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Old 29th January 2013, 07:55 PM   #1
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Default DC coupling with a choke?

Hey, Im trying to figure out a simple way to dc couple an output stage. Here is what I came up with so far:Click the image to open in full size.
Click the image to open in full size.



Do you guys think this would work ok? The second one is with all grids tied together and positive bias as an experiment for a2 or a3 operation or right handed triode or whatever you call it with grid current flowing all the time. Thanks!
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Old 29th January 2013, 09:03 PM   #2
tomchr is offline tomchr  United States
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I'm not sure why you use a transformer as a choke. All you need is a choke. Note that the plate choke results in a LF pole, so you'll need to pick one that supports the lowest frequency of operation.

But you have have the right idea. Provide a DC ground for the grid using the choke. The main drawback is that now the operating point of the driver tube depends on the negative supply voltage.

Another option is to use a circuit similar to that in the DRD 300B.

~Tom
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Old 29th January 2013, 09:35 PM   #3
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I already have the two smallish se output transformers, from an old 6bq5 Magnavox console. Anything wrong with using the primary as a choke?
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Old 30th January 2013, 03:26 AM   #4
tcqanh is offline tcqanh  Viet Nam
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you need a resistor in cathode for power tube if you want it run in class A1. In this case, tube run also in auto bias.

Some people call this circuit "cathode choke for driver". My friend has success with this circuit.

Last edited by tcqanh; 30th January 2013 at 03:29 AM.
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Old 30th January 2013, 06:19 PM   #5
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Quote:
Originally Posted by tcqanh View Post
you need a resistor in cathode for power tube if you want it run in class A1. In this case, tube run also in auto bias.

Some people call this circuit "cathode choke for driver". My friend has success with this circuit.
You are talking about the second schematic? It runs with grid current even at idle as the grids are positive. With all three grids (or even just the screen grid) at the same potential as the cathode, the tube would be in cutoff. So all three grids tied together make a higher mu triode than just one grid being driven but now it requires grid current and positive bias. I got the idea for the second schematic from reading the Tubelab pages. TubelabSE outputs here: "When the grid is negative it is a high impedance, when the grid goes positive it becomes a low impedance. There is a large abrupt change in grid impedance when the grid transitions into the positive region. If your driver stage is not designed for this it will distort badly." and "Since it only needs a few volts of drive the grid is always positive and the transition never happens. The driver tube must be capable of driving a low impedance. Everyone seems to need a name for their unique tube circuit, so we call this A3 operation since grid current is drawn for 360 degrees." I am curious about it and posted both schematics to share the ideas. To try both would be easy enough by switching a couple wires.


The first one might could be called "plate choke for driver" and would run in a1 99% of the time, but could maybe push the grid a little positive on a loud drum sound or something. Thank for the replies!
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Old 30th January 2013, 06:32 PM   #6
TheGimp is offline TheGimp  United States
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I would be concerned that the bias point of -8Vgk and Va=300V on the EL84 in triode mode puts you at 75mA bias current. That is 22.5W or 188% of dissipation rating.

Second drawing.
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Old 30th January 2013, 07:00 PM   #7
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Thanks, I added some resistance and a bypass to make for 300 ohms and 12 volt drop. This is only a rough draft to convey the idea, but I think it could be adjusted this way. Click the image to open in full size.
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Old 30th January 2013, 07:12 PM   #8
TheGimp is offline TheGimp  United States
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I suspect you will have a similar problem with the EL34.

The triode curves here:

http://frank.pocnet.net/sheets/129/e/EL34.pdf

Don't give any hint of being capable of running positive grid bias without fireworks.

If you get it to work I'll be interested in hearing how.
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Old 30th January 2013, 07:58 PM   #9
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Quote:
Originally Posted by TheGimp View Post
I suspect you will have a similar problem with the EL34.

The triode curves here:

http://frank.pocnet.net/sheets/129/e/EL34.pdf

Don't give any hint of being capable of running positive grid bias without fireworks.

If you get it to work I'll be interested in hearing how.
Yup it is an experiment, they don't list curves for this mode of operation. If the screen voltage is low enough then I think positive bias would be necessary, if all three grids are at cathode potential then the tube should be near cut-off. I was reading about this as a way to do a class B2 PA with zero bias and both tubes near cutoff at idle. So to do it single ended then I wouuld need some current flowing and some positive bias. How much I have no idea, I just have to measure the quiescent current as I raise the volts to the grids and stop at the 70 ma or whatever. Here is a webpage that talks a little about it:811A's, Dual Grid and Class B triodes

Last edited by Austin Translation; 30th January 2013 at 08:16 PM.
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Old 31st January 2013, 11:04 AM   #10
DF96 is offline DF96  England
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Some output valves were designed for zero-bias Class B operation for PA and modulator applications. Most were not. Even those that were did not have hi-fi in mind.
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