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Old 23rd February 2010, 10:14 PM   #41
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Yes I did see some of his schematics. There is sand in all of them if you include rectifiers. ;-)

I will continue to consider the idea of common mode choke. How can I model it in PSUDII? I would like to keep ripple <1mV.


Those snowboarders are NUTS!!!!!!. My kids do it (not to that level Thank the Lord!). Me, I'd prefer to keep flying without a plane reserved only for that area in my mind... At least that way I can maintain my dignity! It also doesn't hurt as bad either
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Old 23rd February 2010, 10:47 PM   #42
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boywoder,
As tubelab said. It satifies both worlds. FETS have a VERY small gate current compared to the grid current of driving a kt88 directly, particulary at high output levels. Because of the low gate current, a higher impedance is possible. However, the output impedance of the FET source follower is low and is dictated by the source resistor. What the kt-88 sees as far as drive impedance is the source resistor which still must be <100k per the tube data sheet. IIRC

There is a limit as to how high of an impedance you can go to. It is set by the gate capacitance.
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Old 23rd February 2010, 11:37 PM   #43
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There is a limit as to how high of an impedance you can go to. It is set by the gate capacitance.
In voltage follower service, the crucial parameter for FETs is reverse transfer capacitance (Crss). Select a part whose Crss is low and stable. When Crss is too big, HF info. loss occurs.
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Old 23rd February 2010, 11:46 PM   #44
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Thanks Eli. Your right.
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Old 24th February 2010, 01:03 AM   #45
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Okay here is the latest versions. I am trying to heed all of the advise that has been wonderfully given.

When it comes to the common mode choke, recognizing that the FET heat dissipation can be an issue, I decided against that particular solution for the time being. By not going the common choke path, I don't mean to imply or doubt for one second what George is saying true about that topology.
I feel that a seperate tranny under the hood $10.00 each, schottky diodes (I have a big tube of them Eva which I used on my Alephs), and a regulator will serve the same purpose. I have almost all of those parts in my inventory. I wanted to keep the C- as clean as possible which with the 30H choke and caps is very quiet. With the B1+ on a seperate circuit it will still be quite and I can fit the whole thing including the tranny on a small board under the hood. The other deciding factor was that I am out of room on top at this point using the monster caps.

If they (the monster caps) don't work well like George's experience indicates as a possibility, then I will have a little more room on top.

During the bread board stage of the amp, I plan on evaluating the temp of the FETs on the heatsinks. If the temp is an issue then in goes the b1+ rail supply before it becomes a problem. I want to test both setups anyway and I can do this without the tubes installed.

I have settled on a PT with the following specs. 760Vct @ 180mA / 85 V @ 100mA / 5V @ 3A. If need be I can drop a few volts with a series resistor.
Attached Files
File Type: pdf Visio-Opus5_0_Amp v8.pdf (44.7 KB, 124 views)
File Type: pdf Visio-Opus5_0 Power Supply_v8.pdf (43.8 KB, 105 views)
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Old 24th February 2010, 01:16 AM   #46
smbrown is offline smbrown  United States
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Quote:
Originally Posted by SGregory View Post
Okay here is the latest versions. I am trying to heed all of the advise that has been wonderfully given.

When it comes to the common mode choke, recognizing that the FET heat dissipation can be an issue, I decided against that particular solution for the time being. By not going the common choke path, I don't mean to imply or doubt for one second what George is saying true about that topology.
I feel that a seperate tranny under the hood $10.00 each, schottky diodes (I have a big tube of them Eva which I used on my Alephs), and a regulator will serve the same purpose. I have almost all of those parts in my inventory. I wanted to keep the C- as clean as possible which with the 30H choke and caps is very quiet. With the B1+ on a seperate circuit it will still be quite and I can fit the whole thing including the tranny on a small board under the hood. The other deciding factor was that I am out of room on top at this point using the monster caps.

If they (the monster caps) don't work well like George's experience indicates as a possibility, then I will have a little more room on top.

