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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 04 Jul 2011, 18:28 
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Suncalc wrote:
... an 800Ω cathode resistor, and a 28kΩ plate load.

cheap-Jack wrote:
What will be the cathode bias voltage?

I think I posted it a few pages ago: -3.6V

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PostPosted: 04 Jul 2011, 21:16 
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cheap-Jack wrote:
Hi.
Suncalc wrote:
... an 800Ω cathode resistor, and a 28kΩ plate load.

What will be the cathode bias voltage?

c-J

Here is the load line and associated figures using GE published 12AU7 data and a B+ of 250v. Very close to Mark's figure of 3.6v.
Attachment:
12AU7 Stage.jpg

Matt


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PostPosted: 04 Jul 2011, 22:20 
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Matt: Just had another read of your suggested values for this preamp. I'm keen to try it. I have a complete set of parts. My biggest problem will be what colour to spray the enclsoure!

Oh I don't know, life is full of little challenges!!??!!

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PostPosted: 05 Jul 2011, 17:23 
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I was thinking on using a small triode like a 12AX7 as a rectifier tube for low current projects. If the grid is tied to the plate with a 1K resistor could the tube act as a rectifier tube. I have lots of 12AX7s otherwise I would purchase a small rec tube.

If a 12AX7 can pass 4mA or more why can't it rectify and pass that much current? Or gang the plates and use two tubes, one for each half cycle.

Shoot me down fellas if you think I'm crazy.

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PostPosted: 05 Jul 2011, 21:03 
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mwhouston wrote:
I was thinking on using a small triode like a 12AX7 as a rectifier tube for low current projects. If the grid is tied to the plate with a 1K resistor could the tube act as a rectifier tube.
The short answer is that yes you can do this. However, the limiting factor is grid dissipation. The grid is a very fragile construct and if you dissipate too much power there you'll melt the grid with (potentially) catastrophic results. This is why the 12AX7 has a maximum positive grid bias of 0v. Some tubes are rated to handle some positive grid bias (the 6V6 family comes to mind) but it is generally discouraged for large portions of the cycle.

The way to avoid this is to tie the grid not to the plate, but to the cathode. In this way the grid is at the same potential as the cathode and electrons will simply shoot by because the grid potential will be slightly less than the E-field potential where the grid is located. Then the I-V curve for the "diode" is simply the Eg=0v curve on the plate transfer characteristics. Just make sure that you tie the grid to the cathode at the socket with NO resistor.

Now for the cautions. Watch the plate dissipation. For the 12AX7 the limit is at about 10ma. You will also see a diode voltage of about 93v at 2ma peak plate current. (A very "saggy" diode indeed.) The 12AU7 would be better, here you'll see about a 25v drop at 2ma. The 12AU7 plate dissipation limit hits at about 19ma. So while it is possible, this approach leads to a "diode" which drops a lot of voltage at very low current.

Frankly, for low current jobs like a preamp, I really like the 6H6 used as a full wave rectifier. They are cheaper than dirt, there are millions of them available because they were used as detectors in thousands of small shortwave radios, and their voltage drop is very low at preamp current levels. (I also like the little metal bump of a tube, although the big ST shaped 6H6 adds a lot of panache to a preamp project.) Here is the I-V curve for the 6H6 used a rectifier.
Attachment:
6H6 Plate Curve.png

Of course, if you have a bunch of extra preamp tubes laying about, you could just give it a try and see what you think. Let us know how it goes.

Matt


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PostPosted: 05 Jul 2011, 22:12 
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Matt: Yes the V drop at any sort of current draw may just kill the whole power supply. It was just a thought. Thanks for the explaination.

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PostPosted: 05 Jul 2011, 22:48 
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Of course, I do know of one signal tube that makes a great low current rectifier with even lower forward voltage drop than the 6H6. The venerable 6AS7/6080. As a diode, this gentle giant only has about 4v of drop at 10mA and even at 50mA only drops about 18v. :lildevil:

I know, I know; I shouldn't even say such a thing in jest. But I couldn't help myself. ;)

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PostPosted: 05 Jul 2011, 23:19 
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Suncalc wrote:
Of course, I do know of one signal tube that makes a great low current rectifier with even lower forward voltage drop than the 6H6. The venerable 6AS7/6080. As a diode, this gentle giant only has about 4v of drop at 10mA and even at 50mA only drops about 18v. :lildevil:

I know, I know; I shouldn't even say such a thing in jest. But I couldn't help myself. ;)

I was jsut thinking tube rec. for one of the 4S preamps. I stick with sand for now. My other problem is lighting up the tubes with a 1A 24V wall wart. Not good.

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PostPosted: 06 Jul 2011, 00:10 
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May I suggest the 26Z6GT? The filament is 25v at 300ma. You could run the filament at 24v with almost no loss of efficiency. And you can use it in the voltage doubler mode to get more B+. With the right chassis, this may work well.

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PostPosted: 06 Jul 2011, 00:51 
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Suncalc wrote:
May I suggest the 26Z6GT? The filament is 25v at 300ma. You could run the filament at 24v with almost no loss of efficiency. And you can use it in the voltage doubler mode to get more B+. With the right chassis, this may work well.

I've not seen a double cct. Tube looks good though.

I just read an articel where the writer claims tube rectification produces a cleaner DC than sand. We all know sand is noisy ultra-fast, soft recovery or different. The writer went on to say tube rec. blocks a lot of the crud coming in from the mains.

Any comments?

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