Tube guitar amp blowing fuses - suggestions?
August 13, 2012 6:51 PM   Subscribe

My Egnater Tweaker 40w head is blowing fuses. Possibly lightning-related, or possibly not, but I need to fix it. More details inside.

My fairly new Egnater Tweaker 40w head was accidentally left on in my outbuilding rehearsal room during a thunderstorm. We had a lightning related power outage during it.

When power came back the amp was dead, although it was at least 24 hours before I noticed so it may be unrelated to the storm.

I tracked this down to a blown 3A fuse in the mains socket, which I replaced. I powered it back on with all the tubes out to rule out a dead tube, and the fuse instantly blew again.

I live a couple of hours from anyone I trust to service amps, which is also how far I am from an Egnater service dealer, so if I can solve this myself so much the better. I'm happy working on tube amps and I know how not to kill myself.

Presumably this has to be a problem in the power section. Any clues? There's a surge protection cap which looks fine but is my main suspect at this point.
posted by unSane to Sports, Hobbies, & Recreation (14 answers total) 1 user marked this as a favorite
Response by poster: (I don't have a schematic for this amp but I do have a schematic for the 15w version which may not be too dissimilar in the power stage)
posted by unSane at 6:52 PM on August 13, 2012

I had a tube amp that was blowing the fuse instantly, and I took all the tubes out too, and it still blew. Turned out it was a blown power transformer. That's my guess for yours too. Shouldn't be too hard to source a replacement.
posted by Slinga at 7:20 PM on August 13, 2012

If you haven't yet, I'd also suggest posting this to the Amps and Cabs forum on You may get some more specific suggestions on what to look at or for.
posted by mosk at 9:11 PM on August 13, 2012

Response by poster: Thanks -- for whatever reason, thegearpage seems to be permanently overloaded for me.
posted by unSane at 9:42 PM on August 13, 2012

Looking at the "blown fuse troubleshooting" pages in my copy of How To Service Your Own Tube Amp by Tom Mitchell, it looks like the most likely culprits are bad output tubes (which you've ruled out), bad rectifier diode(s), bad filter caps, and bad power transformer. The flow chart suggests that you replace them in that order (rectifiers, filter caps, power transformer) - I think this is mostly a matter of cheapest/easiest first.

Also, of course, look around for burn marks, loose wires, metal debris that may be causing a short.

You should be able to disconnect the power transformer and do a basic test for internal shorts with a multi-meter.

I dunno about any surge protection cap - all of my experience has been with amps considerably older than yours, but that really doesn't sound familiar. Are you sure that's what it is?

If you can never get to thegearpage, maybe try the Telecaster Guitar Forum, TDPRI. I know they've got a "Tweaker Owners Amp Club" subforum/thread, and a pretty active DIY amp mod & repair community (Shock Brother's DIY Amps).
posted by soundguy99 at 10:26 PM on August 13, 2012

Response by poster: Are you sure that's what it is?

That's what it's labelled on the PCB. It's basically a big ceramic cap in series with one of the hot mains wires. I don't have the value right here. I assume it's a big enough value that the AC passes right through but it will suck up a DC peak. It looks like it's the first thing in line after the 120/250v switch. If there was a DC surge because of a lightning strike it might have taken a hit before the fuse had a chance to blow, I guess.
posted by unSane at 10:36 PM on August 13, 2012

I'd really look hard at the power transformer no matter what. A surge cap is not a lightning arrestor and who knows what kinds of voltages were present in the amp before the fuse blew.
posted by tommasz at 5:25 AM on August 14, 2012

First, do you know electronics? This is a power amp. If there are any large electrolytic rail caps, they could be holding a large amount of energy, which they will *happily* dump into you if you don't know what you are doing. If you don't know, and have the gear, to isolate the amp from mains, you should not be doing this. I'm not going to tell you how, because if you don't know, you should not be doing this.

A instant-blow without the output tubes means there's a dead short from power to ground that doesn't pass through the tube socket. There are two places this is likely to occur -- the output transformers (if present) and rail caps, if any. Other places are rectifiers/diodes creating HV DC -- a blown diode that conducts is basically a wire, after all -- and the preamp transformer, since this amp appears to have one.

First step is a good light. Any caps have black marks or goo under them? Bad, replace. Any smoke marks mean that a lot of voltage went through that component, and if you see a track to ground, it arced to ground there. Anything that's black now and wasn't is under suspicion. Stereo amps have an advantage here in that it's rare to have the exact same failure on both sides.
posted by eriko at 6:33 AM on August 14, 2012

Response by poster: Hey, Eriko, as I said above I know how to not kill myself with tube amps. Nothing looks fried, although some of the components are hidden and I'd have to do some dismantling to get at them. I'll check the caps first and the transformer.
posted by unSane at 6:43 AM on August 14, 2012

Response by poster: Okay, I think I figured it out. The surge suppressor isn't a cap, it's a varistor (they look kinda the same). It only measures about 12 ohms on the meter whidh I think means it's blown and dumping the AC to ground. That would jibe with a big voltage surge during the storm.
posted by unSane at 7:05 AM on August 14, 2012

Response by poster: (of course that might not be the ONLY problem...)
posted by unSane at 7:05 AM on August 14, 2012

Yeah, I was just about to say that I didn't see how a cap simply "inserted" in line on the hot leads would cause a short that would blow a fuse. No power at all, maybe, but no short to ground.

Hopefully it is just a nice cheap part like that varistor. My sympathies that you're having such bad luck with your relatively new toy.
posted by soundguy99 at 7:15 AM on August 14, 2012

Response by poster: So I got a local tech to look at it and it turns out the amp is fried. The transformer is blown and there's some other stuff downstream too, but he couldn't tell how much without first replacing the transformer. It was uneconomic for him to repair. Looking into insurance now but also checking with one of the authorised repair guys.
posted by unSane at 4:58 PM on September 5, 2012

Response by poster: Final update: the amp is working again! It was warrantied by Egnater and all it needed was a new transformer. No charge, apart from paying the original local tech to look at it.
posted by unSane at 7:14 PM on November 1, 2012 [1 favorite]

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