- Schematic - Triode_Kingdom 16:10:21 11/05/09
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- In Reply to: RE: Here's Where It's Going... posted by nullspace on November 05, 2009 at 08:02:23
Here is the basic amplifier schematic. This circuit can drive the 211 into Class A2, and in that mode, it exploits the full 30W capability of the FS-100s. Distortion is very low, even drawing grid current, and this design sounds every bit as open and detailed as my earlier single-tube prototypes. I'll also post the PS schematics if you want, but they are similarly complex due to the multiple voltages required by the amplifier. It's not a project for newer builders.
- RE: Schematic - yes, pls post the PS schematic. - tubejack 17:31:45 11/06/09
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- In Reply to: RE: Schematic posted by Triode_Kingdom on November 05, 2009 at 16:10:21
A post of the PS schematic would be greatly appreciated. Thanks for sharing.
- PS Schematics - Triode_Kingdom 21:52:17 11/07/09
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- In Reply to: RE: Schematic - yes, pls post the PS schematic. posted by tubejack on November 06, 2009 at 17:31:45
Here are the schematics. I've included a brief commentary for each, and I'll be glad to answer any questions.
The original HV supply is shown below. A Hammond 20H choke is located in the negative leg of the supply to stay within its maximum voltage rating. The 5R4GYB is used primarily to delay B+ until the 211 heater is on. It also drops the voltage about 50V, which avoids overstressing the FS-100s. The next version will relocate the 5R4GYB (but not the choke) to the positive leg and will eliminate two diodes from the bridge. In other words, it will be a hybrid SS/tube bridge rectifier. Although I haven't had problems with diode hash, a hybrid supply should be virtually noise-free.
The LV supply uses a trick that I developed specifically for this amplifier. A single transformer drives a capacitor-input filter and a choke-input filter simultaneously. As you can see from the schematic, it only takes a single diode to isolate the higher voltage of the cap-input section from the choke. Unlike supplies that use series resistances to create multiple output voltages, both of the primary sections of this supply (+510V and +360V) can deliver significant current with minimal voltage sag.
The bias supply is straightforward. The -80V output provides the relatively large current needed for the 6BL7 cathode follower; the other output varies grid bias on the cathode follower as a means to adjust the 211's anode current. Note that this is a shunt adjustment. If the adjustment pot becomes dirty or otherwise fails, full negative bias is delivered to the cathode follower.
Finally, here's a rather shoddy photo of the power supply deck in its "trial fit" stage. The deck is approximately 12" x 8". I designed this before I had access to a mill, and it was cut by Front Panel Express from 0.150" aluminum. They did an excellent job. This holds everything shown in the power supply schematics except the 5R4GYB and bias adjustment.
Edits: 11/08/09
- FS-100 Bonus Schematic :) - Triode_Kingdom 22:38:50 11/07/09
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- In Reply to: RE: PS Schematics posted by Triode_Kingdom on November 07, 2009 at 21:52:17
Just thought I'd throw this out for good measure. It shows how a 3P3T non-shorting rotary switch can be wired to select the FS-100 output impedance. This is the best I could come up with - if someone here has devised a simpler switching scheme, please post it!
- FS-100 Bonus Schematic :) - Triode_Kingdom 22:38:50 11/07/09
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- PS Schematics - Triode_Kingdom 21:52:17 11/07/09
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- RE: Schematic - nullspace 08:07:53 11/06/09
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- In Reply to: RE: Schematic posted by Triode_Kingdom on November 05, 2009 at 16:10:21
Thanks for posting that. I had looked around on TubeDIY for info, but the links to the schematic were broken. That looks pretty sweet; I'd like to do an 809 A2 amp at some point in time.
500Vdc or thereabouts is my limit. I'm pretty awestruck by the huge amps seen around here and on the GM70 boatanchors mailing list, but I wouldn't be comfortable working around that much voltage.
Regards,
John
- RE: Schematic - yes, pls post the PS schematic. - tubejack 17:31:45 11/06/09
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