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Old 28th June 2009, 09:31 AM   #111
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Quote:
Originally posted by Wavebourn


You can swap, but you should consider base currents, that means resistors should have smaller values, but the capacitor should have higher value.
Oops... I did not understand your question, I was thinking about swapping of P-type mosfet by PNP transistor, because in microphone preamps I used exactly PNP transistors, before found source of suitable P-type MOSFETs.
No, you can;t. But you can use NPN transistor instead of N-type MOSFET, Base-emitter breakdown voltage is about 5 times less than gate-source bias for given current, so adjustment will be needed. Also, BJT have a secondary breakdown due to uneven base width.
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Old 28th June 2009, 03:29 PM   #112
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Hearinspace
Thanks Michael, I wondered about the grid current . But as far as the power supplies go, the diagram makes it look like current from the 210V loop might also have to go through the 60V supply and if so I'm wondering how that happens. Am I wrong about that?
I suggest printing out the schematic, and completing the loops visually. It's easier to see that way. Remember, all the current from one end of a loop, has to return to the other end. The current from the 210V supply has to return to common. The only way it can do that is through the driver tube. So yes, it has to go through the 60V supply too. But the 60V supply also supplies grid current to the output tube. This current does not return the common, but to the top of the 210V supply, where it returns to the 60V supply. So all of the 210 supply current goes through the 60V supply. But not all of the 60V supply current goes through the 210V supply, some of it circulates is a separate closed loop.

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Old 28th June 2009, 07:20 PM   #113
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Thanks Sheldon. My question is a very simple one and isn't really about the amplifier circuit per se. It's two pronged, first it's that I've never seen current from one supply run through the rectifier of another and wanted to make sure that that was really what was happening before I filed it away in my mind as a workable option. And then I also wondered (if that is the case) doesn't the less filtered 60V supply add noise to the previously filtered 210V current? None of this is meant to be critical or judgmental in any way. I'm just trying to learn.
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Old 28th June 2009, 07:48 PM   #114
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Quote:
Originally posted by Wavebourn
Oops... I did not understand your question, I was thinking about swapping of P-type mosfet by PNP transistor, . . . .
Actually, you DID understand my question - I just asked it wrong.
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Old 28th June 2009, 09:26 PM   #115
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Hearinspace
Thanks Sheldon. My question is a very simple one and isn't really about the amplifier circuit per se. It's two pronged, first it's that I've never seen current from one supply run through the rectifier of another and wanted to make sure that that was really what was happening before I filed it away in my mind as a workable option. And then I also wondered (if that is the case) doesn't the less filtered 60V supply add noise to the previously filtered 210V current? None of this is meant to be critical or judgmental in any way. I'm just trying to learn.
Yes, the current trough the input tube is in series, so has to go through the series path of the supply. Unusual for us, but not really exotic, if you think about it. Turn the 60V supply off - just the secondary, diodes, and any other series resistance in the path.

Hadn't thought about the second part of the question. Interesting. I guess that it would add a little ripple, but not the full ripple amount of the 60V supply. Since they are in series, the two supplies form a divider, so t would depend on the output impedance of the 210V supply, as to how much the ripple in the 60V supply would modulate the voltage on the 210V node. Shouldn't be hard to design for low ripple there. A low impedance supply at 120Hz would have current ripple at that node, but not voltage ripple. Curious as to Michael's take.

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Old 28th June 2009, 09:52 PM   #116
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Quote:
Originally posted by Hearinspace
Thanks Sheldon. My question is a very simple one and isn't really about the amplifier circuit per se. It's two pronged, first it's that I've never seen current from one supply run through the rectifier of another and wanted to make sure that that was really what was happening before I filed it away in my mind as a workable option. And then I also wondered (if that is the case) doesn't the less filtered 60V supply add noise to the previously filtered 210V current? None of this is meant to be critical or judgmental in any way. I'm just trying to learn.
It's a little tricky to get one's head around, I admit. Here's how I think about the current loops. The driver load goes across the 60V and 210V in series. The current through both is the same because they're in series. They are both supplying power to the driver load. The power grid loop is separate as shown. Noise in both loops is isloated by the MOSFET, and the 2 loops are isolated from each other by the bootstrapped MOSFET source resistor (not shown here). The 210V supply needs to be clean to avoid coupling noise through the internal resistance of the driver tube referenced to the power tube cathode.

There are some shortcuts in this circuit because it's on a breadboard. For example, I would prefer the bias referenced to the power tube cathode rather than ground, but this is convenient for testing.

Cheers,

Michael

PS I worry about a short from the top of the 1100V supply to ground so I add a big reverse biased diode across the 210V supply.
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Old 29th June 2009, 05:08 AM   #117
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Michael, Thanks for taking the time to post so much detail. It's really appreciated. I'm digesting it now. Maybe post again once I understand more, thanks!!!
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Old 29th June 2009, 10:18 PM   #118
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Quote:
Originally posted by ilimzn


A common CQY99 IR LED, for which there are probably dozens uppon dozens of equivalents, has a forward drop of 1.25V at 20mA, 1.15V at 10mA, so it should definitely be usable here...
Are there devices like this and the MOSFET (in the way Michael used it on the 417A) that can be used for larger currents and voltages? I'm thinking specifically of the VT-25 cathode - very roughly 30VDC/28mA. I looked for both IR diodes and MOSFETS last night and didn't find any but there are thousands of models and I could have missed them.
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Old 29th June 2009, 10:41 PM   #119
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Quote:
Originally posted by Hearinspace


Are there devices like this and the MOSFET (in the way Michael used it on the 417A) that can be used for larger currents and voltages? I'm thinking specifically of the VT-25 cathode - very roughly 30VDC/28mA. I looked for both IR diodes and MOSFETS last night and didn't find any but there are thousands of models and I could have missed them.
They have cut-off voltages below 4V. Try 27V Zener with 100 Ohm resistor in series.
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Old 29th June 2009, 10:54 PM   #120
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Quote:
Originally posted by Hearinspace


Are there devices like this and the MOSFET (in the way Michael used it on the 417A) that can be used for larger currents and voltages? I'm thinking specifically of the VT-25 cathode - very roughly 30VDC/28mA. I looked for both IR diodes and MOSFETS last night and didn't find any but there are thousands of models and I could have missed them.

I wonder if something like this would work?
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