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Old 28th February 2009, 04:11 PM   #11
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Quote:
Originally posted by Sheldon


True, if you want all tube. But if you don't mind a Mosfet follower to drive the grid, it's three extra cheap parts; two resistors and the FET.

Sheldon

hey-Hey!!!,
Use a cascode FET, deals with the screwey gate-drain capacitance, or maybe a $2 12HL7 pentode. The smaller tubes will be quite able to run the power requirement, and without the filament supply the bigger tubes need.
cheers,
Douglas
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Old 28th February 2009, 05:23 PM   #12
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Anyway did a few sims with a 6C45 driver that indicates A1 over 12W at Ra 4kohm, 600V/130mA, 5% THD. Going A2 with a MOSFET-follower, as suggested, doubles this power.

So if we are talking "SE on a budget" A2 could be a interesting solution.
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Old 28th February 2009, 06:01 PM   #13
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This is a Soundlab-inspired A2 solution. Had no model of 2SK2700 so I took what I had:
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Old 28th February 2009, 09:20 PM   #14
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Bandersnatch
Use a cascode FET, deals with the screwey gate-drain capacitance, or maybe a $2 12HL7 pentode. The smaller tubes will be quite able to run the power requirement, and without the filament supply the bigger tubes need.

In this application, at pretty high voltages and used a a follower, the D-S voltage will be high enough for most FET's that the CRSS will be low and flat. I've used FQPF2N80 for a couple of projects. It's an 800V part, and the CRSS is about 6pF at 25 Vds and doesn't rise much until Vds goes below 6V. It's also an insulated package, which is handy as long as you don't need to run too much bias current.

Sheldon
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Old 1st March 2009, 12:28 AM   #15
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hey Sheldon,
I've tried single FET's for current regulators. Even with lots of voltage across them, the cascode is always the one that sounds better. For a source follower it is a bit more difficult. I'd still protect for it whilst designing the circuit.
cheers,
Douglas
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Old 1st March 2009, 12:46 AM   #16
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Bandersnatch
I'd still protect for it whilst designing the circuit.
Not sure I understand what mean there.

You could substitute a current source for the resistor under the follower. I tried that on one amp with DN2540 or the IX chip for the current source, because both are depletion devices and simple to set up. But with either one it would occasionally oscillate. Tried playing with stopper values but couldn't seem to make it perfectly stable, so I just went back to about a 10k resistor. Sims might help sort that out.

Sheldon

edit: when I said earlier that only three extra parts are needed, I neglected to include the negative bias supply that wouldn't be there unless the design already is fixed bias. Not difficult or expensive to do, but it is more parts.
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Old 7th March 2009, 04:24 AM   #17
avisa is offline avisa  India
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I Thank you all for your kind help.

It looks like the 813, is the tube that will meet my needs - to start with, I could operate it at a lower voltage, and if required, increase the operating voltage to get more power.

I have been reading a lot of info about 813 on this forum, and on the net, in the past few days.

Also another thing about the 813 tube that i have understood is, that the driver stage can remain common for both the higher and lower voltage operating points of the 813 tube.

Thanking you all once again.

- Avi.
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Old 7th March 2009, 05:02 AM   #18
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Quote:
Originally posted by Bandersnatch



hey-Hey!!!,
Use a cascode FET, deals with the screwey gate-drain capacitance, or maybe a $2 12HL7 pentode. The smaller tubes will be quite able to run the power requirement, and without the filament supply the bigger tubes need.
cheers,
Douglas

Doug,

Do you have a schematic to show what you are speaking of with using a 12HL7 or perhaps in my case a 6CL6 (also a video power pentode). Does the FET still have to be used?
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Old 7th March 2009, 09:38 AM   #19
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Quote:
Originally posted by revintage
This is a Soundlab-inspired A2 solution. Had no model of 2SK2700 so I took what I had
Do you mean a Tubelab PowerDrive?

http://www.tubelab.com/PDcookbook.htm
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Old 7th March 2009, 10:07 AM   #20
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Quote:
Originally posted by rcavictim



Doug,

Do you have a schematic to show what you are speaking of with using a 12HL7 or perhaps in my case a 6CL6 (also a video power pentode). Does the FET still have to be used?
Nothing yet published on it, but drawing one is straightforward. 6CL6 is another fine tube; no characteristic pushed to the limit and easy to implement.
cheers,
Douglas
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