6BM8 for Audio

The 6BM8/ECL82 is found occasionally in vintage audio equipment. Its primary applications were in small hi-fi units such as the Sansui 250 and 500 receivers, and in European AM/FM radios as the audio output driver. It is suitable for preamp or driver applications; for a small 1.5-watt SE amplifier of extreme simplicity; or for a low-cost push-pull amplifier capable of up to 10 watts output, as well as for other uses in modern equipment design.

Figure 1 is a line stage with low parts count and no loop feedback. This circuit can produce a maximum voltage swing of 270V p-p on a supply voltage of 450V. Feedback can be added to the circuit with Rf1 and Rf2 as shown. For stability, it is recommended that Rf2 never be made less than four times Rf1, or else oscillation may result. If a volume control pot is desired, it should be installed before Rf1. The resulting circuit voltage gain will be:

AV = -((Rf2 / Rf1) - 1) (approx) for 4<AV<45

If a supply voltage of more than 200v is used, the filament supply should be floated, but with a coupling capacitor from its center tap to ground. This will minimize hum coupling into the audio circuits.

A small power amplifier is shown in Figure 2. This is about as simple as a speaker driver can be built with one tube, given the limited number of audio tube types still available. A single pair of 6BM8s, with a small power transformer and two inexpensive output transformers, can implement a stereo amplifier for less than $100 in parts costs. The output transformer can be a 'universal' type, wired for a 10k ohm primary impedance; or it can be a Hammond 1649, with the primary center-tap unused. The 6BM8 can also drive a Fender-type reverb driver transformer directly. This would be superior to the 12AT7 customarily used, and gives more drive power at a low cost.

A suggestion for a mu follower is shown in Figure 3. This circuit, like Figure 1, can be used either as a line stage or as a driver for a power tube in SE, such as a 300B, SV811 type, or SV572 type. Mu-follower operation offers lower distortion than the typical cascade in Figure 1, without loop feedback. Disadvantages include the very high supply voltage required, and the need to float the filament supply of the 6BM8 to avoid exceeding the heater-cathode voltage rating. This circuit can give the utmost non-feedback performance that the Svetlana 6BM8 is capable of.

Figure 4 shows a simple, low-cost push-pull amplifier using two 6BM8s per channel. This circuit is modeled after the Sansui 500 receiver's power amplifier section. The output transformer is not very critical; any good-quality unit with a primary impedance between 4300 and 10,000 ohms will work. The Hammond 1620 is an excellent choice, will allow up to 10 watts output, and is reasonably priced. The values of the 47 pF capacitor and 10k resistor in the feedback loop may have to be adjusted for other output transformers, to guarantee stability into various speaker loads.

The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information.