short circuit

[raits77]

Hello,

I finished my first two-stroke a couple of months ago. Worked great, but then I got some weird crackling sound until it warmed up, sometimes it didn’t go away at all. Anyway (will work on that later), one day I turned up the volume to max and then suddenly, silence. Checked the fuse, gone. Replaced it with a new one and same thing happened, fuse blown, I replaced all the tubes except the rectifier tube and still it blew the fuse the moment I turned on the amp. Oh yes, I turned down both the volume and tone knobs and boost was off. I just checked all the resistors and they show the correct values, the bigger caps had some voltage in them as well, those must be fine, right? The connections to the tubes didn’t have any visible damage either as well as the connections to the speaker jacks.

Just want to know if any of you has encountered this problem and how I could fix it, my guess is that it’s after the preamp somewhere as it went silent after I picked a string.

Thanks

[Robin]

With all the tubes removed (including the rectifier), does it still blow the fuse with no signal applied? Most commonly, it’s a tube issue but if not, check the filter caps. Confirm the polarity on all electrolytic caps is correct per the schematic.

Did you notice any electrical smell when the fuse first blew? If so, check the power transformer.

[raits77]

Hello,

I did remove all the tubes and same thing happened, the fuse blew. I didn’t even have to apply a signal. Neither the guitar or speakers were plugged in, tone and volume were turned all the way down.

The polarity of the filter caps should be ok as well, have been checking and rechecking the schematic over and over again. I haven’t noticed any smell coming out of the amp during operation or when the fuse started to blow. Sadness

Now I’m out of fuses and wont be able to perform much troubleshooting until next week.

Thanks for the tips Robin.

[Robin]

Keep the speaker plugged in. You need an output load so that when you do pass a signal to the OT, it has a place to go and not fry the OT.
So, you are looking for a short, probably one of the caps or (fingers crossed it’s not) the power transformer. You can check the PT by disconnecting all the secondaries and see if the fuse still blows.
If the fuse does not blow, that’s good. Go ahead and check (carefully) the secondary voltages while you have the transformer isolated from the rest of the circuit. Remember the red pair is B+ voltage (approx 330V).

BTW, you should be using a current limiter anyway to protect the components from a dead short while you are troubleshooting. It will indicate if there is a short or not, so you won’t have to keep replacing fuses while you get things sorted out. There is info elsewhere on this site on how to build a simple limiter.

Which power transformer did you use for your amp?

[raits77]

Thanks for the tips,

I’ll keep you posted how the troubleshooting goes. I’ll start with the PT. I used the EU power transformer from Weber (W022772EU). I’ll look into the current limiter as well, out of fuses already.

[raits77]

Hello Robin, I did as you recommended and disconnected the secondary side of the PT and turned the amp on, fuse still burned. I ordered a new PT from Weber and installed it today, amp works again and the crackling is gone as well, I guess the crackling was from the faulty PT or I wasn’t treating it nicely to begin with. Anyway, thank you for your tips

[Robin]

Congrats on getting it up and running…
Be sure to check through the circuit (with the current limiter attached) and confirm that there are no intermittent shorts. Use a chop stick to push on the solder connections, start with the tube sockets and be sure nothing is touching. Although anything is possible, it’s likely that the issue is not just a faulty transformer. A bad tube can act as your description as well.

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