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Old 25th January 2010, 03:11 PM   #1
dmcgown is offline dmcgown  United States
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Talking Thanks to George for PowerDrive

Just want to thank George for his PowerDrive circuit. I recently built an SV811-10/SV572-10 stereo amp using a triode-connected 7788 driver using the Powerdrive circuit and cookbook on the webpage. The only other change to the PowerDrive circuit is using Gary Pimm's current source on the top of the 7788 instead of a 10M45.

The driver makes the amp. It has tremendous authority and doesn't run out of steam driving my Basszillas. I have used other SET amp driver topologies, and they just do not handle the dynamics anywhere as well as this circuit. It is the best amp sounding I have built by far, and I plan to rebuild my other SET amps using PowerDrive.

The amp uses Tamura 10K output transformers, vintage potted power transformers and chokes for B+, with independent driver and Mosfet and bias power supplies.

Again, Thanks!

David
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Old 25th January 2010, 05:11 PM   #2
wa4swj is offline wa4swj  United States
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David,

I would also like to add my thanks to George. I built a 300B SET amp recently with his PowerDrive circuit and the amp is simply wonderful. I still need to make a nice wood base for it, but here it is such as it is right now sitting on some 2x6's.

Thanks George!

Click the image to open in full size.

Wish I could get the picture to display in a larger format. Here's the URL:

]300B Amp w/866A Rectifiers pictures from hobbies & interests photos on webshots

There are several additional pictures of the amp there plus other amps I've built including a SimpleSE.

This amp uses ElectraPrint output transformers as well as a power supply that uses (obviously) mercury vapor rectifiers. It is amazingly quiet and, like yours, is very dynamic although not as powerful of course. The mounting plate is 3 mm aluminum made by Front Panel Express.

Regards,
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Last edited by wa4swj; 25th January 2010 at 05:16 PM. Reason: Add additional info
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Old 26th January 2010, 04:01 AM   #3
w5jag is offline w5jag  United States
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I would like to see your circuit for the SV811's; I have some of those floating around here somewhere.

What's a 7788?

Win W5JAG
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Old 26th January 2010, 12:35 PM   #4
dmcgown is offline dmcgown  United States
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A 7788 is a very high transconductance Pentode that I am using triode connected (G2 connected through a 100 ohm carbon resistor to the plate). As a triode, it has a mu of around 54 at my op point. Check out Pete Millett's data on similar Pentodes. I am using a Gary Pimm self biased Mosfet current source feeding the tube with about 20mA of current. I have parallel pair of 220 ohm resistors in the cathode (at least as I remember), with two Blackgate 470uF/6.3V N series capacitors as bypass. I have not yet explored using LED, diode, or battery bias for the driver, it sounds pretty damn good right now, I don't want to change anything.

As far as the SV811, I am using the version with a mu of 10, which allows me to get away a single stage driver (not counting the Mosfet source follower). I do not have a schematic of the amp, but please follow the verbal description to pick up the main features, as follows:

B+ power supply: 1360VCT/300mA transformer (RCA Mil-spec potted) into a full wave rectifier consisting of 3 UF4007 diodes in series with 220k resistors in parallel with the diodes. These feed directly through a 6AX4 damper diode, the heater of which is on a 2 minute time delay (Amperite) so that B+ is not applied until after everything is up and running on the amp. Also allows me to easily disable B+ during troubleshooting for safety. Output of the 6AX4 feeds a series pair of 40uF/450V electrolytic caps, then split into separate Left/Right channel LC filters consisting of a 6H choke, a series pair of 330uF/450V electrolytic caps, which are bypassed by a 6uF/660VAC motor run capacitor, plus a .22uF/1000V oil cap. This gives a B+ of around 750V.

C+/- Driver power supply: 660VCT/180mA transformer, which feeds both a positive full wave rectifier (5AR4/5Y3) for the driver, and a negative full wave (UF4007) for the tail of the Mosfet. The driver (C+) consists of a 20uF/600V Solen input cap, split into separate Right/Left channel LC filters consisting of an 8H/200mA choke and a 330uF/450V electrolytic, which is bypassed with a 30uF/330VAC motor run. The negative supply for the Mosfet consists of a 20uF/600V Solen input, into a 10H/125mA choke, into a 330uV/450V electrolytic cap bypassed by a 10uF/400V PP capacitor (I believe it is a Jantzen.

SV811-10 Heater supply: Schottky diode bridge into CLC supply for each SV811-10 heater. I don't have the details (I am at work and reporting this from memory), but I believe it consists of 12000uF/12mH/12000uF, feeding into a common-mode choke at the tube.

Driver Heater Supply: Schottky diode bridge into CRC supply to supply 6.3V to the 7788 drivers.

Bias Supply: Used an inverted 12.6V/120V lowpower transformer to the 6.3 V driver heater supply transformer (connecting the 12.6V winding to the 6.3V winding and using the 120V transformer output. Used UF4007 diodes, CLCLC filter (don't remember the values). It gives me up to around -50VDC, which is enough to set the SV811-10 to a current of 70mA or higher. I like to run the SV811-10 hard (85mA, basically at plate dissipation) to ensure the gettering system is activated. I run the SV572-10 even harder (100mA)

The circuit in comparison is very simple, basically just like the single stage PowerDrive. I tried to keep the current running through the Mosfets down to around 25mA, so sized the resistor in the tail of the Mosfet accordingly.

I hope this helps.

David
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Old 26th January 2010, 06:03 PM   #5
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I thought it sounded fantastic too!!!
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Old 28th January 2010, 12:46 PM   #6
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Quote:
Originally Posted by wa4swj View Post
The mounting plate is 3 mm aluminum made by Front Panel Express.
Just out of interest, how much did that top plate cost? Makes for a very slick looking amplifier.

EDIT:

Wow, looks like you've got a few tube amps there, eh? By the looks of it there would be no need for heating...

Click the image to open in full size.
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Last edited by MellowTone; 28th January 2010 at 12:54 PM.
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Old 28th January 2010, 01:12 PM   #7
wa4swj is offline wa4swj  United States
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Hi Mellow,

Yeah, they help keep the house warm and they also rock the place!

The panel you're asking about is pretty large (21" x 17") so it is sort of expensive. If I remember correctly it was just over $205. This entire amp wasn't cheap, but that wasn't what I was looking for. I wanted a very, very good amp and it turned out to be excellent.

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