Hi everyone,
I'm sorry if anyone has asked about this before, but I've done a lot of searching online and in the archives here and can't seem to find an answer I'm totally confident in. I'm building a copy of a Silvertone 1448 case guitar amp (I will be using an isolation transformer, don't worry everyone 😀) but I'm having trouble with the output transformer. I want to have 8 ohm output impedance, and I'm not sure of the model I'll need. This is the schematic:
http://www.schematicheaven.com/bargainbin/silvertone_1448.pdf
I'm hoping somebody has advice for me here. Also, if I'm not mistaken, am I going to need a 12.6 V filament transformer?
Any help is greatly appreciated!
I'm sorry if anyone has asked about this before, but I've done a lot of searching online and in the archives here and can't seem to find an answer I'm totally confident in. I'm building a copy of a Silvertone 1448 case guitar amp (I will be using an isolation transformer, don't worry everyone 😀) but I'm having trouble with the output transformer. I want to have 8 ohm output impedance, and I'm not sure of the model I'll need. This is the schematic:
http://www.schematicheaven.com/bargainbin/silvertone_1448.pdf
I'm hoping somebody has advice for me here. Also, if I'm not mistaken, am I going to need a 12.6 V filament transformer?
Any help is greatly appreciated!
The data sheet for 50c5 suggests a primary impedance of 2.5k at a somewhat different operating point but from a quick look at the curves I think you should be OK with that.
At the stated 1.9W max output and for guitar use you should be OK with a hammond 125ASE, for example.
/Olof
At the stated 1.9W max output and for guitar use you should be OK with a hammond 125ASE, for example.
/Olof
correction
Sorry I was a little bit too quick and it was late. To cope with idle current in the output tube you'll need a hammond 125BSE or 125CSE, rated for 45 or 60 mA.
/olof
Sorry I was a little bit too quick and it was late. To cope with idle current in the output tube you'll need a hammond 125BSE or 125CSE, rated for 45 or 60 mA.
/olof
Thanks again for the help, I got the OPT with no problems. My only other question here is with the tube heaters. I know that in old AA5 designs, they chose tubes specifically to eliminate needing a power transformer. I know about all the dangers associated with this as well.
The tubes used in this amp (wired in series of course) are the standard 35w4, 50c5 and 12au6. 35+50+12=97 of the 117 (give or take) that this was made to run on. Because of that, they added a filament transformer between the 50c5 and the 12au6. I'm assuming this is converting the 20 (again, give or take) remaining volts into just the 12.6 volts needed to heat the 12au6. If I'm right on all that, am I also right to ask if I could instead put a dropping resistor in series between the 50c5 and the 12au6 to remove the extra 8 volts and eliminate the filament transformer all together?
The tubes used in this amp (wired in series of course) are the standard 35w4, 50c5 and 12au6. 35+50+12=97 of the 117 (give or take) that this was made to run on. Because of that, they added a filament transformer between the 50c5 and the 12au6. I'm assuming this is converting the 20 (again, give or take) remaining volts into just the 12.6 volts needed to heat the 12au6. If I'm right on all that, am I also right to ask if I could instead put a dropping resistor in series between the 50c5 and the 12au6 to remove the extra 8 volts and eliminate the filament transformer all together?
Strikes me that this transformer was about 1:1 and was done in order to isolate the filament from the ac line. You might be able to something similar with a small 12V antek toroid with dual secondaries - just ignore the primaries. (leave floating)
You could perhaps use a larger Antek (say 100VA) with dual primaries and 12V secondaries. Use one primary as the ac input, use the other to provide B+ and filament power to the 50C5 and 35W4 with a larger resistor to accommodate the required drop and one 12V secondary to power the 12AU6 filament. If you choose to do this the chassis must be effectively grounded at all times via a 3 wire cordset. I wouldn't recommend counting on the insulation between the two primaries for that last bit of safety. (It's good, much better than the unsafe original design, but probably will not be as good as a bona fide isolation transformer.)
You could perhaps use a larger Antek (say 100VA) with dual primaries and 12V secondaries. Use one primary as the ac input, use the other to provide B+ and filament power to the 50C5 and 35W4 with a larger resistor to accommodate the required drop and one 12V secondary to power the 12AU6 filament. If you choose to do this the chassis must be effectively grounded at all times via a 3 wire cordset. I wouldn't recommend counting on the insulation between the two primaries for that last bit of safety. (It's good, much better than the unsafe original design, but probably will not be as good as a bona fide isolation transformer.)
Hi Kevin, thanks for your input. If this transformer is a 1:1, and I'm using an isolation transformer at the very start of the circuit, could I then omit the filament transformer completely and not bother with a dropping resistor between the 50c5 and the 12au6?
yerblues said:
I suspect that might be the case, noise might be something of an issue as the grounds are separated between the first stage and output and with the transformer in place the filament of the 12AU6A is referenced to the first stage ground, not the output circuit ground. You could make some changes to the circuit to make it work since you have the isolation transformer. (However it will function and probably sound different than the original - perhaps not a bad thing.)
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