Bottlehead Forum
Bottlehead Kits => Kaiju Stereo 300B amp => Topic started by: Larpy on December 04, 2020, 12:14:24 PM
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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?
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You can drill holes in the wood base to add meters.
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Better to place them in the middle of the wood base than where the input pots are?
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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.
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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?
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Nice job Larry! That looks great and a really nice looking build!
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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.
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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!
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So the appeal of adding meters is mostly cosmetic?
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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.
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It may be mostly cosmetic but they sure look good...John
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So the appeal of adding meters is mostly cosmetic?
Yes.
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Yes.
Is that true of the MonAmour 2A3 as well or is the meter more useful in the 2A3 topography?
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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.
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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.