Tracking Signal Path

[Loon]

How do I work out if a particular wire is in the signal path?

For instance, is the wire between terminals A1 & 5L in the path of the signal?

Could I permanently insert an ammeter in series here without affecting the signal?

Could I permanently place a voltmeter in the circuit to measure one of the anode voltages of one of the triodes of the 12AU7 tube in a manner that would not affect the signal.  Could you please explain to someone of the meanest intelligence (me) how to achieve this in theoretical and practical terms?

The reason for my question is this:

Based on the circuit diagram I assume that the anode voltage of the 12AU7 sets the grid voltage of the 6080.  This is ideally 75V.

I will be inserting a switch into the circuit that will allow me change the plate load of the input tube, setting good operating points for a 6SN7GT and an ECC32.  These operating points *should* result in an anode voltage of 75V.

I want a foolproof way of making sure that the I have the plate load switch in the correct position for the input tube I have inserted at the time.  A voltmeter showing the voltage across one of the anodes in the input tube should achieve this. 

I only want to implement this if I can be sure it will not effect the signal.

I know nothing about electronics, though.  Please simply let me know if it would work, the reason why it would/would not work and, in practical terms, how I would implement it.

Thank you very much for your help.

[Grainger49]

Some additional information for you:

A volt meter has a very high input impedance.  This allows it to measure voltage without appreciably changing the voltage that it is measuring.  In measuring voltage it does draw a very minor amount of current through it.  In the case of an analog volt meter the meter movement is inductive.  It would form a small low pass filter to ground.  Digital volt meters wouldn't have this inductive element.

So you could put a volt meter at some point in the circuit and not affect the sound, even if it is measuring within the signal path.

True ammeters have very low impedances.  They are inserted into the current path to make the measurement.  They would affect the audio circuit.

But some ammeters work from a precision resistor (shunt), they are a volt meter.  Some of the shunts are internal some are external.  They would form a parallel RL path in series with the signal path. 

This makes your choice of an ammeter more complicated.

[Loon]

Awesome.  Thanks Grainger.

Volt meter it is then.

Now for the implementation side of things.  I want to measure the voltage of one of the input anodes.  Would I achieve this by hooking up a volt meter between the anode and ground?

[Paul Birkeland]

Have you considered that most toggle switches will be open when thrown?

Check the thread regarding modifications to run the E80CC in the Crack, this is a good approach.

You'll know that the setting is wrong because the amp will sound quite a bit different.  Plate voltage that's a little high or a little low isn't going to do much harm beyond that.

-PB

[Loon]

Cool. 

I intend to switch using a rotary dpdt that requires a key to turn.

But I would still like someone to answer my question.  This kit is as much about learning as it is about music.

[Paul Birkeland]

Essentially, everything in the amplifier is in the signal path to some degree.  Parts/wires mounted on the power transformer are reasonably isolated from the signal path, but still audible.

An ammeter has incredibly low impedance, so it's not too concerning to just put it where you need it.

[Loon]

Thanks