Most Recent il Monstro 845:845 Schematic

 
Rect -- C --- L --- C --- L --- C --- L --- C --- Choke - 845 Driver
83     3uF   10H   10uF  10H  120uF  12H   24uF    48H
            82ohm       82ohm   |  155ohm         304ohm
                                |                         507VDC
                                |                        .001VAC
                                |                        63V Bias
                                |
                                ----------------- Choke - 845 Output
                                                   48H    552VDC
                                                  304ohm .001VAC
                                                         65V Bias

• B+ uses an oversize Hammond 300BX power tranformer - using the 110V 'world power' inputs. The 300 series Hammonds are built much better than the 200 series power transformers, still, one of the original 300BX trannies died after 6 months use and was replaced by Hammond.
   
  
• The rectifier is a mercury vapor type 83 - available as vintage only, these are probably considered hazardous waste.
   
  
• Small cap input - 3uF 660VAC GE motor-run cap. Using a small cap input give some of the benefits of choke input but allows the designer to adjust the output voltage by varying the size of this cap. The 83 rectifier is not compatible with large input caps at these voltages. Motor-run caps are used for good sonics, availability is various sizes, and toughness.
   
  
• Input Choke - Hammond 193J 10H 200mA, followed by 10uF 660VAC GE motor-run cap.
   
  
• Second Choke - Hammond 193J 10H 200mA, followed by paralleled 60uF 370VAC GE motor-run caps. The filter chokes are rotated relative to each other to reduce electromagnetic coupling.
   
  
• Decoupling Filter - Hammond 193B 12H 100mA, followed by 24uF 660VAC GE motor-run cap. This stage feeds the input 845 only, it reduces B+ feedback between the input and output 845 tubes. This stage is located inside the signal chassis to provide damping for any noise pickup in the umbilical.

• Each 845 has a dedicated Hammond 166R12 filament transformer - 12.6VAC at 8A
   
  
• Full Wave Bridge Rectifier uses 10A 50V Shottkey diodes
   
  
• Smoothing is via CLCLC using 10,000uF 70V CDE computer supply caps and Hammond 193V 0.15H 3.5A chokes. These chokes get very warm in operation, they are also heavy - 11 pounds each. I normally try to derate any Hammond iron since their specifications are generally limits, but there is no other viable off-the-shelf iron for this position.
   
  
• Voltage is adjusted by paralleling the first choke with high-power resistors to push the filament voltage above 10V in operation. This is critical to the sound of the amplifier, starving the filaments will kill the dynamics of the music.
   
  
• Filament caps are located in the raised box with the four filament chokes bolted to the top. The bridge rectifiers and filament transformers are under the top plate so their noise is largely shielded from the filter components. The filament chokes are rotated relative to each other to reduce electromagnetic coupling.

1.0VAC   --+
           |  Altec/Peerless 15095 input
5.0VAC   --+
          ( ) 845 Driver Tube
21.81VAC --+
           |  Magnequest EXO-173ni 
21.67VAC --+ 
          ( ) 845 Output Tube
81.7VAC  --+ 
           |  Magnequest EXO-50 output XFMR
7.6VAC   --+
         (( )) Speaker Binding Posts

• Input connectors - RCA and XLR inputs connect to the input transformer primary through a DPDT mini-toggle with coin silver contacts. The toggle switch inverts the connection to the primary allowing absolute phase control. I've been on an Eichmann-like minimalism kick for connectors going back a few years, but I did use some chunky Cardas input jacks I had in the parts bin.
   
  
• Input transformer - Altec/Peerless 15095 or 15095A models 15k:600 rating, or a 5:1 voltage ratio. 15095 and 15095A have same ratings but slightly different pinout. These small octal-plug microphone transformers are fairly plentiful, popping up on eBay regularly, and they are a real bargain considering the materials and bandwidth. Case looks to be mu-metal with red graphics for pinout and identification.
   
  
• Parafeed Chokes - oneElectron PRC-2 48H at 75mA - these chokes have two sections and can be wired in series or parallel, so they can easily be re-used for another project as 12H 300mA power supply chokes. Not the most beautiful finish on these, the end bells and laminations needed quite a bit of sanding and primer to get a good finish compared to most of the Hammond iron in the project.
   
  
• Interstage autoformer - Magnequest EXO-173ni (nickel) centertapped autoformer. Brass end bells, located under chassis between the 845 sockets.
   
  
• Driver parafeed coupling cap - 3uF 660VAC GE motor-run oil capacitor
   
  
• Output Transformer - Magnequest EXO-50 5k 20W parafeed output tranny. Parafeed output trannies are optimized for zero DC - no air gap and hideous amounts of inductance for extended bandwidth. The EXO-50 has a multi-tapped secondary that can be configured for 4, 8, or 16 ohm speaker connections, so il Monstro has a Euro-style terminal strip between the binding posts underneath to vary the output connections.
   
  
• Output parafeed coupling cap - 2uF 600VAC Sprague 'bathtub' oil capacitor
   
  
• Binding posts - minimal metal, gold plated with plastic hex nuts.
   
  
• Wiring - where possible connections are made directly with component leads. In other places signal wiring is 27ga fine silver and ground wiring is 18ga fine silver. Some teflon tubing is used for short protection on bare conductors.
   
