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Old 19th May 2008, 07:43 PM   #1
hesener is offline hesener  Germany
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Default driving 845's

Looking for a circuit how to drive a 845 in class-A PP, with B+ at 800V. My initial thought was to build a differential amplifier with a 6H30, gain stage with EL84 and drive the gate of the 845 with a source follower (yes, a MOSFET.... - similar to the Kronzilla amplifiers)......

any better ideas? thanks for your help
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Old 19th May 2008, 08:13 PM   #2
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The 845 is a nasty load: high input voltage at the grid due to the low voltage amplification, large internal capacitances, and it likes to pull grid current even before Vgk goes positive. About the only viable choices are a stiffly driven IST or a source follower.

Go with the source follower.
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Old 19th May 2008, 08:46 PM   #3
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I generally think that the requirements for driving the 845 is a little bit exaggerated.
According to the RCA data sheet: Cgp=13,5pf, Cgk=6pf, u=5,3
This makes for a input C of less than 80pf+strays. For the chinese tubes, his is probably on the low side of reality but it is not that far off.
I think the problem is to acheive the big voltage swing into a relatively low grid resistor with decent linearity.
And of course they DO draw some current before 0v at the grid so you have to live with that they clip at Vg 2-3V below zero. Or design a A2 driver.
I have driven 845pp with a 5687 diff pair with good reults. I later tried 12b4 and 6bx7, the latter being VERY good.

/Olof
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Old 21st May 2008, 05:36 AM   #4
Cobra2 is offline Cobra2  Norway
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Is the GM 70 comparable to 845?

Arne K
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Old 21st May 2008, 06:38 AM   #5
Yvesm is offline Yvesm  France
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Let's be somewhat minimalist

According to Pete Millet:

Click the image to open in full size.

or even:

Click the image to open in full size.

More is too much

Yves.
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Old 21st May 2008, 01:21 PM   #6
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C5 looks to be backwards in the second drawing. 'Lytic would die a quick death.
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Old 21st May 2008, 01:31 PM   #7
Yvesm is offline Yvesm  France
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Quote:
Originally posted by zigzagflux
C5 looks to be backwards in the second drawing. 'Lytic would die a quick death.
And the 845 would follow it quickly

You're right ... if it were a 'Lytic !
Bent side is supposed to denote the outside foil.

Yves.
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Old 22nd May 2008, 04:32 AM   #8
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Default Monster MUstage Driver!

Hi Yves,

On your two drawings you could put a choke coupled MuStage Driver like a EL86 or a 7591 and run the same voltage as the output stage !! You would have a monster voltage
swing, about 760v&960v respectively.
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Old 22nd May 2008, 05:24 AM   #9
Yvesm is offline Yvesm  France
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Default Re: Monster MUstage Driver!

Quote:
Originally posted by Ben Reginato
Hi Yves,

On your two drawings you could put a choke coupled MuStage Driver like a EL86 or a 7591 and run the same voltage as the output stage !! You would have a monster voltage
swing, about 760v&960v respectively.
Hi Ben,

Not exactly what you suggest, but I've also tried that wich is also able to deliver huge drive:

Click the image to open in full size.

It had my preference, but the builder was affraid by the choke load

Yves.
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Old 22nd May 2008, 05:35 AM   #10
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Quote:
Let's be somewhat minimalist
Don't get me wrong the 12HG7 has got some huge balls for such a small tube, but isn't the 813 an easier to drive than the 845. I could very well be wrong. I am sure something of the sort could drive a 845 though. If you want over kill you could drive it with a KT66 or KT88 triode strapped, or you could leave it a pentode, could choke-load or CCS-load for max effect!

Quote:
Is the GM 70 comparable to 845
Somewhat, the GM70 is a monster, it is on my list to build. The GM70 as far as I know, is easier to drive than the 845; it also has a higher max plate dissipation at 125watts vs. 100watts. Designed right and it can kick out close to 50watts of SET magic. One of the special things about this tube is that it can be run at very low voltages to start. You can put 400v-450v through the GM70 and you will basically have an over-grown 300B. To get the max power you need closer to 1kv, but unlike the 845 and 211 it doesn't really NEED 1kv to "come alive". They are also a lot cheaper than 845's, but the base is very unique.

Could you just use an interstage, or still not meaty enough?

Cheers

James

edit: ^^^ got to it before me with the 6V6, especially choke loaded it could handle it with ease.
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