Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cfortner on November 05, 2023, 03:21:03 am

Title: BIAS measuring resistor and grid3 at KT120
Post by: cfortner on November 05, 2023, 03:21:03 am

Hi,

I am building an amp with 6 KT120. To be able to biasing it correctly I want to insert resistors 1 Ohm 5W 1% from cathode to ground as usual.

No I read an old article from 1957, which describes amps from 5 to 1100 (!) watts. There was a schematic with a different circuit (jpg 1), which shows the shunt resistor (here 10 Ohm) between cathode and ground, grid 3 connected directly to ground.

I attach two schematics at this post, the first as shown in the article from 1957, the second with grid and cathode together attached at the 1 ohm resistor.

The result will be equal in both schematics? Or do you a huge difference? We are talking about 840V Ua and 200-220mA Ia.

Whats the grid 3 for?

Thanks a lot

cfortner

Title: Re: BIAS measuring resistor and grid3 at KT120
Post by: pdf64 on November 05, 2023, 05:16:51 am

Does the KT120 have its suppressor grid (g3) or beam forming plates brought out to pin 1?

Typically beam pentodes have their beam plates internally connected to the cathode.
Their pin 1 is often not connected (an exception being the 7027).

Title: Re: BIAS measuring resistor and grid3 at KT120
Post by: sluckey on November 05, 2023, 05:33:33 am

What he said. Look at attached pic...

Title: Re: BIAS measuring resistor and grid3 at KT120
Post by: cfortner on November 05, 2023, 05:35:04 am

Hi,

I'm a stupid, thanks for that advice!

Title: Re: BIAS measuring resistor and grid3 at KT120
Post by: pdf64 on November 05, 2023, 06:13:33 am

Just to expand and provide background to that schematic, in normal intended usage, g3 is akin to g1, in that current flow there is expected / assumed to be negligible.