What's Vaa for a KT120? I'm trying to find out the optimum B+ voltage for 6 KT120's in push/pull.
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.
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?
I've got a lot to learn about AB2. Anybody got a link to a good tutorial?
I understood his aim is instrument/guitar amplifier ?
safe sounds like a joke with 850V 😱 😀
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_Folders/mosfet_folly/mosfetfolly.htm)
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_Folders/mosfet_folly/mosfetfolly.htm)
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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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.
I was thinking more like 600 v.
I have attached quick estimation , Ub=600V at full power, Raa=1K2. You should get about 440W. The values are shown for 3 KT120 parallel per side. The average cathode current at the border to clipping will be around 210 mA per tube, so there is still a small margin with regards to the maximum allowed average cathode current of 250 mA.
One hint from my experience: the KT120 need up to 5 min before you will get full power out of them. If you often do a "coldstart" the lifetime might be reduced significantly.
One hint from my experience: the KT120 need up to 5 min before you will get full power out of them. If you often do a "coldstart" the lifetime might be reduced significantly.
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