Over on YouTube user Full spectrum technician has uploaded an interested video where he shows how he used a beam deflection tube to create an upconverter for his RTL-SDR. A beam deflection tube is a type of vacuum tube that can be used as a mixer. If you aren’t aware, a vacuum tube (a.k.a tube or valve) is an electrical component that was used in electrical equipment heavily back in the first half of the 1900’s. They could be used to implement circuits like amplifiers, mixers, switches, oscillators and more. Even today they are still used in some high end audio equipment because many people believe they produce superior audio quality. Full spectrum technician writes on his video:

A simple test using a 6ME8 beam deflection tube as a balanced mixer up converter for an RTL-SDR to enable HF reception.

The only problem I had was too much conversion gain. Even with a relatively short antenna, and literally starving the tube for voltage, the signal output levels were high enough that I had to crank back the gain of the RTL SDR and/or use padding on the input of the RTL-SDR.

The LO was feed to grid 1 for common mode input.
The antenna was feed to the two deflection plates via a transformer as a differential input.
The output was taken from the two anode plates via a transformer as a differential output.

That resulted in the LO balancing it’s self out on the output so that the LO would not overload the front end of the receiver.

Operating voltages at the time were..
20V anode.
5V deflection plates.
20V accelerator grid.
Cathode tied to ground.