DPDT on-off-on switch wiring for ammeter help

[Jerry garrcia]

Hi and sitting bored in the car and contemplating a wiring for an ammeter in an integrated tube amp that is cathode biased.
Might not use it since I will probably get an old west German percentage meter instead, that I probably would need help with setting up.

But regarding this ammeter thing. In an integrated SE cathode biased amp and I have one ammeter and want to be able to check the current draw for each channel with a toggle switch and in the middle position I want the ammeter bypassed. So safest would be the ammeter placement in series before the resistor I guess.
The amp has a large bypass cap separately connected to ground.
How would one wire the switch with these conditions? I have a brain meltdown. If anyone has the time and lust to help with a drawing/layout I have something to contemplating during this 12h drive while the sone is doing his first long haul.

I know a voltage meter would be better but this is was I have at the moment and more to calm down my mind since I don’t get how to bypass the meter in the middle switch position.

[martin manning]

This is easy to do with a voltmeter by paralleling it across the Rk's or a series 1R. A current meter has to be inserted (or pre-inserted shorted and then un-shorted), not something I see a way to do using a single center-off DPDT.

[Jerry garrcia]

Yes that’s my problem. I only have an ammeter (at the moment). Wouldn’t it work if a large bypass cap is out of circuit?
What I got for suggestion was:
Looking at older designs: Cathode->ammeter->resistor-> ground. Cap own ground.
1 Cathode 1 resistor + 220 µF cap node (bypass path)
2 Cathode 2 resistor + 220 µF cap node (bypass path)
3 Cathode 1 tube pin
4 Cathode 2 tube pin
5 Ammeter → Cathode 1 resistor → ground
6 Ammeter → Cathode 2 resistor → ground
Would that work with a DPDT switch?
I’m awful with spatial switch wiring.

D3590576-3E75-42DC-8777-5E38AF7938BD.png

[johnnyreece]

Wouldn't that link the cathodes? That's what I couldn't get my head around; keeping the cathodes separate, because the meter would link them.

[Jerry garrcia]

Wouldn't that link the cathodes? That's what I couldn't get my head around; keeping the cathodes separate, because the meter would link them.

I think so to. In the off position they should be tied together through the ammeter. 3PDT to isolate

[johnnyreece]

I'm sure someone much smarter than me can figure it out, but I don't know what switch will do it...

Edit: Best I can come up with works with an on-on 3PDT. If it was a center off switch, center off would truly be "Off"

[Jerry garrcia]

I'm sure someone much smarter than me can figure it out, but I don't know what switch will do it...

Edit: Best I can come up with works with an on-on 3PDT. If it was a center off switch, center off would truly be "Off"

. I’ll get me a 100mVDC meter. Seems easier

[cdemike]

This is easy to do with a voltmeter by paralleling it across the Rk's or a series 1R. A current meter has to be inserted (or pre-inserted shorted and then un-shorted), not something I see a way to do using a single center-off DPDT.

This seems like the easiest and most reliable solution.

If you're dead set on getting the ammeter to work over a voltmeter, though, you could route each of the cathodes' ground references each through a parallel pair MOSFTs wired as remotely-operated switches (basically applying the standby switch from MOSFET Follies: http://www.geofex.com/Article_Folders/m ... tfolly.htm). MOSFETs per side makes the most sense to me since their gates shouldn't draw any current, but you could probably also make the same concept work with other transistors or with vactrols or optocouplers. Relays might also work too, but I think you may have a tough time getting the switch not to pop. But each tube would have one of the two MOSFETs at its cathode resistor set up as a normally-closed switch connecting its cathode resistor to ground when the amp is in normal operation (i.e., 3 position switch in neutral position). Flipping the 3 position switch would then re-route one tube's cathode current from its normal path to ground to the ammeter by turning off that tube's normally-closed MOSFET while simultaneously applying the FET bias voltage to that tube's normally-closed FET switch.

The capacitors on the normally-closed FETs' gates probably aren't necessary, and the values for the ground reference RC network connecting each of the normally-open FETs' gates were chosen based on a rough estimation of what a desirable tau range would be in order to avoid switching noise but also maintaining some accuracy for the ammeter if you're switching quickly between sides. I haven't tested the values for the RC networks at the normally-open FETs, though, so I suspect you'll need to tune their values.

IMG_2073.jpg

[Jerry garrcia]

This is easy to do with a voltmeter by paralleling it across the Rk's or a series 1R. A current meter has to be inserted (or pre-inserted shorted and then un-shorted), not something I see a way to do using a single center-off DPDT.

This seems like the easiest and most reliable solution.

If you're dead set on getting the ammeter to work over a voltmeter, though, you could route each of the cathodes' ground references each through a parallel pair MOSFTs wired as remotely-operated switches (basically applying the standby switch from MOSFET Follies: http://www.geofex.com/Article_Folders/m ... tfolly.htm). MOSFETs per side makes the most sense to me since their gates shouldn't draw any current, but you could probably also make the same concept work with other transistors or with vactrols or optocouplers. Relays might also work too, but I think you may have a tough time getting the switch not to pop. But each tube would have one of the two MOSFETs at its cathode resistor set up as a normally-closed switch connecting its cathode resistor to ground when the amp is in normal operation (i.e., 3 position switch in neutral position). Flipping the 3 position switch would then re-route one tube's cathode current from its normal path to ground to the ammeter by turning off that tube's normally-closed MOSFET while simultaneously applying the FET bias voltage to that tube's normally-closed FET switch.

The capacitors on the normally-closed FETs' gates probably aren't necessary, and the values for the ground reference RC network connecting each of the normally-open FETs' gates were chosen based on a rough estimation of what a desirable tau range would be in order to avoid switching noise but also maintaining some accuracy for the ammeter if you're switching quickly between sides. I haven't tested the values for the RC networks at the normally-open FETs, though, so I suspect you'll need to tune their values.

IMG_2073.jpg

Thanks for all info. I’m convinced. Gonna get me a voltmeter. The FET way seems interesting/functional but way to complicated to achieve the result I want. Thanks for adding the time and effort.

[martin manning]

Gonna get me a voltmeter...

Hang on, this will do it. An analog voltmeter will have its own issues with loading. They are all current meters, really.