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Old 27th February 2003, 01:43 PM   #1
JoeBob is offline JoeBob  Canada
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Default Help with common-cathode circuit.

Alright, I've got a 6922 tube that has to run off 150V (I already have the psu for it since I used it before in another configuration). Since it has a mu of 33 I'd like to get the highest gain possible, which I gather will be 33. I can't find a datasheet on the 6922 so I don't know it's plate resistance or any other specs and I forget what they are. I gather I'd have to bypass the cathode resistor with a cap to attain my desired voltage gain. I just don't know what values of the cathode resistor and plate load resistor would be optimal for a 150V operating voltage. Any help would be apreciated. By the way the load it will be hocked up to will be a cathode-follower if that makes a difference. Thanks alot again for any help I get. I'd be using a single half of a tube, no balanced operation here. Or would I need a ccs to attain the gain I want?
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Old 27th February 2003, 02:06 PM   #2
Joel is offline Joel  United States
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The only way you can get the full 33 out of it would be with transformer or choke loading. Yes, the cathode resistor will need to be bypassed by a large cap ~50uF.

I would think a 100 Henry choke in the plate circuit will get you close to a v.g. of 33.
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Old 27th February 2003, 02:18 PM   #3
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Default 6922

Hi,

For an online datasheet check here:

6922

Cheers,

P.S. A CCS will get you close to the max amplification factor as well, you'll need about 15mA.
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Old 27th February 2003, 02:24 PM   #4
SY is offline SY  United States
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Frank is correct about the CCS, but I do wonder why you want to get ALL the gain you can? Any of those configurations to maximize gain will compromise distortion and bandwidth. If you need more gain, why not use a higher gain tube and run it in a lower distortion, higher bandwidth mode? Or use a cascode topology?
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Old 27th February 2003, 02:31 PM   #5
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Default Cascode.

Hi,

Quote:
Or use a cascode topology?
How does a cascode sound?

Doesn't it acquire curves as if it were a penthode?

I have no listening experience on this type of circuit so I wonder if it is worth tinkering with...

Cheers,

EDIT:It also requires both sections of the tube...
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Old 27th February 2003, 02:34 PM   #6
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Default Re: 6922

Quote:
Originally posted by fdegrove
A CCS will get you close to the max amplification factor as well, you'll need about 15mA.
Looking at it on the CAD, the only place I can actually find a mu of 33 is when the grid is zero, and the current is up around 15mA with Vak of about 50V. In most of the places where you will actually operate it, it's more like 27 - 29, and the Rp is about 3 - 4k. So even with a CCS, you're not going to get a gain of 33 from it.

datasheets galore
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Old 27th February 2003, 02:48 PM   #7
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Default 6922.

Hi,

Quote:
the only place I can actually find a mu of 33 is when the grid is zero
Which is a rather useless bias point unless you want to use it for moving coil amplification. (As I do).

Cheers,
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Old 27th February 2003, 03:39 PM   #8
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Others have said that this tube is unreliable at full disipation but since you will be using only one section this will help.

In my experiance using a 6DJ8 at some 80% disipation (both sections working) did cause a degradation on the tube, what went wrong I can't say.

I have been using 6922 at around 1/2 disipation with no problems.
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Old 27th February 2003, 06:03 PM   #9
SY is offline SY  United States
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IME, a cascode can easily be made to have no particular sound, i.e., output indistinguishable by ear from the input. Joe Curcio has done some excellent cascode-based designs; his power amp driver design, a cascode diff-amp, is particularly good. My own phono preamp uses a FET-tube cascode; quite linear and very low noise.
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Old 27th February 2003, 06:21 PM   #10
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Default 6922

Hi,

Thanks SY.

Tony,

Quote:
I have been using 6922 at around 1/2 disipation with no problems.
They'll only sound half as good at that dissipation.(Pun not intended)

I can't recall any reliabilty problems with the 6922s.

Cheers,
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