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All,

I am starting on a new project updating the pair of 845/211 SET amplifiers that Grover built for Bob Olson back a couple decades ago. The amplifier has had some issues with excessive noise (hum), which has not been solved despite Charlie Phelps and Dave Berning taking a look at it to figure out why they are so noisy. Anyway, I have agreed to take on the task with the current owner to get these quiet, making the construction a bit safer (Grovers' builds at this time were infamous for breadboard construction with flying leads and generous application of RTV silicone applied for insulation from the 950V B+. Not something you want to stick your hands in when running. Also, at the request of the current owner, the amp will be optimized for 211 tubes rather than maintaining 211 and 845 convertibility. The voltage swing for a 211 is much less than for an 845, but it is power limited unless one drives it into A2 operation. Therefore, my approach is to use a single front end tube (D3A or similar) for voltage amplification, with a MOSFET source follower direct coupled to the grid of the 211 to supply grid current as it transitions in to A2 operation. This is the "Power Drive" concept that George at TubeLab advocates, as well as other builders such as Al Moglia as Bartola Valves. The amp would go from a 12W Class A1 to a 30W Class A2 output capability.

I have designed a couple of power supply boards using DC Link capacitors and KEMET high reliability electrolytic capacitors for the 211 SET rebuild project I am working on for Myles Burgess. I am doing the final check on them before sending them out to fabrication, but I have designed the Driver Power supply board to be applicable for other designs up to 600V. Since per board costs in qty are pretty cheap, I can make extras for use in other builds.

A few words about the boards.

The HV board is a CLC circuit for up to 1200V output using a FW rectifier with three (3) stacked UF4007 diodes per side with 0.01uF/1000V bypass caps to ensure equal voltage handling on each diode. Using a series SiC diode at the output of the FW rectifier to prevent any reverse recovery spike on the UF4007 diodes, optionally an offboard damper diode can be used for the same function plus soft start. Included is the provision for offboard power chokes, either on the HV side or the Ground side depending on whether your chokes can take the elevated B+. The DC link capacitors are Vishay 20uF/1200VDC, the two series connected electrolytics are nominally 390uF/600VDC KEMETs (15000 Hr). Of course using voltage equalizing resistors in parallel with the capacitors.

The Driver board is a CLCRC circuit for up to 600V output. Rectifier can be either bridge (no CT) or FW (with CT, omitting two diodes). Again, using UF4007 diodes with SiC or tube damper diode at the output of the rectifiers. Instead of a choke, the pad spacing will accept a power resistor in the first Pi filter. The design uses Panasonic 40uF/600VDC DC link capacitors and KEMET 220uF/600V long life electrolytic caps. There are two DC outputs after each filter stage.

Attached are some pictures of the design, the layout is compact and should fit nicely in a chassis. The boards are designed with the traces on the back side but through-hole pads.

Let me know if you are interested in a few boards. I do not expect the cost to be more than a few $ per board.

David

More pics

Now with the Driver Supply Board (more general application)

Last file

Right on, Dave. Another great project amplifier that will undoubtedly entice you to build a pair for yourself!:-D They are noisy devils and needed someone to do a rework on them. Look forward to hearing them.

Jim,

No 211 amplifier for me...I have a GM70, plus SE output transformers for a transmitter tube amplifier are very rare and very dear. I am using the MQ FS-100 from Grover's amp. I have a pair of Tamura outputs on my GM70's.

What this project will propel me to do is rebuild my GM70 amps to a more compact (and more portable) size and rework the driver stage for A2 operation to get additional power. My current transformer coupled single stage driver does not have the voltage swing or current drive capability for much of any A2 operation (it is at least is immune to blocking at the transition), I need a two stage driver with PowerDrive to get there. The boards I am designing will work with that setup, should I choose. I am also looking at an all poly cap power supply arrangement, but probably not worth the extra cost.

The boards are still being refined. I caught an error in the direction of rectifier diodes in the bridge on the Driver Board (quickly corrected), but based on some feedback from Charlie, I reworked the HV Power Supply board rearranging the rectifier and adding another filter stage. This will work better as a generic power supply board, basically a higher voltage equivalent to the Driver Power Supply board which is similarly a CLCRC arrangement.

David

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Walt

Walt,

I am using Rod Coleman filament regulators. I have them in my GM70 amps and they are reliable, and dead quiet at 20V/3A filaments. I would not use anything else for a transmitter tube amp.

David

If anyone can do it, it is Dave. Dave's tube amps are the quietest I have ever heard. Like dead quiet, even into 100db eff horns.

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Walt