Bottlehead Forum
General Category => Technical topics => Topic started by: HF9 on March 12, 2011, 04:49:39 AM
-
So I've seen a few amps with the analog panel meters mounted up front. It seems like most of them measure DC in Amps. It's kind of a cool look I think. Anyone here happen to use them on their equipment? How did you wire it up?
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.parts-express.com%2Fimages%2Fitem_standard%2F320-231_s.jpg&hash=d008c6dbdca9bc54b23e14deb2000d5cc581b30c)
-
Every DC power supply in Bottlehead equipment could be monitored, both high voltage and heater current. They shouldn't change. They also can not be adjusted. But the meter looks cool.
Way too much information about amp meters:
There are three kinds of current meters, one with a shunt, one without a shunt (it is internal) and industrial current meters with current transformers (never mind those).
Meters never have circuit current through the movement. The movement is a voltage meter.
A meter that has an external shunt will have something like "100mV=FS" on it. That means full scale on the meter is 100mV. A shunt is a precision resistor that produces a predictable voltage at a specific current. For example 100mV for 1000A DC. I have seen these.
Those that don't use an external shunt have the current, much lower current, going into and out of the meter. They can measure AC or DC, the AC is rectified because the meter movement is DC. These meters have internal precision resistors that the volt meter, movement, reads across. An example is the meter you pictured above. It doesn't have 10A going through the meter movement but doesn't have a volt/full scale marking on it either.
-
... How did you wire it up?
What do you want to measure? :^)
Most often, such meters are there to adjust the plate current in amplifiers with fixed bias, where a small bias voltage error can lead to output tubes that self-destruct. This is especially important in class B and AB push-pull tube amps. Bottlehead amps mostly use so-called "automatic bias" which is self-adjusting. (Some of our drivers use current source loads which prevents excessive plate current in a different way.)
It is a cool look, and some classic amplifiers - mostly solid state - have power output meters, which bounce around with the music. Very cool to watch if you're stoned (yes, I remember the Sixties. Some of them anyhow.)