Deepdark
Well-known member
Hi guys
Here is my Federal AM864 build. I think it could be a good idea to list all issues I got and what were the solutions, so if someone encounter the same ones, there will be a resource for solutions, since a lot of am864 thread have some lost informations, outdated, etc.
So, here are the specs:
Input tranny: Edcor XSM 600/10K
Output tranny: Edcor XPP1 600/10K (push pull)
Power tranny: Edcor XPWR012
Meter* Hairball audio 8020
Case: Collective case blank case
Layout: Nearly identical to the one we found Inside the manual. I interconnected some elements on the board, instead of wiring everything to the tubes. R6 is on the main board, at the other extremity of the turret board. Caps are all Panasonic polypropylene.
PSU: Diode rectifier. I replaced the entire psu section with a RC topology, as linked below. Super quiet, lot of filtering.
Heaters: Ac heaters. The power transformer has a center tap so I strap that to ground. I run both wires to heaters pin, so basically, I didn't followed the schematic. I did it the classical way. No hum. Tightly twist and far from B+ and all other circuit wires
Ground: I made a little ground buss, taking all ground connections of the audio circuit in a hierarchical maner, then strap the ground buss to the psu 0v, and then straight to the main bolt. Shields (6SK7's and 6SQ7, xlr's shielded wires) are short, strap on their respective mounting bolts.
Motorboating: I had motorboating that was related with filtering. I had to add another 47uf filter cap on the psu and rerout a little my wires to get rid of it. The only remaining motorboating is related to the 6SK7. They are pretty well balance, but one can make a balance pot. The easiest and most effective way is to play a little with the curent ctrl. I ended up setting my curent ctrl to about 2.5v, and play with threshold to achieve what I want. I set it to about 10db of gain reduction.
Meter: I read over the internet that it needed a 2ma fs (DCma), type of meter. Once searching over some original unit's pictures, the meter were rated 1.010ma fs. What I did, is taking a Hairball standard vu meter. It's basically rated 200μA, so 1/5th of the original spec (speaking of 1.010ma fs). R2 is set to get about 1.2 vdrop dc across that resistor, with the proper meter strap in ||. With mine, I got about 0.28 vdrop, so the needle sit really low on the scale. I basically change the 240ohm resistor for one that is about 5 times it's value. I ended up using 909R instead of 240ohm, and the needle sat nearly 0. I trim the little bolt on front of the meter and Voila!. the 0 has been adjusted to my 2.5v curent ctrl. if you change that setting, you have to recalibrat the meter. Does it change anything to the circuit? No. no motorboating, no anything else bad.
Schematic: I used the one with 12K plate reistors (R3/R4)
Voltages Values: Basically, if you use the manual to validate your voltages, beware of the fact they were measured with 47K plate résistors, and calibrate as speak in the manual. If you use 12K plate resstors, then your plate voltages will not be 155v but will sit higher, about 185v (if you search over the other thread, lot of peoples get those values). V1 and 2 pins 4 are about -0.92v, instead of -1.3. I didn't noticed any issues with that value. I know some guys were aware of that value, not being the same as the one found on the manual. Other than that, voltages are pretty spot on. My B= is 191v. It runs good.
Here is my Federal AM864 build. I think it could be a good idea to list all issues I got and what were the solutions, so if someone encounter the same ones, there will be a resource for solutions, since a lot of am864 thread have some lost informations, outdated, etc.
So, here are the specs:
Input tranny: Edcor XSM 600/10K
Output tranny: Edcor XPP1 600/10K (push pull)
Power tranny: Edcor XPWR012
Meter* Hairball audio 8020
Case: Collective case blank case
Layout: Nearly identical to the one we found Inside the manual. I interconnected some elements on the board, instead of wiring everything to the tubes. R6 is on the main board, at the other extremity of the turret board. Caps are all Panasonic polypropylene.
PSU: Diode rectifier. I replaced the entire psu section with a RC topology, as linked below. Super quiet, lot of filtering.
Heaters: Ac heaters. The power transformer has a center tap so I strap that to ground. I run both wires to heaters pin, so basically, I didn't followed the schematic. I did it the classical way. No hum. Tightly twist and far from B+ and all other circuit wires
Ground: I made a little ground buss, taking all ground connections of the audio circuit in a hierarchical maner, then strap the ground buss to the psu 0v, and then straight to the main bolt. Shields (6SK7's and 6SQ7, xlr's shielded wires) are short, strap on their respective mounting bolts.
Motorboating: I had motorboating that was related with filtering. I had to add another 47uf filter cap on the psu and rerout a little my wires to get rid of it. The only remaining motorboating is related to the 6SK7. They are pretty well balance, but one can make a balance pot. The easiest and most effective way is to play a little with the curent ctrl. I ended up setting my curent ctrl to about 2.5v, and play with threshold to achieve what I want. I set it to about 10db of gain reduction.
Meter: I read over the internet that it needed a 2ma fs (DCma), type of meter. Once searching over some original unit's pictures, the meter were rated 1.010ma fs. What I did, is taking a Hairball standard vu meter. It's basically rated 200μA, so 1/5th of the original spec (speaking of 1.010ma fs). R2 is set to get about 1.2 vdrop dc across that resistor, with the proper meter strap in ||. With mine, I got about 0.28 vdrop, so the needle sit really low on the scale. I basically change the 240ohm resistor for one that is about 5 times it's value. I ended up using 909R instead of 240ohm, and the needle sat nearly 0. I trim the little bolt on front of the meter and Voila!. the 0 has been adjusted to my 2.5v curent ctrl. if you change that setting, you have to recalibrat the meter. Does it change anything to the circuit? No. no motorboating, no anything else bad.
Schematic: I used the one with 12K plate reistors (R3/R4)
Voltages Values: Basically, if you use the manual to validate your voltages, beware of the fact they were measured with 47K plate résistors, and calibrate as speak in the manual. If you use 12K plate resstors, then your plate voltages will not be 155v but will sit higher, about 185v (if you search over the other thread, lot of peoples get those values). V1 and 2 pins 4 are about -0.92v, instead of -1.3. I didn't noticed any issues with that value. I know some guys were aware of that value, not being the same as the one found on the manual. Other than that, voltages are pretty spot on. My B= is 191v. It runs good.