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Monday, March 29, 2021

Tube of the Month : The 75TH


For this months tube presentation I picked a rather unusual specimen from my collection. The EIMAC 75TH.

The 75TH is a transmitting tube designed for amplifier (RF as well as audio) Oscillator or Modulator applications.

The tube has a thoriated tungsten filament which operates at 5V and consumes a hefty 6.25A. It has a maximum plate voltage of 3kV and can dissipate 75W. The plate glows orange in normal operation. For audio amplification the tube was intended to be used in pairs in a Class B output stage with the grids being  driven positive. A pair 75TH can deliver 300W in that configuration. Some intrepid audio designers use this tube in single ended operation. In fact EIMAC tubes have some followers who swear by them. For complete technical information look up the data sheet. As can be seen in the pinout diagram on the left the tube has a UX4 base but only 2 pins are used for the filament. The plate is brought out at the top while the grid is connected from the side just below the spherical part of the bulb. The connections do not have caps but the bare rods which protrude through the glass are accessible. There had been accessories available to make the connection. These will be shown below.

I only have 3 pieces 75TH and never built anything with them. With just so few samples at hand I do not plan to use them. So many tubes, so little time. Nevertheless it is an interesting tube which deserves it's spot in the tube of the month series.

The tube is basically a sphere which extends from a glass tube. Certainly very aesthetic.

The plate is cylindrical and situated within the sphere:

The grid is a mesh which is also cylindrical and the filament is a spiral.

The plate connection:

Grid connection:

This is the accessory to make connections to plate and grid:

It is a small heatsink which helps to cool the connection rods which get very hot. It is secured to the rod through an allen screw while the wire can be connected to the other screw on the side.

More views of the box:

Some more shots of the tube:

The tube in operation. First just the filament lit up:

Applying plate voltage to get the anode to glow:

Quite a sight to behold!

One of my samples unfortunately has some gas and does not draw current anymore, but gives a nice blue glow instead:

I hope you enjoyed this little excursion into the EIMAC world

Best regards



  1. Nice post Thomas. I’ve been wondering when you were going to get around to some EIMAC tubes. I have several of the 75tl version but haven’t come across
    any 75th yet. Very beautiful tubes.

  2. Serious nice tube Thomas, highly appreciated.
    I'm using the smaller cousing (35TG) which (surprise surprise) has a 35W anode dissipation rating. But all these tubes can take serious beating....
    I love those transmitting triodes although you can not implement standard schematics as most of them want to be driven in A2. But when you have found that dominating drive you're up for a welcome suprise. Currently i'm using my 35TG monoblocks (around 8W) for the amplification of the woofer in my tre-amped systems. Works great.
    Next stop is a major overhaul to accomodate the HK54 (electrically the same; physically annoyingly different) and better OPT's.

    A shame those 75's and 35's are not easily sourced anymore.

    Enjoying your blog to the max.
    Regards, Reinout