During the bread board stage of the amp, I plan on evaluating the temp of the FETs on the heatsinks. If the temp is an issue then in goes the b1+ rail supply before it becomes a problem. I want to test both setups anyway and I can do this without the tubes installed.

I have settled on a PT with the following specs. 760Vct @ 180mA / 85 V @ 100mA / 5V @ 3A. If need be I can drop a few volts with a series resistor.
So the 1.5mh and 1nf cap are forming what you are calling a "hash filter"? To me, that just mean not showing up at mom's house on Sunday evening for dinner. However, if you use an oil cap in the first position, can you avoid a hash filter? Seems too complex. Maybe I've just not heard the sonic benefits. Secondly, and personally, I'm thinking the advantages of the FET are being outweighted by the circuit complexity. Seems like you gain a few DB in headroom in exchange for a seperate PS, FETs, sonic impact in the circuits, etc... I think when I build my version of this it will be a garden variety fixed bias supply to which I can add the FET follower at some later date to see what it does. When I tried this in the TubeLab SE amp it did add bass weight from other SE 300b designs but I felt I gave up a lot in midrange tone, realism, etc. (no offense George, just my experience). I also wonder if some kind of simple protection circuit can be devised - a fuse is too simple. A circuit that protects the tubes and Fet seems to be in order.
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Old 24th February 2010, 01:48 AM   #47
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When I tried this in the TubeLab SE amp it did add bass weight from other SE 300b designs but I felt I gave up a lot in midrange tone, realism, etc. (no offense George, just my experience).
No offense taken. When I tried the same amp with and without PowerDrive the differences ranged from minor (freedom from overload recovery on a 45 TSE) to dramatic (more power authority and better transient behavior on a KT88 amp). Loss of realism and tone wasn't noted.

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Okay here is the latest versions. I am trying to heed all of the advise that has been wonderfully given.
I just noted another little thing. The math shows that a 1/2 watt resistor on the screen grids of the KT88's is sufficient. Experience has proven otherwise. I have blasted even 1 watt resistors on a Simple SE with EL34's by overdriving the amp with my guitar preamp. Definitely worse case, but resistors are cheap. I have been using 2 watt screen resistors in everything since then, and haven't blown one up since.
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Old 24th February 2010, 01:59 AM   #48
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However, if you use an oil cap in the first position, can you avoid a hash filter?

It's not the dielectric that matters. The large capacitance immediately following the rectifier is what matters. Large value cap. I/P filters are associated with tiny conduction angles. Part and parcel of a tiny conduction angle is a sharp, highly "triangular", ripple waveform. Fourier's Theorem tells us that such a waveform contains overtones of the ripple fundamental extending well up into RF. The capacitance associated with the many turns of wire associated with "typical" filter chokes shorts the choke out at RF. The "hash" filter kills the RF crud, before it gets to the choke.

Get the Physics right, before worrying about refinements!
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Old 24th February 2010, 03:10 AM   #49
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George, I will trust your experience. A two watt will be fine.

smbrown. Your opinion is certainly valid for your build. In this one the Hash filter and FETS stay. Complexity is a personal experience and preference. The power supply is built around what the circuit needs to perform at its optimum. Some well spent time planning the layout will alleviate the complexity. That is my next step. Maybe there are different ways to skin the cat to achieve the same result, but I like and understand this way enough to have a reasonable chance of success.

With the Solid State and Hollow State amps that I have built (Only SETS so far in tubes) IMHO, the more quite and lowest impedance supplies win in the musicality department. Now if you wanted to comment on the merit of the monster caps I am using at least up to this point in design, I am not sure I would have a leg to stand on. They are "Over the Top, in fact way over the top". A good example of flying without an airplane...... I'll let my osc scope tell me how I land.
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Old 24th February 2010, 03:22 AM   #50
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Here is the PSUDII simulation of the waveform entering the main choke to support what Eli is saying.
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File Type: pdf Chart1.pdf (34.4 KB, 103 views)
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