Amp blowing fuses when turned off, only when not set to standby first?

[Low_fidelity2100]

What would cause an amp to blow fuses when turned off? If it's switched to standby first (i.e. No sound is passed), and then powered off, the fuse doesn't blow. But if the standby switch is in the "on" position (still makes sound), and then the power is turned off, the fuse blows.

Any ideas?

Amp is a Mesa heartbreaker. Fuses are correct 3A slow blow (what's indicated in the manual and on back of amp). Amp functions totally fine, has normal volume, and has no problems even when run for several hours, does seem to run a it hot, but not insanely hot. Just got the thing used, and beyond blowing fuses when it's turned off and not left on standby to do so, it's great and seems totally fine otherwise.
I've never had an amp that blew fuses when turned off, and not switched to standby. Seems weird.


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[dave74]

Re: Amp blowing fuses when turned off, only when not set to standby first?

That's weird indeed. I've read so many times that it makes no difference at all how you power down, except for when you want to drain the charge it should be turned off when Not on standby.

 

[Myaccount876]

Re: Amp blowing fuses when turned off, only when not set to standby first?

First thing to do is to replace your power tubes with a known working set. Power tubes can do all kinds of weird symptoms when they're on their way out, even though the amp still sounds ok (for now).

If it's not the tubes, then I'd check to make sure there isn't a short on the filter caps; when you turn the amp off without going to standby first, you and partially draining the filter caps and if there is a short/bad cap, that will blow a fuse. When you go to standby first, the only things in the circuit receiving power from the power transformer are your rectifier and tube heaters (the standby switch should disconnect the connection that sends power through everything else). Since your filter caps are taken out of the circuit in standby, I'm assuming that's why the fuse survives if you go to standby before turning the amp off.

 

[Low_fidelity2100]

Re: Amp blowing fuses when turned off, only when not set to standby first?

That's weird indeed. I've read so many times that it makes no difference at all how you power down, except for when you want to drain the charge it should be turned off when Not on standby.

Yeah same here, always heard it made no difference and never had it make any difference before.

First thing to do is to replace your power tubes with a known working set. Power tubes can do all kinds of weird symptoms when they're on their way out, even though the amp still sounds ok (for now).

If it's not the tubes, then I'd check to make sure there isn't a short on the filter caps; when you turn the amp off without going to standby first, you and partially draining the filter caps and if there is a short/bad cap, that will blow a fuse. When you go to standby first, the only things in the circuit receiving power from the power transformer are your rectifier and tube heaters (the standby switch should disconnect the connection that sends power through everything else). Since your filter caps are taken out of the circuit in standby, I'm assuming that's why the fuse survives if you go to standby before turning the amp off.

Hmm. I don't have a spare set of 6l6's at the moment. I suppose I could use the ones from two different amps for a minute without it causing any issue (be 2 matched pairs of 6l6's from two different amps that function fine). Although all the tubes in the Mesa are lighting as they should, not red plating or anything. So I dunno.

The shop I got it from did give me a 30 day guarantee on it. If swapping tubes doesn't do anything I should probably have them look at then I'm thinking? (I know enough about amps to know, I don't know enough to go messing around inside something this complicated haha)

 

[Nostalgic Distortion]

Re: Amp blowing fuses when turned off, only when not set to standby first?

My Silverface 72 Bandmaster Reverb started doing that a while back. For the most part I just started switching it & all my others to standby for 30 seconds or so before turning them off. Although like you both already mentioned I've always been told this shouldn't make any difference or hurt the amp in anyway? A few months down the road I was getting a little bit of tube rattle from the 5U4GB rectifier & it was sorta annoying at lower volumes so I changed it.

I haven't had a problem since!!! I had no idea what was causing it & still don't? All I do known is swapping the 5U4 did something positive but for the life of me I still don't see the connection??? The power tubes certainly would have made more sense but whatever, it works... I guess if I were you I'd start with the tubes as well & if you find your still having issues bring it back to be looked at, especially if you still have some kind of warranty on it. Let someone else go poking around in there & save yourself the hassle & inherent danger!!!

 

[SlyFoxx]

Re: Amp blowing fuses when turned off, only when not set to standby first?

If it's still under some kind of warranty and you feel the shop has the correct people diagnose the issue I'd start there. If they sold you an amp with only a few hours left on the power tubes or some other component you might make a good argument for them to eat the whole repair or at the very least a good break on the price.

 

[Myaccount876]

Re: Amp blowing fuses when turned off, only when not set to standby first?

My Silverface 72 Bandmaster Reverb started doing that a while back. For the most part I just started switching it & all my others to standby for 30 seconds or so before turning them off. Although like you both already mentioned I've always been told this shouldn't make any difference or hurt the amp in anyway? A few months down the road I was getting a little bit of tube rattle from the 5U4GB rectifier & it was sorta annoying at lower volumes so I changed it.

I haven't had a problem since!!! I had no idea what was causing it & still don't? All I do known is swapping the 5U4 did something positive but for the life of me I still don't see the connection??? The power tubes certainly would have made more sense but whatever, it works... I guess if I were you I'd start with the tubes as well & if you find your still having issues bring it back to be looked at, especially if you still have some kind of warranty on it. Let someone else go poking around in there & save yourself the hassle & inherent danger!!!

A failing rectifier tube can also blow the fuse. The rectifier is immediately after the power transformer and before the filter caps. With a failing rectifier, the wrong type of current may be sent/too much power etc and short the filter caps, mostly when there is a huge voltage change for the rectifier (i.e. Power on vs off) and blow the fuse. If it's not the filter caps, check the rectifier whether it's tube or SS. Does the Mesa Heartbreaker has a tube rectifier?

