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Friday, July 29, 2022

Tube of the Month : The RV218 / RV258


Here are two more triodes borrowed from my friend Josef in Vienna to show them to you. The RV218 and its successor the RV258.

These are two very old directly heated triodes made in Germany. The RV218 was introduced in 1928 and later replaced by the RV258.

They have a peculiar base which - contrary to most tube bases - is female. This means they do not have pins extruding from the bottom but connector jacks which conveniently fit 4mm banana plugs. They were intended as audio amplifier tubes. They are directly heated by a thoriated tungsten filament which runs at 7V and needs 1.1A of current. Some data sheets list slightly different filament voltages. I have seen 7.2V and even 7.5V. The filament power is actually close to that of the 801A or 10Y. The amplification factor of 7 and plate resistance of 3500 Ohms are quite similar to the values of the 801A as well. The RV258 is an uprated version which can handle up to 800V on the plate and 32W dissipation. The large metal collars of the bases have an additional jack for a grounding connection for shielding purposes. Interesting and long forgotten tubes. Due to their scarcity probably hardly used in any amplifier nowadays. As always let's have a look at the plate curves which I took from the RV218 with the tracer:

The grid lines are spaced in 5V steps. The plate voltage is on the horizontal axis, 50V/step and the plate current vertically at 5mA/step.

And now some photos of the two specimens Josef lent me, starting with the RV218

This tube was made by Siemens.

A few close ups:

Next the RV258, made by Telefunken:

And some fotos with the filament lit up:

Hope you enjoyed the presentation of these old gems. Stay tuned for more samples from Josef's collection.

Best regards


Friday, July 15, 2022

The All Silver D3a Differential Phono Preamplifier


Just finished a D3a differential phono stage with all signal transformers, plate chokes and LCR coils in silver.

This phono stage will be used together with previously finished 10Y differential line preamplifier and 300B mono blocks

To house all the components needed the signal unit has to be 6cm longer than the usual standard size.

The power supply unit is in the usual 40cm long portrait chassis.

View from the top:

These are all the silver wind goods used in this phonos stage:

Some impressions of the construction process:

Power Supply:

The phono stage is meant for use with MC cartridges. It has both XLR and RCA in and outputs.

Best regards