adding a mA meter to the Kaiju

[Larpy]

I love the idea of adding a couple of mA meters to my Kaiju, but there's not much room under the chassis.  If I were to remove the input pots, is there a way to install meters in their place, or is that spot too sensitive in terms of picking up noise from the cathode?

 

[Paul Birkeland]

You can drill holes in the wood base to add meters.

 

[Larpy]

Better to place them in the middle of the wood base than where the input pots are?

 

[Paul Birkeland]

If you are comfortable enlarging the holes in the chassis where the pots are, you could probably do that.  You'd have to relocate the pots or eliminate them from the design. 

 

[Larpy]

I added a couple of meters.  Pretty straightforward procedure.  Now for one last question:  both channels of my Kaiju have 72 mA of current going through the cathodes.  I assume that as the tubes age, the mA measurement will go down(?). At what point should I become alarmed and think about getting replacement tubes?

 

[johnsonad]

Nice job Larry! That looks great and a really nice looking build!

 

[Paul Joppa]

I added a couple of meters.  Pretty straightforward procedure.  Now for one last question:  both channels of my Kaiju have 72 mA of current going through the cathodes.  I assume that as the tubes age, the mA measurement will go down(?). At what point should I become alarmed and think about getting replacement tubes?

There are many ways a tube can fail. But if it is simply worn out, the cathode loses emission. An often used criterion is when the transconductance drops to 70% of new. For Kaiju, that amounts to about 4mA reduction in current. This is a small change and hard to see on a meter! Even worse, it's sensitive to power line voltage - a 5% change in the power line has the same effect as a 30% change in transconductance.


The reason it's so small is the self-adjusting nature of cathode resistor bias.

 

[Paul Birkeland]

I assume that as the tubes age, the mA measurement will go down(?). At what point should I become alarmed and think about getting replacement tubes?

I have some very dead 300Bs that still draw about 70mA, it can be pretty tough to tell in a Kaiju!

 

[Larpy]

So the appeal of adding meters is mostly cosmetic?

 

[Larpy]

Nice job Larry! That looks great and a really nice looking build!

Thank you, Aaron.  I still plan on springing for the Takatskis you recommended.  A matter of when, not if.

 

[2wo]

It may be mostly cosmetic but they sure look good...John

 

[Paul Birkeland]

So the appeal of adding meters is mostly cosmetic?

Yes.

 

[Natural Sound]

Yes.

Is that true of the MonAmour 2A3 as well or is the meter more useful in the 2A3 topography?

 

[Paul Joppa]

The MonAmour has a switch which changes the 2A3 plate current, and in the high-current mode it requires a modern 2A3 with higher plate dissipation rating than old-stock 2A3s. So the meter indicates the current to help prevent accidentally risking older tubes.

 

[Natural Sound]

The MonAmour has a switch which changes the 2A3 plate current, and in the high-current mode it requires a modern 2A3 with higher plate dissipation rating than old-stock 2A3s. So the meter indicates the current to help prevent accidentally risking older tubes.

Thanks for the explanation P.J. That is a very useful feature indeed. You guys always think of clever stuff like that.