• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Vaa for KT120

es345

Member
2012-01-29 9:41 am
Ulm
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.
 

es345

Member
2012-01-29 9:41 am
Ulm

es345

Member
2012-01-29 9:41 am
Ulm
I understood his aim is instrument/guitar amplifier ?

safe sounds like a joke with 850V :eek: :D

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 :D

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.
 

Attachments

  • KT120 Raa 6K 300% drive.jpg
    KT120 Raa 6K 300% drive.jpg
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es345

Member
2012-01-29 9:41 am
Ulm
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.
 

Attachments

  • 6x KT120PPP  Raa 1K2 600V.jpg
    6x KT120PPP Raa 1K2 600V.jpg
    134.6 KB · Views: 122