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Years ago I remember working on a tube amplifier that used EL84/6BQ5 output tubes in push-pull, but instead of applying about 300 or 400VDC to screen grids and plates, the plate circuit went to chassis ground and the cathodes were operated at a high negative voltage. Naturally the sound was just as good as the more conventional approach. I tried some searches for such an amplifier and so far have not been able to determine who made such an amplifier.

The cathode circuits of the outputs had electrolytic capacitors to bypass that circuit installed in reverse polarity from that seen in most amplfiers, as did the power supply which used solid state rectifiers. There was also a negative bleeder circuit for the heaters of the output tubes to help avoid cathode to heater shorts.

Do any of you remember who made such an amplifier?

Joe

I can't find any info for one, other than car radios with vibrator supplies seem to sometimes used positive ground

I can think of two reasons for this:

Technical - You can get away with OPT's of cheaper insulation. An example is ham radio HV supplies putting the choke in the negative lead.

Culture - If your engineers are used to designing PNP transistor amps back in the 60's, they're going to naturally *think* positive grounding.

Cheers!

Those were some of my thoughts too. Not having high voltage on transformer windings might extend their life some. There was also a common practice in older radios where a capacitor with high voltage rating was placed across the primary of the audio output transformer to limit the AC peaks at high frequencies. The lower AC peaks helped the transformers to last longer without insulation failure. This was especially true of radios back in the period prior to 1950 or so.

Using this negative voltage to cathodes approach was definitely not a common one and I doubt it saved any component cost in manufacturing. I seem to remember that the amplifier in question used a circuit board rather than point to point wiring. The approach is not one that had any great following and went the way of Do-Do birds. It was an interesting variation on design approach though.

Have fun!

Joe