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Old 18th May 2010, 06:31 AM   #81
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Hi Ian, have tried base resistors. It depends very much on Cob whether you get much control that way. With the low Cob types needed here, HF control is not very effective until higher values are used, which risks spoiling performance at 1.2A Ic.

I found the best approach was to use transistors with similar HF rolloff, or the same power transistor in both positions. A slow power transistor coupled to an HF-capable driver transistor is certain to lead to a 'tail-chase' as the slow transistor tries to keep up with rapidly changing command!
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Old 21st May 2010, 03:58 AM   #82
cfan is offline cfan  United States
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Quote:
Originally Posted by Rod Coleman View Post
Hi Ian, have tried base resistors. It depends very much on Cob whether you get much control that way. With the low Cob types needed here, HF control is not very effective until higher values are used, which risks spoiling performance at 1.2A Ic.

I found the best approach was to use transistors with similar HF rolloff, or the same power transistor in both positions. A slow power transistor coupled to an HF-capable driver transistor is certain to lead to a 'tail-chase' as the slow transistor tries to keep up with rapidly changing command!

Hi Rod, you call the positive leg of the circuit a Gyrator but it looks like a capacitor multiplier. Are they same? Thanks.
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Old 21st May 2010, 07:32 AM   #83
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Yes!
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Old 21st May 2010, 08:17 AM   #84
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Effectively the same as cap multiplier, yes.
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Old 21st May 2010, 08:35 AM   #85
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Default Mark II version

I have been working on a Mark II version of this circuit, mostly because Kurt (Zigzagflux) showed me that it measured poorly if the MJ150xx transistors (that I suggested) were used in place of the Japanese originals.

The 2SC5511 is now obsolete.

While looking through the alternatives (which must have unusually high Hfe at only 2 or 3V Vce, and LOW Cob) I have looked again at the schematic and found ways to reduce the output ripple voltage.

I'll post a schematic if it achieves the aims, and looks to have the same stability that the Mark I has - no notable drift in six years working life.

It's remarkable that I have had requests for PCBs to support this circuit all this time, and I feel that if the new circuit is a success, I'll finally do a batch of PCB.

And since the transistors are not easy to find, I'll buy a stock of those too, and supply it as a set (PCB + transistors) or assembled/tested. (any preferences?)

I assume that most builders will use them for 300B/2A3 power amps or preamp duty (26, 10Y, LS8, LP2 &c &c) with current of 1 to 2.5A

Is that a good idea? Post here or PM if you are interested in some hardware.
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Old 21st May 2010, 09:10 AM   #86
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I definitely will order several sets. I have no preference between a kit or assembled/tested. If possible, I like to use them for 3C24 requiring 6.3V @3A filament current. I am not sure if 3A is significantly higher than your 2.5A current limit.

Last edited by agent.5; 21st May 2010 at 09:16 AM.
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Old 21st May 2010, 09:11 AM   #87
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Hi Rod !
You have new design schematic for DHT????
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Old 21st May 2010, 11:47 AM   #88
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Originally Posted by Rod Coleman View Post
And since the transistors are not easy to find, I'll buy a stock of those too, and supply it as a set (PCB + transistors) or assembled/tested. (any preferences?)

I assume that most builders will use them for 300B/2A3 power amps or preamp duty (26, 10Y, LS8, LP2 &c &c) with current of 1 to 2.5A

Is that a good idea? Post here or PM if you are interested in some hardware.
Sounds like a great idea to me Rod, I'd be interested in a few.
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Old 21st May 2010, 12:10 PM   #89
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Would there be any problems configuring the boards for 71A/112A tubes? 5 volt 0,25 Amps.

Geir
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Old 21st May 2010, 03:41 PM   #90
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I would consider buying a stash if you can prove good performance with the Japanese xistors. Still would surprise me to see good HF performance and high gain at low Vce, but my experience with BJT's has been limited - just pump the base current to get them to perform.

For the time being, I've resorted to the simple LT1086 CCS, which works nicely, all things being equal. But I suspect your upgraded boards would fit in my existing space, so they could potentially be retrofitted with no changes.
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