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Old 17th April 2015, 04:33 PM   #1
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Default Calculating Cathode Bias w/o tube curves?

I'm putting together the parts to make use of a pair of 6AV5's that I have (this will probably be built as a test-bed for other octals, too), but I can't find triode curves for the 6AV5. I do see plenty of OP's suggested in other posts, but I like to be able to calculate these things instead of blindly following suggestions.

So, without triode curves on a given pentode, how does one calculate proper cathode bias resistors? I suppose I could apply a known B+ with a 'safe value' resistor, measure and adjust, but it seems like there should be a better (algebraic) way.

Is there some formula of Vp, Rk, Rp, etc that I've been missing?
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Old 17th April 2015, 05:00 PM   #2
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The problem with sweep tubes in triode is the low Vg2(max). That will limit the use- but had you considered using it as a zero-bias triode? In that case, you can use the set of curves given for different screen voltages at Vg1 = 0.
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Old 17th April 2015, 05:10 PM   #3
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Hey SY! Hope you're well.

Zero-bias triode? Is that this Class A3 thing I read about?
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Old 17th April 2015, 05:15 PM   #4
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6AV5 triode characteristic:

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Old 17th April 2015, 06:36 PM   #5
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Jazbo,

You are a scholar and a gentleman (or classy lady). Thank you!
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Old 17th April 2015, 06:41 PM   #6
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Whoa, where did you come up with that?

Tim, to run the tube as a zero-bias triode, you connect the control grid to the cathode (i.e., Vg1k = 0), then drive the screen grid. In single-ended operation, it's class A2. The disadvantage is that you need a low source impedance driver (like a source follower) since the screen will suck current. And the mu tends to be a bit low. The advantage is that you can get great linearity and increased output power since the plate voltage can be made quite high (much more than the 175V G2 limitation).
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Old 17th April 2015, 07:21 PM   #7
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Hmm...that's a very interesting concept SY.

I have a couple of 125ESE's for multiple pri impedances, a nice big empty chassis, and a humongous choke to build a beefy power supply. I'm planning to build this amp as a kind of sandbox for single-ended tests. I just need to figure out the power transformer(s) to provide a good range of B+ values.

The zero-bias option will go on the list of things to try. When you say 'high plate voltage' how high do you mean?
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Old 17th April 2015, 08:05 PM   #8
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Here's an old thread that recently came back to life: 6CD6GA in Enhanced Triode Mode for SE Amp

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Old 17th April 2015, 08:24 PM   #9
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Thanks, Jeff. I had started reading that the other day, but I wasn't quite sure what I was looking at (enhanced triode...huh?).

Will give it a good read!
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Old 17th April 2015, 09:59 PM   #10
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Quote:
Originally Posted by SY View Post
Whoa, where did you come up with that?

Tim, to run the tube as a zero-bias triode, you connect the control grid to the cathode (i.e., Vg1k = 0), then drive the screen grid. In single-ended operation, it's class A2. The disadvantage is that you need a low source impedance driver (like a source follower) since the screen will suck current. And the mu tends to be a bit low. The advantage is that you can get great linearity and increased output power since the plate voltage can be made quite high (much more than the 175V G2 limitation).
I did some experimenting with this, for a transconductance headphone amplifier. The zero bias triode curves show a high impedance at the plate and they are more linear than traditional pentode mode. Furthermore, using a gyrator circuit on the driver tube, one has both load for the driver tube as well as the driver for the G2, and this all from a single supply!

Transconductance amplifier for headphones.

As you may see, I used two 6,2V zeners in the cathode, so grid and cathode are not strictly tied. I did that to allow a bit higher voltage at G2, without exceeding max plate dissipation. Because, indeed, for class A operation I find that the datasheet curves with Vg at 0V are not very promising. I attached the curves of the 6W6GT, as example. For an operating point of 250V and 40mA, the tube expects about 40V on G2. But there are only curves for one direction - it is not clear what happens when G2 approaches 0V.

any tip is welcome! I also have a utracer on the way, so I hope to be able to plot some curves myself for these sweep tubes with some fixed negative voltage on G1, and varying G2.

Cheers, Erik
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File Type: png 6W6GT curve.png (108.1 KB, 97 views)
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