DrVincelette
New member
I have successfully built and perfected an 833A pair of monoblocks and I am satisfied I have perfected it after more than two years experimentation. WAVAC made an 833A stereo they sold for $350,000 and it is nothing more than the 833A triodes driven by 300B SETs. My 833A final stage is driven by a 45 SET with a Lundahl LL2765 transformer taking the 45 through the 5K winding configuration stepping it down to 600 Ohms. The 600 Ohms is connected between the 833A grid and the same ground the directly heated cathode is connected to. At 1000 plate Volts and zero grid bias the 833A draws about 120 ma which does not strain this tube enough to turn a spot on the plate red-hot the way I saw in an AM radio station that was using this tube which is made for AM radio station transmitters. For the output of the 833A I use a Hammond 1642SE transformer which is rated for 300ma before core saturation. This is bigger than any other transformer you can buy for a big SET. The 1000 Volt power supply I use 2300 Volt rated polypropylene filter capacitors with a 10 Henry choke between them with the choke on the negative sides of the capacitors so they are connected to ground rather than the dangerous 1000 Volts. I put fast reacting solid state diodes together in parallel for surge current to the first capacitor when the plate Voltage is first turned on. While I had to build an elaborate pre-charging 10 Volt cathode heater for the 10 Amps the 833A cathode requires and 4.5 Farad final filter capacitors, I recommend using seated lawn mower batteries with 0.3 Ohm dropping resistors for simplicity. Tungsten cathodes with the least ripple in the DC they require will make the 120 Hz buzz in the speakers.
The cost of parts to build this final stage is well under $2000; the Hammond output transformers are the most expensive at just under $300 each, the Lundahl transformers cost under $200 each, you can get new 833A vacuum tubes on ebay for about$200 each and the 1000 Volt power supply capacitors, Hammond 1500 Volt center tap transformers and Hammond 10 Henry chokes from Mouser electronics. A Bottlehead headphone amplifier should drive it if you use a 600 Ohm output impedance direct to the 833A grid and you won't need the Lundahl transformers or you could use a Bottlehead amplifier with the Lundahl replacing the output transformer.
I have been playing with the possible idea of putting together kits for an 833A output stage of amplification like my prototype or building them for a modest price of my labor. My background for this is over 20 years building SET amplifiers using 300Bs 2A3s and 245s. I have done avionics work as an FAA licensed aircraft mechanic and I got my PhD in interdisciplinary applied mathematics and mathematical physics with emphasis on physics from Delaware State University class of 2009. I taught as an adjunct and visiting professor but the cost of commuting and apartment rent took my take-home pay so low that I quit. I start working as a janitor which pays better than university teaching. In short, I have the kind of experience to be able to make nice 833A booster amplifiers for a minuscule fraction of what they sell for anywhere else they have been built and sold. The 833A I have found perfect for Magnepan speakers, more than loud enough if I turn up the volume. One final note, I tried driving the 833A grid with parallel feed but it sounded watery and weak even if it was turned up loud. An air gaped transformer sounds much richer and full-bodied driving this tube and there is neither a parallel feed transformer and choke big enough for the output of an 833A.
The cost of parts to build this final stage is well under $2000; the Hammond output transformers are the most expensive at just under $300 each, the Lundahl transformers cost under $200 each, you can get new 833A vacuum tubes on ebay for about$200 each and the 1000 Volt power supply capacitors, Hammond 1500 Volt center tap transformers and Hammond 10 Henry chokes from Mouser electronics. A Bottlehead headphone amplifier should drive it if you use a 600 Ohm output impedance direct to the 833A grid and you won't need the Lundahl transformers or you could use a Bottlehead amplifier with the Lundahl replacing the output transformer.
I have been playing with the possible idea of putting together kits for an 833A output stage of amplification like my prototype or building them for a modest price of my labor. My background for this is over 20 years building SET amplifiers using 300Bs 2A3s and 245s. I have done avionics work as an FAA licensed aircraft mechanic and I got my PhD in interdisciplinary applied mathematics and mathematical physics with emphasis on physics from Delaware State University class of 2009. I taught as an adjunct and visiting professor but the cost of commuting and apartment rent took my take-home pay so low that I quit. I start working as a janitor which pays better than university teaching. In short, I have the kind of experience to be able to make nice 833A booster amplifiers for a minuscule fraction of what they sell for anywhere else they have been built and sold. The 833A I have found perfect for Magnepan speakers, more than loud enough if I turn up the volume. One final note, I tried driving the 833A grid with parallel feed but it sounded watery and weak even if it was turned up loud. An air gaped transformer sounds much richer and full-bodied driving this tube and there is neither a parallel feed transformer and choke big enough for the output of an 833A.