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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


Friday, March 19, 2021

The ELROG ER5U4G Rectifier Tube


Today I have the pleasure to announce the availability of the newest member of the ELROG family. The ER5U4G.

After the introduction of the ER274B and ER274A this is the third rectifier in our line up and the 14th tube type.

The ELROG ER5U4G is fully compatible with the 5U4 specification and can be used in any applications designed for 5U4, 5U4GB or 5U4WG.

Like all ELROG tubes the ER5U4G has a thoriated tungsten filament which gives off a bright light.

The two plates are arranged vertically one above the other so that the filament is visible between them.

Here a comment from a lead customer who uses one of the first ER5U4G rectifiers in his Audio Note Meishu:

After a good afternoon and evening yesterday of listening, I’m pleased with the tube - i would sum up the tube as bringing another sense of clarity and solidity to the music. 

The first series production batch is already almost sold out. Get in touch if you want one.

Best regards


Thursday, March 4, 2021

The Mono Phono Preamplifier for Historic Records


Just finished a new variable EQ phono stage and took some photos before it got shipped to its new owner.

Like previous builds of  mono phono preamplifiers, this unit has separate controls for the bass turn over frequency and the roll off at 10kHz.

This allows for a variety of settings to cover a wide range of EQ curves from early 78rpm records to 33rpm vinyl mono LPs.

The actual phono stage and power supply have separate chassis similar to the stereo LCR RIAA phono stage.

The EQ networks are done in LCR topology as in the stereo phono stages. The inductors are mounted directly behind the face plate besides the EQ setting switches.

Some photos from the construction steps. Here the top plate with vibration damped mounting of the tube sockets.

Some parts mounted:

Output and interstage transformers as well as the B+ smoothing chokes are mounted on the top side under transformer covers:

There is a second support plate for B+ smoothing capacitors, a choke for the heater supply and the remaining parts of the EQ networks.

About one third of the EQ networks mounted :

The power supply top plate:

Chokes go on a separate plate...

.... which is mounted to the underside of the top plate:

Finished preamplifier:

Bass turn over selector:

Treble roll off selector:

Vibration damped mounting of the D3a tubes:

Side view of phono stage:

Back side:

Some sound clips with a similar phono can be found in the post about a 78rpm demo at the Munich High End show.

Best regards