Hi guys, I wanted to share with you some work I did on this amps. As far as I know, this amp was produced from the beginning of the 60's to the 90's. It was mounted inside an organ and was design by Allen Organ and Acrosound. The schematic looks very similar to the published TO350 amp from the Acrosound brochure, except the PS and the entry stage with additional tubes. Some of you know my interest to the Acrosound family so I couldn't resist to try that one. Because Acrosound's involvement in this project, the early model have the desired Acrosound monster OT in square can. Then after Acro's debacle, Allen was forced to produce their own OT, in house and from other company, often with rounded can. I have both models and I can tell you that I did not measure or heard differences between the original Acrosound and the latter Allen OT. The older model have a PS with under chassis WW power resistors, poorly mounted in contact to the chassis for heat dissipation. Later model have modern resistor bolted to the chassis. There are slight difference in resistance value but that's beyond the point. I added an heat sink to provide more chassis contact and additional heat dissipation. Everything in this amp has been design with reliability in mind. The PS with a big choke input is feed by an oversize power transformer, which barely gets warm after 5 hours of listening. The PS design is interesting, with a constant current "wasted" in heat, to get the choke going constantly. The amp design is excellent. Fixed bias, only 2 couplings caps to separate the 2 stages, some direct coupling driver tube/splitter, tube balancing via 6SN7, bias point. All is good here. For HiFi usage the first tube 12AT7 can be removed as there is no need for the super high gain factor the 2 entry driver tubes provide. I only use the EF86 as first stage with 470k to ground, 10K to grid, and relocated it to the 12AT7 which was originally mounted on a rubber platform. Very nice. The wiring is impeccable, with thick 1 mm solid copper plastic/cloth wire with good mechanical contact under the solder. On a funny note, the chassis were signed by Betty, Ana, Anne. These ladies did an amazing work and deserve our respect. This amp has 2 fuses, one on primary, and one on the HV CT to ground. So I rebuild the PS and bias supply with good caps, good SS diodes, added bypasses, relocated the balancing pot to the front panel as some version had it inside the amp with no throughout access, added RCA connector, removed the 12AT7 tubes etc etc.. I used KT120 on this baby, and get a solid 68 W < 1% THD. The square wave is excellent, thanks to these monster OT. Now on the listening side, this amp is amazingly detailed and dynamic. Top to bottom, it is well integrated and the imaging is superb. It is one of my best amp. So I decided to make them pretty, I asked my friend Chris to make me some wooden enclosure. It really makes these amps pop. So in conclusion, this amp deserves much more credit than the little you can find on the internet. You can find the Acrosound's touch in this amp, with its distinctive dynamic. Once rebuild, it competes with the top amps of that era, without a doubt. Alright, I recorded most of the important info for this fantastic amp so you can get a pair and get going. Thank you for reading. Brice. My Notes as is: Modification Allen Organ 75 C13 and C15 bumped from 140uF 350V to 680uF 400V Mouser 80-ALC40A681DL400 C13 and C15 bypassed with 2.2uF 630V Panasonic film caps Mouser 667-ECW-FA2J225JB Old rectifier bridge removes, replaced with bridge IXYS 21 Amps 1200V added Mouser 747-VBO21-12NO7 Rectifier bias supply replaced with UL4007. Bias supply caps 2x30uF 200V replaced with German FT-Cap 2x100uF 350V Added bypass Panasonic film cap 4.7uF 250VDC bias supply Mouser 667-ECQ-E2475KF Relocated internal bias adjustment pot to from panel. Input stage 12AT7 removed Relocated EF86 to the original location of the 12AT7 with vibration dumping grommet. R43 removed (supply 160VDC not needed anymore). Adjusted R42 from 12k1 to 39K as a result of removing R43: 205 VDC point 4 in specs. C15 40x10x40uf 475V replaced with an Authenticap KTL41 200x50uF 550VDC same location and an Authenticap KON5650 50uF 350V at the EF86 original position. Fits to cover the hole. Added 470k to ground to RCA input jack and 10K to grid to EF86. Added film capacitor 0.47uF 1200V before choke, to adjust the B+ and tame the choke buzzing. Added LED 6.3VAC with 3.9K resistor 1/2W serial. Added LED on 205VDC supply with 3.9M 1/2W resistor serial. Removed the 0.01uF 600V ceramic from primary to chassis: dangerous. Use the 125VAC red/black primary tap instead of the 115VAC white/black tap. Use of Tunsgol KT120 instead of 6550. Bias the same or higher. Change power cord and add aviation connector 3 prongs.