3

The pros and cons of triodes vs screen grid tubes like pentodes and tetrodes are well documented elsewhere.

When it comes to low and medium power tubes both triodes and screen grid tubes are readily available. At the really high power levels like kWs it seems to be an all triode show.

  • Are high power say pentodes difficult to manufacture?
  • Is it inconvenient to have more connections to the glass envelope when voltage clearances are considered?
  • Are screen grid tubes more likely to flashover?
JRE
  • 71,321
  • 10
  • 107
  • 188
Autistic
  • 14,616
  • 2
  • 28
  • 65
  • 1
    I remember using the 4CX1000A. That's a tetrode and very commonly used decades back (in my day.) Still bigger were available, though I've no experience with larger. I think the plate impedance used the usual RF output network to match the antenna's input feed to the amplifier's anode. Have tetrodes like this, and larger, become unavailable now? – jonk Apr 22 '17 at 07:04

3 Answers3

5

Another consideration is that the screen grid connection (generally to a high and constant voltage) may consume 20-30% of the cathode current in a tetrode circuit.

In a triode, substantially all the cathode current reaches the anode.

I haven't done the math on high power tube amplifiers, but that seems likely to impact the power supply design, power dissipation, and overall efficiency.

  • @ Brian Drummond Interesting Answer +1 .So what you reckon is that the overall efficiency is higher with a triode when screen power is accounted for .Maybe when people said that triodes were less efficient they were talking about anode efficiency and did not take the screen into account . – Autistic Apr 22 '17 at 10:44
  • I can't really comment without knowing where it's said that triodes are less efficient. They may not have been talikng about Pout/Pin but effectiveness - with lower gain (lower Zout) you need more stages. Interesting question. –  Apr 22 '17 at 10:49
  • .Music people did proper tests of anode efficiency and found pentode 1st and triode 2nd and triode connected pentode 3rd – Autistic Apr 22 '17 at 10:52
  • Aaah right. Williamson et al. I suspect RF amps with tuned loads, and audio with broadband transformers may be different here, but I'm not sure. For one thing, audio applications recover a lot of the screen grid power via the ultra-linear connection, there are very few pure pentode connected amplifiers. Are you looking towards audio or traditional RF amps at these powers? –  Apr 22 '17 at 10:58
  • I was thinking of say 50KW MW AM broadcast where I guessed most of the high power valves went so you see Pushpull triode modulators and class C RF finals . – Autistic Apr 22 '17 at 11:05
  • Right. Class C finals. Audio work on efficiency (for a given degree of linearity) definitely doesn't apply. If you can get the anode voltage swing you need from a triode, you probably don't want to waste 10kW on the screen grid. Have you seen the output circuit on those things? Plumbing. You can probably cool the anode load and make whisky at the same time! –  Apr 22 '17 at 11:14
  • @ Brian drummond Right ...low mu triode will conduct heavily and therefore give the needed anode swing with of course no screen to waste power .Gain is not important becuase number of valves is not a biggie like say a domestic radio . – Autistic Apr 22 '17 at 11:21
3

Could be a matter of matching. Pentodes have stiffer Rout. To match to 50_ohm cables requires a smaller cap on plate and a larger inductor to resonate, moving energy into the large cap wired to the output cable.

Triodes just may be more match-friendly.

analogsystemsrf
  • 34,253
  • 2
  • 19
  • 48
  • +1 .I can see the lower output impedance being a benefit on the Triode .I think that there could be more reasons. – Autistic Apr 22 '17 at 06:30
  • I suspect - especially where the load is 50 ohms - the anode load would usually be a (probably tuned) transformer, allowing matching via turns ratio. –  Apr 22 '17 at 10:45
  • Is this an answer or a guess? Good answers need to be supported by evidence, not speculation. – Dave Tweed Apr 22 '17 at 13:58
  • I've done matching, but not with kilovolt triodes. – analogsystemsrf Apr 23 '17 at 07:12
1

Check out this article about beam power tetrodes: http://www.r-type.org/articles/art-024.htm titled "The Beam Tetrode".

According to the last 2 paragraphs, triodes now seem to be able to give better linearity and still reasonable efficiency. The RCA808 power triode is designed for the grid to be driven positive (up to +300V), specified up to 210V, to get reasonable saturation. For class C operation though, distortion wouldn't be a problem, and beam power tetrodes would be more efficient, especially the new ones that have cylindrical helix grids and don't need beam forming plates (actually beam confining plates(see article)). The beams are actually the sheets of electrons going between the grid wires and converging to act like a suppressor grid. The so called beam forming plates just confined the beams to the area where the flattened helix grids worked best.

Peter R. McMahon
  • 391
  • 2
  • 7