• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Distorting 6L6 amp

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peterthetrumpeter

Member
Joined 2018
Hi All,

I'm a newbie. Last night I completed the build of my first amplifier, a 6V6/6L6 SE monoblock - this is a project from the book "Tube Amplifier Basics". I turned on the amp (was using 6L6), was listening to music, but the amp was clipping on low volumes (at least I thought the volume was low for 8W). At some point I wanted to hear the distortion more clearly and I increased the signal level pretty high. The volume died down in about 3 seconds and then the fuse was blown (actually it's a 1A circuit breaker). I tried to turn it on multiple times after it cooled down but the circuit breaker trips after about 5 seconds every time (the filament supply LED is lit during this short period of time).

For troubleshooting, I removed the 2 diodes in the power supply, and they are OK. I also detached the power transformer secondary to measure voltages, but I can't because the circuit breaker trips before I could measure anything (although the filament wires are still connected without tubes).

Is my transformer shorted? I'm assuming it's safe for the transformer to power the primary, leave the secondary open and measure voltages that way?

I did not get a chance to measure any voltage while the amp was working more or less correctly, so I'm aware I'm not giving out a lot of info here, but could you guys throw some ideas at me as to what could have gone wrong?

Thanks a million,

Peter
 
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analogadikt

Member
Joined 2011
To be sure disconnect the heater supply also and test the transformer with an incandescent bulb wired in series with the primary. If the transformer is ok then there shall be a very weak glow. A shorted transformer shall result in the bulb glowing brightly.

Regards,

Anwesh
 
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analogadikt

Member
Joined 2011

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peterthetrumpeter

Member
Joined 2018
Built the lamp test, unfortunately the 40W lamp is brightly lit with all secondary connection removed. So I have the replace the power transformer.

It would be nice to know what caused the problem so that it doesn't happen again. The lamp setup (with a 75W bulb) will hopefully prevent the transformer disaster next time.

Thanks for the tips.
 
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peterthetrumpeter

Member
Joined 2018
I'm wondering about what wattage of bulb to use as current limiter when I install the new power transformer for a normal operation. According to Edcor specs, this PT can provide 500V@60mA. That's about 25W? But there are losses so, is 30W a more correct estimate? Double that, so maybe a 60W bulb will run it safely?

analogadikt said:
If the lamp glows brightly, you need to check all the components and connections in the supply path. The fault may not be with the trafo, but a shorted power cord or something else.

Regards,

Anwesh
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peterthetrumpeter

Member
Joined 2018
What I meant to say by "running" is that the amplifier circuit is connected to the power supply - I've done a test earlier where the secondaries were all detached just to see if the PT is shorted (it is).

Thanks for confirming the wattages.

DF96 said:
The bulb is for safe testing, not running. 40W or 60W should be OK.

Difficult to see how such a simple circuit could blow a power transformer primary.
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