Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on May 09, 2017, 05:53:59 pm
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Im running a 12au7, and 2 12ax7s with the heaters in series and CT disconnected (12v right on 4 and 5) off of one 7812 regulator. Datasheet says its rated for 1A and the tube datasheets says that they run at 300 milliamps (series) so it should be ok right? Furthermore, this is in basically a triple valvecaster circuit, so could the low plate voltage (15v) cause heater to draw more current? Regulator is heating up a bit and i measure 8v on the output, is it more likely shorted out somewhere?
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Wired for 12V, the 12A_7 types pull 0.15A each.
Times 3 is 0.45A.
The 7812 can stand that (will sag at turn-on). It WILL run hot: 3V drop at 0.45A is almost 1.5 Watts, so you want a heatsink.
If you only got 8V out, see how bad the INput to the 7812 has sagged.
> could the low plate voltage (15v) cause heater to draw more current?
No.
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Theres no plate voltage either, but the 7815 regulator is just getting warm, im in school at the moment i will check input voltage when i get home (should have thought of that :facepalm:
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Only getting 10-11v off the rectifier, i know it should be about 20 mabey a little higher, im not reading any shorts to ground, i have two 2200uf filter caps, is that too much?
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6v~ coming off the transformer, bad transformer?
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Post a schematic.
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This is the problem area
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Maybe you're just trying to pull more power from the transformer than it can deliver, with voltage sag as a result.
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Ive ran a single 12au7 and 4 5 watt audio amp chips at once off of the same transformer (school has a bunch) i think ill just pop in a new one.
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i have two 2200uf filter caps, is that too much?
I used that circuit for DC fil, on 2 builds, I used 2200uF on input, and 2 X 4700uF on output.
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Doug sell a relay supply PCB that has substantially the same circuit (different voltages)
if you wanto to give a look at the circuit here it is
http://el34world.com/projects/relay_switch.htm (http://el34world.com/projects/relay_switch.htm)
you can order the PCB and use it also as Heater Supply (for few small tubes)
Franco
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Space is *very* limited in the enclosure so another board is not an option. Its probably just a bad transformer, ill get another tomorrow