Planar/Lighthouse triodes for Audio?

[triodes4ever]

Is there anyone who ever experiemented with planar/lighthouse/disk triodes like 2C43, 2C39, EC157 etc? These are coaxial built GHz devices, with quite impressive linearity, amplification factor and other goodies. General issue is the socket which is usually exotic and would need to be handcrafted in most cases.
Curious, I saw one phono preamp article somewhere using this type of tube but can't find that anymore.

Leonard

[piano3]

I saw a circuit on a Russian site but I cannot find it anymore either

[SY]

Originally Posted by triodes4ever

Is there anyone who ever experimented with planar/lighthouse/disk triodes like 2C43, 2C39, EC157 etc?

I've used them in transmitters back in my ham days, but not in audio. Let's take the 2C43 as an example. Dissipation is 12W, so it's in a similar class with EL84. But the 2C43 is made for pulsed Class C service, so its linearity at low currents is pretty poor- its grid has to be driven positive to get into the more linear region. But then, the grid current is nearly as high as the plate current, requiring high drive power (a few watts) for a pretty mediocre output power (maybe 9-10 watts). If you drive it harder, you'll kill the tube- it's meant to run higher currents at a very low duty cycle, characteristic of Class C. So as an audio tube with a high duty cycle, you end up being stuck in the non-linear parts of the tube curves.

2C39 has similar issues, just at higher plate dissipation- because of the duty cycles, it can't be driven into the nice linear region of operation without killing it. The curves get more evenly spaced at 300 mA, but the max current is 125 mA, unless the duty cycle is really, really low. At 100mA, it looks like a variable mu triode, quite nonlinear.

[piano3]

What about 7077 for preamp use though?

[SY]

That could certainly work- maybe running it at 7-8 mA, 150V on the plate. The low swing required will mean that the tube's mediocre linearity won't be as important. Transconductance is good, maybe 1/3 of a D3a, half of a 6AQ4. You'll get less Miller capacitance with the 7077, but at the cost of poorer linearity. It looks much cooler, though, and it's hard to put a value on that.


edit: Actually, about the same gm as a 6AQ4, not half.

[jlsem]

Interestingly, G.E. developed a small planar triode for audio that had nearly identical parameters as the type 45. It never went into production. If you are interested in measuring the 7077 for distortion, I have a pair with sockets I'll send to you. They were pulled from FAA direction-finding equipment and I have no use for them.

John

[LAJ]

Like 6CW4. They're hi-mu and have an integral shield and will run off 150V or so. Getting sockets might be difficult though.

[Michael Koster]

3CX300 is a mu~=7 planar triode adapted for audio use.

300W dissipation, if you use forced air cooling, gets you up in the linear part of the curves.

It's rated for 30W using convection cooling (not sure how they do this as Eimac insists on cooling air for their metal tubes even if only the heater is powered)

Also the 3CX400 (8874) is a high-mu planar triode that would work for audio in a local feedback circuit ala Schade.

[DF96]

A possible snag with these valves is that they are designed to amplify at UHF and microwave frequencies. This means that for stability you may have to build to this level of sophistication, as the valve doesn't know you only want it to amplify audio. Just slapping on a grid stopper might not be sufficient. A bit too much inductance in the cathode lead, and you have a Hartley oscillator!

If you know how to use these for their intended purpose, then you could probably use them successfully for audio too. It does seem a bit of a waste, though! Why not get an amateur licence and build a transmitter?

[Michael Koster]

Originally Posted by DF96

A possible snag with these valves is that they are designed to amplify at UHF and microwave frequencies. This means that for stability you may have to build to this level of sophistication, as the valve doesn't know you only want it to amplify audio. Just slapping on a grid stopper might not be sufficient. A bit too much inductance in the cathode lead, and you have a Hartley oscillator!

If you know how to use these for their intended purpose, then you could probably use them successfully for audio too. It does seem a bit of a waste, though! Why not get an amateur licence and build a transmitter?

True, these tubes are meant to be applied as a system, consisting of the tube itself, the socket, the ground plane, airflow, and some simple wiring practices that are good for any high-gm low capacitance tube. The ones I called out are good to about 250 MHz in normal service.

Why use the 4CX250 et al?

Relatively inexpensive -- the 4X150 is about $20 NOS plus about $30 for the socket. Long life if well cooled.

Great gm for use in local feedback circuits.

Nice gm curve for AB crossover.

Modest heater power, 6V/2.6A (less than a 6KD6 sweep tube).

Easy to drive; no grid current, modest drive voltage due to high gm.

They enable 200-300 WPC power amp with 2 output tubes per channel.

The drawbacks - high anode voltage (900V or more) needed to get good power efficiency due to high Vak(min), cooling air, preheat timer needed.