SET vs. Triode Mode

[Heideana]

What's the difference between SET and Triode mode? It sounds like they are the same, but I don't seem to see the terms used in the same sense...

Thanks for any elucidations...

[pauln]

More informed may chime in, but this is how I understand it:

SET (Single Ended Triode) means two things:

1) The tube is a triode; or is a tetrode or pentode 'strapped' be operate as a triode)

2) The operation mode is single ended, which means that the full wave of the signal is carried by the tube with the middle line (with the upper part of the wave above and the lower part of the wave below) called the operating point set in the middle range. The tube in this way is always delivering signal. Single ended is opposed to balanced in which a pair of tubes would be used - both of their operating points would be at minimum and each takes half of the signal (upper and lower) and an additional element called a driver passes the upper and lower parts of the signal alternatively to one tube, then the other. Both upper and lower signals are combined in the output transformer to yield the full signal for output to the speakers.

Triode mode means that either the native mode for the tube is to operate as a triode, or that the tube used in the amp is not a triode, but has been 'strapped' (rewired in the external circuitry) to function electrically as a triode. This can be done with tetrodes and pentodes by neuutralizing some of their additional elements.

Triodes (3 electrodes) = one pair of electrodes (plate or anode & cathode) conducting, with the 3rd electrode (control grid) controlling the current flow between them

Tetrodes (4 electrodes) = add an electrode (screen) as a shield for the grid to increase performance

Pentodes (5 electrodes) = add an electrode (suppressor) to shield the other grids by catching 'stray' electrons from the plate.

Triode 'mode' is simply a tetrode or pentode with the screen grid disconnected so as to operate more or less as a Triode.

[Heideana]

Thanks! If that's the case, when folks are talking about SET, then they're talking about running in class-A...is there a difference between folks getting really low power from their SET amps or say a kt-88 amp running triode?

[Daddy Dee]

An SET amp will use a single power tube per channel.

A kt-88 amp would use two power tubes per channel. They act together in what is described at PP or Push-Pull which dramatically increases the power. Some amps like this are switchable between triode or pentode mode. There is some difference in the sound. The conventional wisdom is that triode is sweeter, but I haven't always found that to be the case. I had a Cary Rocket 88 which was switchable in this way, and even though I am a fan of SET sound, I found this amp to sound notably better, to my ears, in pentode mode.

My technical understanding is quite limited, but what I understand from some people who do know the methodologies and such, a switchable scheme from triode to pentode isn't necessarily a great deal. The amp circuit would need to be optimized for operation in triode or pentode. The results of switching would be determined by how the circuit is optimized.

[Mark Hardy]

Some comments:

1) A single-ended amp can have more than one output tube (per channel); it is possible to parallel outputs. This is true for a push-pull amplifier

2) A push-pull amplifier can operate in Class A, Class B, or (most typically) Class AB.

3) You can have as much power as you want in a single-ended or push-pull (tube) amplifier; depending on the availability of a suitable output power tube/tubes, how many tubes per channel, and how much B+ (plate voltage) you're willing to have.

Here is a primer (edit: some primers!) on amplifier classes and topologies that may be helpful.

http://www.aikenamps.com/ClassA.htm

http://sound.westhost.com/amp-basics.htm#amp-basics

http://sound.westhost.com/class-a.htm

[Heideana]

Thanks all for your comments and information!

Once again, the forum is a great place to learn about audio...