tube amp prototype with pc

[hpupo]

Hello

I am testing a project which for now it just one triode from a 12ax7 tube. I have 350V feeding it, a standard 1Meg, 68K network on the input, a 2.7K and 25u on the cathode and a 0.022u on the output. this is my very first project and hopefully will be my first tube guitar amp. I have the whole circuit on a breadboard. In order to test it, i used my PC speaker output and directed connected on the 1Meg/68K junction. Problem is when i turn the 350V power supply on, it trips the breaker. I have no idea why...I believe it has something to do with the impedance of the pc output... do i need to place a coupling capacitor for the input? If that's the case, why dont i need that for the guitar?

Thanks

[cogsncogs]

I would say the problem most likely lies in the power supply. Schematic, diagram, napkin?

[hpupo]

Sure, here they are. Please disregard the values on the Ac source on the power supply. They are just tests. also i am not using discret diodes, but a single rectifier bridge.
If the problem is the power supply, why i have no trouble when there is no input to the circuit? Tube glows, DC values are correct, etc....

[hpupo]

Sorry, the link for the images were incorrect on the last post. these are the correct ones...

[DF96]

The PC output may have a standing DC voltage in addition to the signal. It is always good practice to include a DC blocking coupling capacitor at the input of equipment.

However, a 12AX7 running through a 100K anode resistor is not capable of tripping a breaker so something else is happening.

Your power supply appears to be a voltage doubler from a single secondary, using two half-wave supplies in series. It should work OK, although you have some hum due to inadequate smoothing. The diode bridge is being used as a pair of diodes, not a bridge. Is this what you intended, or is the PSU diagram not actually what you built?

[hpupo]

I agree that something wacky is happening. I will test with a blocking capacitor to be sure, but i could not find any DC component on the oscilloscope...somehow it looks like when i have a signal in, the power supply shorts out for some reason....
The design of the power supply was intentional. Basically i had some parts here that i put together quickly. I do understand that's not optimal, but like you said, it should be ok for testing....

[tubelab.com]

Are you using a transformer between the wall outlet and your power supply diodes? If so it may be bad, or it may not have isolation. If not, do not plug this thing in again until you get one. You could be feeding serious power back into your PC. If you are tripping a circuit breaker, a large amount of current is flowing in an unexpected path. That path is likely to ground through the PC. If the PC wasn't well grounded the path could be through YOU!

[DF96]

Oops, it never occured to me that he might be running without a transformer. That would certainly explain it. If so, he may be lucky to still be alive to ask the questions!