aon
Well-Known Member
Figured I might as well make a new thread since it's been so long and it's not really the same amp anymore...
In October I bought a slightly faulty Fender Super 60 with the intention to remove the guts and build something along the lines of the RR763 Blackvibe which is a simplified single-channel AB763 circuit without reverb and tremolo. I'm using the choke, transformers, chassis, cabinet, power / standby switches, pilot light and speaker (for now) from the Super 60.
So I built most of it ready in November. Tested it, didn't work, put it aside. Started to get a bit fed up of looking at the chassis in the corner (and telling my son not to touch it), and a few days ago my former boss was asking how my amp build is doing. That was the last straw, I thought I'll be damned if I can't get it working
So I made some time for it yesterday.
Found two wiring mistakes, fixed those and it came to life!
That made me have enough confidence to put in my good JJ power tubes and start biasing them. After getting them to 19-20ish W of plate dissipation I noticed that there's quite a noticeable AC hum present. The bias supply had quite a lot of ripple so I added an extra filtering cap and an additional series resistor to drop the current draw somewhat. Didn't really help that much but I left them in anyway. Moving the input wire to the phase inverter grids further away helped a lot more but I'm still not completely happy with it. Doesn't bother me when playing but it's clearly there when you put your ear next to the speaker with the input unplugged...
The bias modifications are a bit ugly, for instance the additional filter cap is a 31 year old used radial Illinois Capacitor, so I'll have to take it apart soonish and clean that up a bit. I think I'll add DC elevation to the heaters to see if that gets rid of the rest of the hum.
When looking for the hum I replaced the old Fender 12AX7 and 12AT7 with a new EH 7025 and 12AT7. Left them in for now. It stays completely clean to about 5 on the volume control which is already pretty loud.
Other stuff I'm planning on doing besides cleaning up the bias circuit / modifying the heater supply:
I'm playing this as my main amp now. Sound samples will be coming when I get around to recording something again...
Current revision of the schematic is attached in case anyone is interested.
In October I bought a slightly faulty Fender Super 60 with the intention to remove the guts and build something along the lines of the RR763 Blackvibe which is a simplified single-channel AB763 circuit without reverb and tremolo. I'm using the choke, transformers, chassis, cabinet, power / standby switches, pilot light and speaker (for now) from the Super 60.
So I built most of it ready in November. Tested it, didn't work, put it aside. Started to get a bit fed up of looking at the chassis in the corner (and telling my son not to touch it), and a few days ago my former boss was asking how my amp build is doing. That was the last straw, I thought I'll be damned if I can't get it working
So I made some time for it yesterday.Found two wiring mistakes, fixed those and it came to life!
That made me have enough confidence to put in my good JJ power tubes and start biasing them. After getting them to 19-20ish W of plate dissipation I noticed that there's quite a noticeable AC hum present. The bias supply had quite a lot of ripple so I added an extra filtering cap and an additional series resistor to drop the current draw somewhat. Didn't really help that much but I left them in anyway. Moving the input wire to the phase inverter grids further away helped a lot more but I'm still not completely happy with it. Doesn't bother me when playing but it's clearly there when you put your ear next to the speaker with the input unplugged...
The bias modifications are a bit ugly, for instance the additional filter cap is a 31 year old used radial Illinois Capacitor, so I'll have to take it apart soonish and clean that up a bit. I think I'll add DC elevation to the heaters to see if that gets rid of the rest of the hum.
When looking for the hum I replaced the old Fender 12AX7 and 12AT7 with a new EH 7025 and 12AT7. Left them in for now. It stays completely clean to about 5 on the volume control which is already pretty loud.
Other stuff I'm planning on doing besides cleaning up the bias circuit / modifying the heater supply:
- reverb, probably with the original tank and a single 6BM8 (already have the tube socket and heater wiring in place)
- solid-state bias vary tremolo
- replace the speaker with something more british sounding, maybe Eminence Legend GB128
I'm playing this as my main amp now. Sound samples will be coming when I get around to recording something again...
Current revision of the schematic is attached in case anyone is interested.
