Tube output: Voltage or Current source?

[Bobby Dipole]

Now that I’m retired, I’ve taken up the hobby of restoring vintage radios and audio equipment. I’m new to tubes, despite being in the electronics industry for 32 years. I’ve worked on a few 30’s radios, and now a 50’s mono and 60’s stereo tube amplifier. I find it interesting that the amplifiers have negative feedback around the output transformer. (I didn’t think this possible, but evidently it is.) Does this change the speaker output from a current source to a voltage source? If you remove the speaker from a 30’s vintage radio, the tube pushes the audio current though what now looks like a large inductor. The voltage across the transformer primary goes wild and can damage the tube or transformer. What would happen with the feedback design? Does the output voltage stay the same when you remove the load?

Our company’s early transistor car radios had the same problem. Our output design had a single large PNP germanium transistor operating class A. The collector had a large choke going to ground with the speaker in parallel with the choke. If you turned up the volume with the speaker disconnected, the high voltage across the choke would fry the transistor.

Bobby Dipole

[bob91343]

Well, it's not quite that simple. Certainly voltage feedback lowers the output impedance of the amplifier and moves it toward being a voltage source. It's always considered bad to remove the load from an output transformer, whether it be driven by tubes or transistors, for the reasons you state.

Some old units would put a dummy load across the speaker terminals, say 100 Ohms, to reduce the possibility of transformer arcing.

Indeed you can put feedback around a transformer but you have to know what you are doing and make sure the amplifier is stable even at frequencies where the transformer loses its performance.

[tubelab.com]

I find it interesting that the amplifiers have negative feedback around the output transformer. (I didn’t think this possible, but evidently it is.)

There is a limit as to the amount of feedback that can be applied to reactive elements due to phase shift. This limit is far below what is needed to create a good voltage source. The output impedance of many tube amps is still near 1 ohm or so.

Our output design had a single large PNP germanium transistor operating class A.

I learned electronics by tinkering with stuff I dragged home from the trash dump or the junkyard. I blew up quite a few of those big germanium transistors trying to get more power. 2N174 maybe? It was a long time ago. I used to feed my solid state experiments with an old Lionel train transformer. Used it to light up tubes too.

[Wavebourn]

Power source, ideally...

[Bobby Dipole]

Originally Posted by tubelab.com

There is a limit as to the amount of feedback that can be applied to reactive elements due to phase shift. This limit is far below what is needed to create a good voltage source. The output impedance of many tube amps is still near 1 ohm or so.

Thanks for the info. The modern car radios I worked on use chip-amps. These are power op-amps, more or less, and have output impedance in the milliohm range. These are definitely voltage sources, and make for a good damping factor.

Bobby Dipole