Spencerleehorton
Well-known member
Thought I’d document this as haven’t really used this U47 as when I built it many years ago I didn’t really use it as it ended up being noisy, a rogue hum.
Original The psu had a couple of normal transformers, both 15v, one at 12va and one at 8va. I’m assuming the 12va (0.8A) one had 2x6va windings and the 8va (0.5A) had 2x4va windings.
12va was one the heater and 8va was on B+.
The pcb for the psu was the same one as per vintagemicrophonepcbs.com U47EF as well as the microphone pcb.
The tube is a EF800 and uses 275mA.
From the build thread it says to use a 12R-30R load resistor.
The reed relay is only 5v and uses a 10k in R5 and 510R in R9 to get the 5v, relay uses( 10mA) and has 12k as load resistor.
B+ started with 220vac x 1.4 after diode bridge gives 308vdc.
B+ had R1 as 4.1k and R3 as 3.9k and R8 at 10k trimmer and I could wind down to 105v.
H+ started at 15vac x 1.4 which gave 21vdc after diode bridge had R2 as 5.2R, R6 as 13.R and R11 as 13.1R and had 25ohm 5W pot which could adjust easily to 5.05v.
All pins connected apart from pin 3 on mic socket.
Psu and mic sockets
Pin 1 to pin3 xlr
Pin 2 to pin2 xlr
Pin 3
Pin 4 to H+
Pin 5 to B+
Pin 6 to pattern (5v) for relay
Pin 7 to gnd
All seemed correct but getting hum?
So I have taken apart psu as case wasn’t ideal and started again.
My new psu is slightly different
B+ transformer is now 110vac (measures 125vac)
H+ transformer is 18vac (measures 22vac)
Original The psu had a couple of normal transformers, both 15v, one at 12va and one at 8va. I’m assuming the 12va (0.8A) one had 2x6va windings and the 8va (0.5A) had 2x4va windings.
12va was one the heater and 8va was on B+.
The pcb for the psu was the same one as per vintagemicrophonepcbs.com U47EF as well as the microphone pcb.
The tube is a EF800 and uses 275mA.
From the build thread it says to use a 12R-30R load resistor.
The reed relay is only 5v and uses a 10k in R5 and 510R in R9 to get the 5v, relay uses( 10mA) and has 12k as load resistor.
B+ started with 220vac x 1.4 after diode bridge gives 308vdc.
B+ had R1 as 4.1k and R3 as 3.9k and R8 at 10k trimmer and I could wind down to 105v.
H+ started at 15vac x 1.4 which gave 21vdc after diode bridge had R2 as 5.2R, R6 as 13.R and R11 as 13.1R and had 25ohm 5W pot which could adjust easily to 5.05v.
All pins connected apart from pin 3 on mic socket.
Psu and mic sockets
Pin 1 to pin3 xlr
Pin 2 to pin2 xlr
Pin 3
Pin 4 to H+
Pin 5 to B+
Pin 6 to pattern (5v) for relay
Pin 7 to gnd
All seemed correct but getting hum?
So I have taken apart psu as case wasn’t ideal and started again.
My new psu is slightly different
B+ transformer is now 110vac (measures 125vac)
H+ transformer is 18vac (measures 22vac)
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