Kt120 screen current

[tim614]

Kt120 I'n my PP UL amp draws 10ma on the screen, is this normal? kinda high compare to kt88.

 

[kevinkr]

Provide plate voltage, operating point, and screen voltage and someone may be able to answer your question. Other circuit details like screen (UL, pentode) configuration and series resistance if any, and fixed/cathode bias, and OPT winding DCR if UL would also be useful.

 

[tim614]

Provide plate voltage, operating point, and screen voltage and someone may be able to answer your question. Other circuit details like screen (UL, pentode) configuration and series resistance if any, and fixed/cathode bias, and OPT winding DCR if UL would also be useful.

Push-pull ultra linear with 1k inseries to the screen.
578v on plate
578v on screen before the 1k resister
568v on screen after 1k resister (10 volt drop across the 1k=10ma)
cathode bias @ 70ma per tube
5k OPT (Hammond 1650R)

 

[Kay Pirinha]

568v on screen after 1k resister (10 volt drop across the 1k=10ma)

This means screen dissipation = 5.68 W. What's the maximum screen dissipation rating for this tube?

Best regards!

 

[Frank Berry]

If the tube is a Tung-Sol, the maximum screen dissipation is 8 watts.

 

[tim614]

If the tube is a Tung-Sol, the maximum screen dissipation is 8 watts.

yes, Tungso kt120.
just want to make sure before i call this amp done

 

[Frank Berry]

Enjoy the amp!

 

[tim614]

alright!
thanks guys.
i can wait till i get off work and do some listening.

 

[TonyTecson]

Push-pull ultra linear with 1k inseries to the screen.
578v on plate
578v on screen before the 1k resister
568v on screen after 1k resister (10 volt drop across the 1k=10ma)
cathode bias @ 70ma per tube
5k OPT (Hammond 1650R)

tube dissipation at 44.8watts.....would be interesting to find out full power B+...

 

[TonyTecson]

you may also want to try pentode operation with screens regulated at say 500volts......i find out that my 6LU8 pp amps give better bass than my EL34, 6P3C pp amps run ultra-linear....

 

[jazbo8]

Screen at 500V?! Surely that's too high looking at the datasheet, have you tried it?

 

[Frank Berry]

In ultralinear mode, the screen can handle up to 600 volts.

 

[jazbo8]

But AJT suggested pentode operation not UL.

 

[Frank Berry]

For pentode mode, maximum screen voltage is 650 volts. Maximum anode voltage is 850.

 

[jazbo8]

May be I should re-phrase the question, it is not the voltage itself that bothers me, how do you keep the screen dissipation under 8W with the present plate voltage and load?

 

[DF96]

To reduce screen dissipation you need to reduce screen voltage and/or screen current. These are likely to reduce anode current too.

BTW if pentode mode gives 'better bass' then this may just be 'one note bass' from an underdamped speaker resonance due to low damping factor.

 

[jazbo8]

Perhaps AJT can clarify his recommendation for Eg2 at 500V on the KT120, I tried to draw some load lines -> ka-boom...

 

[TonyTecson]

a lot of good stuff here.... loadmatch5-beam-tetrodes-about

 

[jazbo8]

Thanks, based on Mr. Turner's graph, I added the estimated screen grid dissipation per RDH4, at ~15W, it is way over the 8W maximum, but that is assuming continuous usage at maximum power, which never happens for hi-fi use, but it is a bit scary still... Please also note that the cathode idle current has been reduced to 50mA from the 70mA used for the UL connection.

An externally hosted image should be here but it was not working when we last tested it.

 

[TonyTecson]

TBH, i have yet to build a KT88 amp, but i will probably run the screens at 400 volts regulated instead of UL...agree that 500v is kinda high....

the KT120 is more like an EL509ish than a real KT88 from what i gather from the net, i could be wrong......so the things can be done differently...

there are teeve tubes with reinforced plates for more power handling, i've seen a 6LQ6 done like that...