[Niteshade]

Power Pentodes: 6L6, EL84, KT88, etc...

What most of us call "garden variety" audio output tubes. Clearly they are the opposite of esoteric to a tube guru.

I have called power pentodes integrated circuits a few times. That is because a power pentode has two shared gain stages. Both share the same plate. (AKA: Have the same output structure.)

(My favorite drawing of a power pentode.)

Beam power pentodes have two confinement structures that are tied to the cathode. This is also referred to as a suppressor grid in non-beam pentodes. The supressor grid/beam electrode is responsible for containing/directing stray electrons towards the plate and away from the screen grid. This dramatically improves the tube's performance. Triodes do not have a suppressor grid or beam focusing structures and stray electrons can bounce back to the grid, which is unwanted.

The added amplification of a pentode comes from the screen grid. This acts as an accelerator, giving electrons an extra push on their way to the plate. This electron acceleration is what adds a considerable amount of gain to the tube.

Due to the nature of a power pentode, they require VERY FEW external components to augment. Since there are two amplification stages integrated inside the tube, the added gain means less external circuitry. Fewer circuits equates to better sound when they are made appropriately.

[Niteshade]

I didn't mean to upset any power triode enthusiasts. Smaller triodes, such as the 45 & 2A3 will have more gain than a 300B, GM70, 845, etc... and can incorporate a circuit that is more along the lines of good, logical audio design for low power applications.

Need ALLOT of horsepower? We MUST look into tubes such as the 572B and GM70 in push-pull configurations with pentodes driving them. There's no better way to achieve about 200+ watts/channel economically. I say economically because after 100 watts/channel, pentodes, IMO, are not a good choice....that is, unless you want house guests to mistake your amp for a pinball machine.

[FullRangeMan]

Thanks Niteshade for these posts, very instructive and to be considered by audiophiles indeed.
Regards

P.S.:> You mean say connect the Pentode as Pentode or as Triode??

[Niteshade]

Triode mode is something to be avoided at all cost. A power pentode is typically best connected in tetrode mode or ultra-linear.

[FullRangeMan]

OK, Thanks.

[guest1632]

Power Pentodes: 6L6, EL84, KT88, etc...

What most of us call "garden variety" audio output tubes. Clearly they are the opposite of esoteric to a tube guru.

I have called power pentodes integrated circuits a few times. That is because a power pentode has two shared gain stages. Both share the same plate. (AKA: Have the same output structure.)

(My favorite drawing of a power pentode.)

Beam power pentodes have two confinement structures that are tied to the cathode. This is also referred to as a suppressor grid in non-beam pentodes. The supressor grid/beam electrode is responsible for containing/directing stray electrons towards the plate and away from the screen grid. This dramatically improves the tube's performance. Triodes do not have a suppressor grid or beam focusing structures and stray electrons can bounce back to the grid, which is unwanted.

The added amplification of a pentode comes from the screen grid. This acts as an accelerator, giving electrons an extra push on their way to the plate. This electron acceleration is what adds a considerable amount of gain to the tube.

Due to the nature of a power pentode, they require VERY FEW external components to augment. Since there are two amplification stages integrated inside the tube, the added gain means less external circuitry. Fewer circuits equates to better sound when they are made appropriately.
 

Hi Blair,

So could these 6L6 tubes be pressed in to service as a preamps. My thinking is that since it has two built-in stages, but probably is of a mono construction. So say for a beacon III type config, you'd just use two tubes per side. So if the Pentodes are better tubes, then what makes the triodes so wanted other than the simplisity of circuit design?

Ray Bronk

[Niteshade]

I have used 6L6's in a preamp...exactly once and I learned allot since then. The preamp came out VERY well...such sweet sound!

However, I have discovered that your basic triode is ideal for preamplification. First, the 6L6 or even a small pentode used in the IF stage of something like a Fisher 500C has way too much horsepower for this application. It can and has been done- but I can provide the same results with a good triode. Same goes for using 300B's as preamp tubes. They're impractical. Application mismatches complicate circuitry and will not simplify it.

As a power tube, the 6L6 simplifies circuitry v.s. something like a 300B or 845 and therefore provides more potential to the designer to make a superior amplifier. Outdated triodes used in power amp applications have low gain and that is why they complicate things. Many designs use a pentode to drive a triode. In this case...what is the point? 

Preamplifiers spend 95% of their time attenuating signals. The proof: Hook a source straight to a power amplifier and you'll see how loud it actually is. In fact, that is a good way to damage your amp and speakers along with your hearing. A pentode (think IF stage pentode) would have considerably more gain than is ever necessary for a preamp. A 6L6 would have way too much current for this application. Amps require hardly any to drive.

[guest1632]

I have used 6L6's in a preamp...exactly once and I learned allot since then. The preamp came out VERY well...such sweet sound!

However, I have discovered that your basic triode is ideal for preamplification. First, the 6L6 or even a small pentode used in the IF stage of something like a Fisher 500C has way too much horsepower for this application. It can and has been done- but I can provide the same results with a good triode. Same goes for using 300B's as preamp tubes. They're impractical. Application mismatches complicate circuitry and will not simplify it.

As a power tube, the 6L6 simplifies circuitry v.s. something like a 300B or 845 and therefore provides more potential to the designer to make a superior amplifier. Outdated triodes used in power amp applications have low gain and that is why they complicate things. Many designs use a pentode to drive a triode. In this case...what is the point? 

Preamplifiers spend 95% of their time attenuating signals. The proof: Hook a source straight to a power amplifier and you'll see how loud it actually is. In fact, that is a good way to damage your amp and speakers along with your hearing. A pentode (think IF stage pentode) would have considerably more gain than is ever necessary for a preamp. A 6L6 would have way too much current for this application. Amps require hardly any to drive.

Hi Blair,

So have you ever tried using a 6146 as a audio power tube for an audio amplifier? They're transmitting tubes, but I thought I'd pose the question anyway out of curiousity.

Ray Bronk

[Niteshade]

I have used 807's and similar tubes with great success but not the 6146 series. They're a fantastic tube, however- they are not as easily found as 807's and other cousins to the 6L6, so I never made an amp with them. It wouldn't be a problem to use 6146's.