  
• Bias network - Hum pots are 100 ohm 3W with discrete 100 ohm 3W WW resistors between wiper and end terminals. Cathode bias resistors are 1k ohm 50W wirewound pots. Big suckers. The cathode resistors are bypassed with SCR 400V polypropylene film capacitors.

• 'Buddha' CMC at input - see Mambo schematic for details - this is a cap and CMC filter where the AC enters the PS chassis
   
  
• RC filters in B+ - Sometimes you get the bear and sometimes the bear gets you. CLC pi filters are generally stable and reliable in high-voltage filter designs, but sometimes things go bump in the night and there is an unforseen interaction - a resonance that must be dealt with. For some reason I had a tank resonance in il Monstro's B+ filtration, but with some help from Voltsecond I was able to add some simple RC filters in parallel to dampen the resonances without affecting the low impedance of the original PS design.
   
  
• Switchable 50k ohm 50W bleeder resistor in B+ at the decoupling filter stage (switch in ground leg)
   
  
• High-voltage supply switch on AC supply to the Hammond 300BX power transformer allows testing of the filaments without firing the 83 rectifier or high-voltage
   
  
• Shielded 18ga twisted pairs run the filament DC to umbilical connector
   
  
• Shielded AC lines to switches at front of PS - using grounded copper braid tubing

• Chassis plates - 13ga copper. Signal chassis plates are 12 x 18 inches and PS plates are 18 x 18 inches. PS also has a diagonal brace of heavy aluminum angle to help support the massive transformers. Chassis plates are backed with 1/8 inch sorbothane sheet for vibration control.
   
  
• Signal chassis sub-chassis - made from .250 inch thickness polycarbonate, the subchassis in each signal box holds the 845 sockets, the bias and hum pots, the EXO-173 interstage autoformer, and the test points. This provides additional dampening of vibration and allows the main signal components to be removed all in one assembly for repairs or changes. Stainless socket head bolts with stainless nylock nuts were used for all chassis plate attachments.
   
  
• Transformer detailing - The Magnequest transformers already had good finishes and brass hardware, but the oneElectron and Hammond iron had a variety of finishes and hardware. Drips of varnish were common on bell ends. Urk. It was a significant effort to take every transformer apart, to sand and repaint all of the end bells (bronze hammertone) and also the laminations (semi-gloss black). During reassembly gaskets were cut from automotive cork/rubber gasket sheet and installed between the end bells and laminations to control resonance and hum (note that power transformers were grounded as delivered). All screws were replaced with brass allthread and acorn nuts with rubber washers. Transformers were mounted to chassis with hidden sorbothane pads under the lamination stacks and rubber/composite washers. All end bells had one bolt mounted with toothed washers to ground the iron to the chassis plate for safety.
   
  
• Tube sockets - 83 rectifier uses a standard ceramic UX-4 socket, rubber mounted to the PS chassis plate. 845 tube sockets are gold flash ceramic sockets from tubesandmore.com - as noted above they are attached to the polycarbonate subchassis in the signal boxes. There is an additional octal socket on the power supply beside the 83 socket that had been used for a timer relay but is now empty.
   
  
• Umbilicals - One of the more complex areas of the assembly is the umbilical design. Female (Neutrik NL8MPR) jack is mounted to the PS chassis, umbilical is permanently connected to the signal chassis with a cable connector (Neutrik NL8F) on the end of the umbilical attaching to the PS.
   
  Umbilical is long enough to place the PS under the signal chassis in a stand, or they can be laid out side by side. PS and signal chassis are mirror images and interchangeable.
   
  The Speakon connectors are only rated for 300VAC or so, but if you look at the design they are well-insulated and each conductor has it's own screw-down clamp with full plastic fences between the attachments. All contacts are recessed in both sides of the connectors, clever design. Very nice for the money, much less $$ than most of the other connectors I looked at.
   
  I used plenum rated video coax for all lines - this stuff has foamed teflon insulation, very tough. 6 coax lines:
  • + driver fil 18ga solid copper plenum coax
  • - driver fil 18ga solid copper plenum coax
  • + output fil 18ga solid copper plenum coax
  • - output fil 18ga solid copper plenum coax
  • + high voltage 20ga solid copper plenum coax
  • - signal GND 20ga solid copper plenum coax
  • all shields tied to chassis GND
  
   
  7 contacts are used on the Neutrik connector - wires bundled into 3 pairs of coax covered with fiberglass sheathing (fiberglass I have on hand will not stretch over more conductors). This leaves one contact free in case I decide to try fixed bias for the output tube later.
   
  
• AC Wiring - wall supply cable is hard-wired to amplifier with 12ga solid core THHN house wire, braided with fiberglass sheath and Hubbell plug. Internal wiring is also 12ga solid core copper.
   
  
• Chassis GND posts - each chassis has a gold speaker binding post bolted directly to the chassis in case of external ground loops. Not needed so far, but nice to have.

It truly was a great meeting. I was at the Mill 3 years back but haven't had a chance to return since. As others have pointed out, the gear sounded very good with many of the amps doing a fine job. The Mill is such a big space that even with the huge RCA horns, an 8W 300B amp isn't enough power. The big 845 SE amps or the push pull amps really were needed to bring things to life.
 
Once the amps were matched to the speakers (either Jonathans's big horns) or the RCA LC-9s produced truly amazing sound. The sound of Louis Armstrong singing St. James Infirmary on vinyl made you think he'd come back from the dead.