Bottlehead Kits > Kaiju Stereo 300B amp

Use of Bias Probe

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drewh1:
Anyone have experience using a bias probe with 2a3's or 300b?  Wanting to know more about its use and efficacy.  thanks

Paul Birkeland:
Your voltmeter can act as a bias probe to measure plate current in your output tubes.

In the Kaiju, the cathode bias resistor is 1,000 Ohms, and Ohms law tells us that I=V/R, so if you had 70V DC across the 1,000 Ohm resistor, that would equate to 70mA of current.

drewh1:
Wow, I think I might be learning something here.

So to convert that to watts, I use  I= P * E?  so if I have 70 mA with 400 volts, I get 400 * .07 equals 28 watts.  so for a target of 22 watts (recommended for the Meshplate), with 400 v, I would want 5.5 mA.  I would use the trimpots to get my desired current.

Did I get that right? 

I guess the one advantage to a bias probe, is I don't have to mess  around under the hood and can do it with the amp in place once I know the voltage?

Paul Birkeland:
You're getting a little ahead of yourself.  The voltage is plate to cathode voltage, not just the voltage that's at the actual plate.

I don't have the Kaiju in front of me, but I would expect roughly 355V plate to cathode/75mA or so of plate current, which is around 26.5 Watts of dissipation. Maximum plate dissipation for a 300B is rated at 36 Watts, with more like 30 Watts as the recommend maximum dissipation under the specific operating points listed in the datasheet.

It is worth noting that the actual EML 300B lists 40 Watts as the maximum plate dissipation.  All the other EML tubes that call themselves 300B's have enough deviations in various ratings that they should be considered separate tube designs entirely. 

I don't think you will need to perform any modifications to run the 300B mesh in the Kaiju, I would just plug it in and use it.   A bone stock Kaiju ought to run just fine with any of the EML 300B variants that I see for that matter.

drewh1:
thanks again Paul - I was going by Jac's recommendation for 22 watts plate dissipation for the 300b Mesh. Maximum is 28 according to him so 26 should be just fine. 

Although I know a fair amount about digital circuits, analog has been a mystery to me, so I appreciate the opportunity to learn more about this stuff.

with that, I'll stop bothering you for awhile  :)

drew.

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