Vintage PA Tube Head Output Question

[cryptozoo]

I was checking out an old PA head and it's got two octal outs for speakers on it. In addition to multi-ohm taps from the OT, there's also a "0" tap, a "GRD" tap (?) and a few different voltage taps. Is the "0" tap the ground for the OT? Were these powered speakers? I don't have enough familiarity with something like this to know the purpose of those taps, so if someone could shed some light, I'd appreciate it. Also, in order to get any signal out it seems both the "GRD" and "0" have to be grounded. Kinda odd...

[pdf64]

See Constant voltage speaker system - Wikipedia, the free encyclopedia

[cryptozoo]

Ah, that explains it... thank you.

I should be able to run a speaker off the common and one of the secondaries without much issue. Any idea what the "GRD" tap is?

[pdf64]

Just a guess, maybe it allows the output to be run without a ground reference for some reason, eg avoiding ground loops?
Pete

[Mick Bailey]

You could establish the relationships of the connectors with a multimeter and also determine what relationship GRD and 0 have with respect to each other and to the chassis.

May be worth checking the back of the connector to see if the wiring is original -it may have been altered over the years.

GRD may mean 'Guard' rather than 'Ground' - which you'd conventionally expect to be labelled as GND. Could be linked to a protection circuit.

[Tom Phillips]

Attached is the schematic for a Stromberg-Carlson PA amp that has a similar set of output selections. With the BLK being the same as your "COM" you can see that it uses a separate ground terminal. This confirms Pete's guess description.

[Enzo]

For old PA amps like that, I generally assume the "0" or "COM" (common) to be the low end of the output transformer winging. I assume GND or Gropund to be the chassis. SOme are marked "chassis" even. Usually we connect the 0 and GND together, but there are applications where they won't want the speaker lines grounded, so they give you the option.

[Mick Bailey]

I don't get why the 0v tap and GRD would need to be connected to get output on this particular amp.

[trobbins]

The 0V and the GND don't need to be connected per se (as per schematic link) - if they aren't connected then the speaker terminals are floating with respect to earth. There are safety and fault issues with making an earth connection to somewhere amongst the speakers taps. Typically the 0V end of the winding is earthed, but that may not be the best safety option. Same type of issues and concerns as you get with AC mains powering and connecting Neutral to Protective Earth, or leaving Neutral floating (eg. 2 wire powered equipment).

[Mick Bailey]

Cryptozoo didn't say what amp it was. So I guess we have to ask "what make/model is it?" Then we could find a schematic.

I've come across old theatre amps where you pull the output plug and it cuts the power.

[Enzo]

Like many old jukebox amps. ANd many of the Ampeg flip top amps.