Need Help - Ran Amp with No Load

[SBax]

Hi All.

I built a Bassman power section and a Thundertweak TopFuel 50 preamp.

Ran with no load by accident. I was playing full out with the amp super loud and a speaker wire became loose and fell off.
The amp died with a burnt smell. A replacement fuse blew right away. I need a mentor to help me fix it. I plan on doing the work over the Christmas holiday so I need to order parts.

What should I try? Isolate and check for shorts in the OT transformer primary or just order a replacement OT?

[Prairie Dawg]

Hi All.

I built a Bassman power section and a Thundertweak TopFuel 50 preamp.

Ran with no load by accident. I was playing full out with the amp super loud and a speaker wire became loose and fell off.
The amp died with a burnt smell. A replacement fuse blew right away. I need a mentor to help me fix it. I plan on doing the work over the Christmas holiday so I need to order parts.

What should I try? Isolate and check for shorts in the OT transformer primary or just order a replacement OT?

Unless you like spending money on stuff you may not need, test it for shorts. You don't even have to remove it.

[stokes]

Open it up and see if you just burnt the screen resistors,not uncommon when tubes go south.Dont know what kind of OT you have there but a good quality should stand up to some abuse,the power tubes are more likely to have blown.See if you can see what caused the burning smell.Do you have a light bulb current limiter?Very helpful for troubleshooting shorts.

[Enzo]

Pull your power tubes. Unplug from the wall. Measure resistance from pin 3 of each power tube socket to ground. ANy continuity is bad. I am assuming your home built did not include flyback diodes. If you DID install flybacks then MAYBE one of them gave its life to save your tranny.

For that matter, measure for resistance from the standby switch terminals to ground. ANything other than filter caps charging is bad.

If powering the amp up with no power tubes still blows the fuse, I start to suspect your output transformer has died. You transformer could be arcing inside. You can't measure that with an ohm meter while not powered, but it will still blow fuses when powered. Or can.

But in general, with tubes out and no power, using an ohm meter, we want NOT to find continuity from windings to frame or from winding to winding.

[SBax]

Thanks a lot guys. The OT is a Hoffman Bassman with 2,4,8 ohm taps. Ceramic screen resist look good.
With tubes out, open line from both pin 3 to ground. Also open line from the standby switch to ground. I put in known good tubes and the fuse blew right away. I am out of fuses and will get more this weekend.

[Chuck H]

You need to TEST the screen grid resistors. They do usually look burned if they are, but test them anyway.

[stokes]

Make a light bulb current limiter,simple one is just a light bulb wired in series with one side of an ac cord to an outlet that you plug your amp into.You can then diagnose shorts without blowing fuses.If you are interested in doing that just let us know,most anyone here can explain it in detail.Like chuck says,check those screen resistors,the flameproof type dont always look burnt,they're flameproof.

[SBax]

OK, thanks. I'll be making a current limiter this weekend.

[Jazz P Bass]

Yes but have we powered the amp without the tubes?
As Enzo stated, if it then also blows the fuse, the output transformer is suspect.

[Chuck H]

FWIW there is also the possibility that one of the old tubes shorted and took out the bias supply. If this happened then the new tubes will be drawing enough current to pop the fuse. The amp should be fired up with no tubes. Then, if the fuse DOESN'T blow, check voltages. The current limiter isn't needed for this.

[Enzo]

Generally, if you lose bias, the tubes run REAL hot, red plate, and then blow the fuse. In my experience, an amp with missing bias doesn't usually pop a fuse right off the bat.

[Chuck H]

Agreed. Wishful thinking. Every time someone posts in fear about blowing their OT I REALLY want it to be anything else. So far, so good. If none of the OT primaries are shorted to the laminates, frame or chassis then it's probably NOT the primary. Secondaries don't blow very often. Even if the primary is shorted between wraps, that's probably not going to instantly blow a fuse. So... Shorted screen resistor, charred tube socket, burned wire contacting something, shorted cap (and they DO short). It shouldn't be hard to diagnose.

[stokes]

Usually I find when people think the OT is blown,more times its not.A good quality OT like Fender used or most Hammonds usually take a lot of abuse.It seems this one is an unknown origin,well he knows it came from Hoffman,I'm not familiar with where he gets his.If its Hammond,he may be okay,if its one of those asian numbers...well.

[Enzo]

I agree the OT should be near last on the list, but his OP was classic for destroying one.

I agree that secondaries rarely short, other than the occasional primary short TO it. Usually turn to turn shorts/arcs on the primary result in it not working, rather than blowing fuses. But shorts to frame or arcs to frame will ground out the B+ and take fuses. Primary to secondary shorts/arcs will also, since the secondary is usually at ground.

[Leo_Gnardo]

ChuckH has a good point here: charred tube socket. Just had a Marshall late 70's lead amp on my bench yesterday. Owner was wailing on it when one of his bandmates tripped over the speaker cable & disconnected it. Saw a big flash from the back of the amp, and upon examination, charred arc mark on one tube socket and on the bottom of the tube. I was able to buzz away the char with a Dremel tool and the amp's back to working. At almost no cost too. You may have something similar, so examine those output tube sockets carefully.