Use of Bias Probe

[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.

[Paul Joppa]

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.

You did the right calculation, though the equation is P=I*E. I'm guessing algebra was a long time ago?  :^) More accurately, as PB pointed out, is 0.07 amps times 350 volts equals 24.5 watts. This is within the absolute maximum for the EML mesh (28 watts), but not the recommended maximum of 22 watts. It is, however, one of the recommended operating points according to EML.

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.

To maintain the performance, it's better to keep the ratio of voltage to current constant. This ratio is technically called the "beam resistance" since it has the dimension of resistance - in this case 5000 ohms. I get 66.3mA at 331.5 volts.

None of the three trimmers/potentiometers per channel has any effect on the plate current or voltage of the 300B. The simplest way to reduce the dissipation while preserving the beam resistance is to reduce the plate-to-cathode voltage to 330 volts. The 1000 ohm bias resistor will automatically  reduce the current to correspond. There are some resistors in the power supply that can be increased a bit to get there if it seems necessary.

It is possible to increase the bias resistor a bit to bring the current down - I get 1161 ohms. The beam resistance then is about 5500 ohms, which is higher than I normally design for, but it's not far off. This is one of the EML recommended operating points, so you may be happy with it.

My recommendation would be to run it stock at first, and post the voltages you get with your power line voltage. Then adjustments (if desired) can be based on facts rather than predictions.

Incidentally, the filament current in this tube is 1.4 amps, not the 1.2 amps of a standard 300B. The filament voltage will be reduced about 5%, so the series filament resistors might also need to be changed. Again, it's best to measure it stock before modifying so that the mod is based on actual measured voltages.

As you can see, using a non-standard tube is a little complicated!

[drewh1]

Yep - algebra was about about 45 years ago I am better on concepts than details . . .

Thanks for this detailed info. I will do as you suggest. Jac definitely recommends keeping the filament within 4%, so after everything is built and measured, I'll get back to you for the details on the possible modifications to get it within spec.

I haven't ordered anything yet, so I could still punt and get the standard 300b's but I would like to give the mesh a try if I can do it with a reasonable effort.