Pantode and Tetrode What is the difference?

KT88's often are described as clearer and more linear, pentodes like an EL34 or something, are often described as warmer or lusher. If you bought the amps new, you should have them replace the tubes. You shouldn't have to pay. If you don't have individual biasing for each socket, yes, check the manual. If it says in pairs then match. If not, don't bother.

Let me put on my 1960's tube hat for a minute....

A triode has one grid, called the control grid. This is where signal is applied for amplification, and bias voltage is applied.

A tetrode has a second grid called the screen grid, between the control grid and the plate of the tube. It gets a positive bias (but no signal!) to cause more electrons to flow from the cathode to the plate, thus increasing current flow through the tube.

A pentode has a third grid, called the supressor grid, placed between the screen grid and the plate. When electrons hit the plate at high velocity they knock other elctrons loose from the metal, forming an electron cloud near the plate that can reduce current flow due to electron replulsion. The supressor grid is tied to the cathode and reflects these electrons back to the plate (they would normally be attracted to the positive charge on the screen grid). This improves efficiency.

If that's not enough there is another type of tube called a beam power electrode tube. Instead of a supressor grid it uses specially shaped metal electrodes to focus the electron stream onto the plate.

The 6L6/5881/KT66 is a beam power tube. The 6550/KT88, EL34 and EL84 are true pentodes.

As mentioned above you can use a tetrode, pentode. or beam power tube as a pseudo-triode by using only the control grid. If my memory is correct one normally ties the unused screen and suppressor grids to the plate.

If you have mixed up your tubes, you will need a tube analyzer to pair them again. If you can bias tubes individually on your amp you need not have matched tubes.

The KT88 is a kinkless tetrode, i.e. the cathode and one of the grids are shorted. In spite of being called a tetrode, it has three grids like a pentode. Ditto for the 6550 tubes, which are known as a beam tetrode. EL34s are 'pure' pentodes. I have a couple of amps where I can use KT88s, 6550s or EL34s (and I guess a couple other tubes as well)

Depending on how the amplifier is wired, you can run the KT88 in triode, tetrode or ultra linear. You can run any the majority of tetrodes and pentodes in these three configurations. (There may be ones you cannot, I don’t know)

Running a pentode/tetrode in triode mode normally gives a forward sounding midrange, but you pay a price as you restrict the output of the vacuum tube. Most folks seem to prefer this configuration. Ultra linear and tetrode mode allows the tube a higher output, but is not generally a preferred configuration.

To my ear a tetrode wired pentode/tetrode mode sounds the most dynamic and 'peppy'; consequently I prefer this layout. My all time favorite tube amp is a single ended tetrode wired 6550.

Run in single ended triode mode, I generally prefer the sound from EL34s.

In a push-pull configuration with so many tubes, you may as well stick to the KT88 as they have lots of oomph and given the configuration, will give more or less the same detail as any other tube.

Regards
Paul

"What is the difference between Pantode and Tetrode tube?"

What is the difference between the analog forum and the amp forum? Maybe you should concentrate on THAT one first.......

Oz

ya just gotta love the forum police