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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Foolproof Fixed BIAS Simulator with LTSpice for PP

Hi !

I made a foolproof fixed BIAS simulator with LTSpice for push-pull output stage (LTSpice is freeware). The idea of foolproof BIAS circuit was inspired by the Soloist amplifier designer. Even if BIAS potentiometer go south with total annihilation of the wiper, vacuum tube will not overheat and burn. Additionally, it allows to fit available components. For example, I have a box of 33K potentiometers which I would like to use for BIAS adjustment of PP KT88.
Entire archive enclosed, you will need to open *.asc file in LTSpice, edit values, types of tubes, etc., and run simulation. For output tubes other then KT88 - download, copy and include another *.lib file from within *.asc.

I changed resistors to lower values (compared to Soloist), as IMHO Soloist's grid leak final value is too high.

Self-explanatory screenshots attached. 2nd simulation is with broken wiper.

Enjoy !


  • Screenshot at 2022-08-03 00-54-57.png
    Screenshot at 2022-08-03 00-54-57.png
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  • Screenshot at 2022-08-03 00-55-48.png
    Screenshot at 2022-08-03 00-55-48.png
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  • BIAS-Calc.zip
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  • Like
Reactions: 1 user
Independently reinventing a solution to a problem is a good thing. It demonstrates a creative mind. Truly new ideas are few and far between in the tube audio world. I convinced myself that I have invented something new several times only to discover its been done before. It is still fun and valuable to exercise the prob!em solving mind.


2008-08-16 4:57 pm
Great but which part is new? It is a common practice to place an additional resistor between the grid and negative bias supply, in case the wiper goes floating.

New is a freeware turnkey simulator available for download to quickly check design values, currents, voltages, etc.
As I wrote in my 1st post, I took foolproof circuit idea from Soloist amplifier.