obtain the triode curves of the 865 ?
I thnk in use 300V in the plates.

Aldovan
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Old 17th June 2013, 02:54 PM   #4
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Sounds pretty good!
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Old 18th June 2013, 11:49 AM   #5
aldovan is offline aldovan  Brazil
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Quote:
Originally Posted by Bas Horneman View Post
Sounds pretty good!
Thanks, Bas !
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Old 19th June 2013, 03:17 AM   #6
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Quote:
Originally Posted by aldovan View Post
Can you obtain the triode curves of the 865 ?
I thnk in use 300V in the plates.

Aldovan

I was able to plot at higher voltages. Note this is probably a large file.
Let me know if it does not really work.

https://dl.dropboxusercontent.com/u/...ode-curves.JPG

I went up to -150 volts on the grid.
Screen grid current was still reasonable at higher plate voltages.
You should be able to do SE up to about 500 volts before the tube gets less linear.

Here is an older link at lower voltages.
https://docs.google.com/file/d/0Bzuo...JVSkxiaVE/edit

I am about to finish an amplifier using 865s. Just need to put some fininshing touches.
I am using 6sl7, 112, 865 and a little negative feedback.

You do not get alot of power from the 865 without needing to push the control grid positive. One good thing about the 865 is it does not draw alot of control grid current. You can see control grid current from a regular spec sheet.
I plan to just use a 112 with a fairly low impedance capacitor coupled driver. I know bias may shift when attempting to push the grid positive.

I may also just go ultralinear SE instead. I am just debating how much power do I really need. My guess is 5 watts is enough.
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Old 19th June 2013, 12:38 PM   #7
aldovan is offline aldovan  Brazil
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Join Date: Jul 2005
Location: Curitiba
Default 865 curves

Quote:
Originally Posted by daqvin_carter View Post
I was able to plot at higher voltages. Note this is probably a large file.
Let me know if it does not really work.

https://dl.dropboxusercontent.com/u/...ode-curves.JPG

I went up to -150 volts on the grid.
Screen grid current was still reasonable at higher plate voltages.
You should be able to do SE up to about 500 volts before the tube gets less linear.

Here is an older link at lower voltages.
https://docs.google.com/file/d/0Bzuo...JVSkxiaVE/edit

I am about to finish an amplifier using 865s. Just need to put some fininshing touches.
I am using 6sl7, 112, 865 and a little negative feedback.

You do not get alot of power from the 865 without needing to push the control grid positive. One good thing about the 865 is it does not draw alot of control grid current. You can see control grid current from a regular spec sheet.
I plan to just use a 112 with a fairly low impedance capacitor coupled driver. I know bias may shift when attempting to push the grid positive.

I may also just go ultralinear SE instead. I am just debating how much power do I really need. My guess is 5 watts is enough.

Thanks for the curves.
In my experiences, this tube sound really fine with 300V in plates.
In the graphic, the ideal bias is 350V x 50mA in my opinion.
You work with 17,5w of plate dissipation and is easy for driver the 865 in this point.
You can obtain about 3W of high quality sound.
The positive grid bias do not is recommended, because the sound quality do not is too good.
If you need more power, use a 813 in triode mode.
This 865 sound really hifi in my experiments. I use a E180F in triode mode as driver. Really a fine and cheap tube !

Aldovan
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