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Old 28th March 2013, 06:18 PM   #1
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Default Vaa for KT120

What's Vaa for a KT120? I'm trying to find out the optimum B+ voltage for 6 KT120's in push/pull.
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Old 29th March 2013, 09:16 AM   #2
es345 is offline es345  Germany
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Please define Vaa, thanks
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Old 29th March 2013, 05:47 PM   #3
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Optimal plate voltage? I guess that's cathode to anode?
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Old 29th March 2013, 06:24 PM   #4
es345 is offline es345  Germany
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I have built a powerstage for evaluation with 2 KT120 PP. It is running with a plate voltage of ~850V idle and ~750V under full load. The OT has a separate winding for G2 (40%) running the G2 at ~600V. Primary impedance of the OT is 4K. The max clean power is about 200W working AB2 mode (grid current). This is from my experience the upper limit and only safe for operation without any clipping.
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Old 30th March 2013, 06:39 AM   #5
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Holy moley! The specs say they will take that much voltage, but that's higher than the graphs i've seen.

I've got a lot to learn about AB2. Anybody got a link to a good tutorial?
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Old 30th March 2013, 09:10 AM   #6
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by es345 View Post
I[COLOR=black] ...only safe for operation without any clipping.
I understood his aim is instrument/guitar amplifier ?

safe sounds like a joke with 850V
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Old 30th March 2013, 09:12 AM   #7
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by cyclecamper View Post
I've got a lot to learn about AB2. Anybody got a link to a good tutorial?
Quote:
Originally Posted by cyclecamper View Post
What's Vaa for a KT120? I'm trying to find out the optimum B+ voltage for 6 KT120's in push/pull.
maybe try with 2x KT120
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Old 30th March 2013, 09:17 AM   #8
es345 is offline es345  Germany
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In AB2 you must be able to drive G1 with current.

You can do that using a smaller power stage driving the big powerstage via a transformer, e.g. like in the Fender 400ps

or

with direct coupling of cathode followers (e.g. as shown here http://ken-gilbert.com/the-big-***-guitar-amplifier, the schematic is just before " 5")

or

with MOSFET followers (have a look here http://www.geofex.com/Article_Folder...osfetfolly.htm)
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Old 30th March 2013, 10:02 AM   #9
es345 is offline es345  Germany
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Quote:
Originally Posted by tinitus View Post
I understood his aim is instrument/guitar amplifier ?

safe sounds like a joke with 850V
OK, I should be more precise and the correct usage of "safe". For sure building and testing this machine is a not a task for late evenings after some bear

Regarding the configuration: I am just showing the limit. Interesting to see, the real limiting factor in that configuration is the maximum allowed average cathode current of that tube. Using that config in overdrive will damage the tubes because the cathodes will be worn-out after a short time.


If the goal is to build a guitar amplifier I would stay with the high voltage (no problem at all) but change to Raa 6K. The configuration is attached (my design estimation sheet) .You can drive up to ~300% Signal (100% is the limit of clean Signal) without violating the allowed average cathode current

If the goal is to build a bass amplifier - typically not used in overdrive - you can go more to the limit. I would combine that with a limiter/compressor adjusted to max clean power.
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File Type: jpg KT120 Raa 6K 300% drive.jpg (139.5 KB, 129 views)
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Old 1st April 2013, 07:03 PM   #10
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All my bass amps have "direct coupling of cathode follower" but some have it in the splitter and some have two after. That's because they all use 6 output tubes, 3 push and 3 pull.

I was thinking more like 600 v.
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