 

[Myaccount876]

Re: Amp blowing fuses when turned off, only when not set to standby first?

Just checked, your amp has a 5AR4 rectifier tube. Replace that along with the power tubes and see if the problem persists. Those will be the easiest things to check.

 

[Low_fidelity2100]

Amp blowing fuses when turned off, only when not set to standby first?

It has a switch to select between the tube rectifier and solid state. I'm assuming if I were to switch to solid state and try it, that'd show if it was the rec tube, right? An if it doesn't blow then I'd know it was the rectifier. Or am I totally wrong in thinking that?

UPDATE: so I switched the amp to the ss rectifier and turned it on, let it warm up, and turned it off without turning it to standby, and the fuse held. So I tried it a couple more times and the fuse held through them all. So I'm guessing it is the rectifier tube?

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[Demanic]

Re: Amp blowing fuses when turned off, only when not set to standby first?

That would stand to reason.

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[Nostalgic Distortion]

Re: Amp blowing fuses when turned off, only when not set to standby first?

Nice... I figured it was probably a tube & I'm glad you got it worked out. Being able to switch between the tube rectifier & a S.S. rec. is a pretty cool feature, one I wish I had on a few of my amp's.. At this point I've just gotten in the habit of turning all of my amp's to standby to warm up and before I power down. I know they say that it's harmless but so is the standby switch & "they" say a lot!!! Erring on the side of caution & all that....

 

[Low_fidelity2100]

Re: Amp blowing fuses when turned off, only when not set to standby first?

Yeah I think I'll continue switching to standby at power down for now, just to be safe for now. Still gonna call Mesa on Monday an see what they have to say too.
Thanks for all the help everyone.


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[Myaccount876]

Re: Amp blowing fuses when turned off, only when not set to standby first?

It has a switch to select between the tube rectifier and solid state. I'm assuming if I were to switch to solid state and try it, that'd show if it was the rec tube, right? An if it doesn't blow then I'd know it was the rectifier. Or am I totally wrong in thinking that?

UPDATE: so I switched the amp to the ss rectifier and turned it on, let it warm up, and turned it off without turning it to standby, and the fuse held. So I tried it a couple more times and the fuse held through them all. So I'm guessing it is the rectifier tube?

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Yep it is the tube rectifier then. Since you have the option of switching rectifiers, this would be a bit redundant, but for everyone else with tube rectifiers, I'd recommend getting your standard tubes and one retrofit solid state rectifier that plugs into the rectifier tube socket. Those are really handy and work in a pinch/troubleshooting. They obviously don't sound the same, but that's really a matter of taste on whether or not you like the saggy response or not of the actual tube in the rectifier socket.

 

[Boogie Bill]

Re: Amp blowing fuses when turned off, only when not set to standby first?

I doubt that it is the rectifier tube....usually they work or they don't. My layman's guess is that it is the ON/OFF switch causing the problem; an internal short between the contacts blows the fuse. You could pull the chassis and inspect the switch's wiring for bad solder joints or frayed or burned cable. You might even try some contact cleaner in the the switch. It may just be working better, as a result of switching it on and off a few times. Perhaps it sat for a while? But I suspect that Mesa will tell you to replace the switch. Do call them.

Good luck. (BTW, I love the Heartbreaker, been looking for one for a while. Enjoy!)

Bill

 

[crusty philtrum]

Re: Amp blowing fuses when turned off, only when not set to standby first?

I doubt that it is the rectifier tube....usually they work or they don't. My layman's guess is that it is the ON/OFF switch causing the problem; an internal short between the contacts blows the fuse.

Well he's switched the amp into s/s rectification and in that mode, he is able to switch it off any way he wants without blowing the fuse. The issue only occurs when in the tube rectification mode.

Mesa-labelled tubes have usually been, in my experience, just cheap crappy tubes. I would start by trying a different rectifier valve, and if that solves the problem, buy a high-quality replacement. Looking at the Mesa schematic, it appears to use a 5AR4 (GZ34) ... I have two amps in which I use Sovtek 5AR4s, and they have proven to be strong and reliable, and they're not outrageously priced.

 

[Low_fidelity2100]

Re: Amp blowing fuses when turned off, only when not set to standby first?

Talked to Mesa. Told them everything I've mentioned here (blows fuses when turned off w/out going to standby, but only in tube rectifier mode, etc). They said that in the heartbreaker the 5AR4 is being worked really hard when in 100watt mode with 6L6's and this has been known to happen with a lot of modern production 5AR4's. It's a voltage spike, due to the 5AR4 trying to pull more voltage than available as power decreases. Or something to that effect. Said the solution is to either run SS rectifier when using 6L6's @ 100watts, or switch the amplifier to standby when turning it off. He also said they recommend always switching to standby to turn off (or on obviously) with all of their amps regardless since it saves the tubes from any sudden power surges (thus making them last longer).
Also said while it's clearly not awesome that it will blow fuses with certain rectifiers if turned off without switching to standby, that it does absolutely no damage to the amp. Other than blowing fuses and making the rectifier tube die quicker.

So yeah. Makes me feel a lot better coming from Mesa themselves.

Semi unrelated he told me that if I do use EL34's, and only use two (60watt mode) that they recommend using a 5U4G instead of 5AR4 (says that in the manual too).


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[Demanic]

Re: Amp blowing fuses when turned off, only when not set to standby first?

Seems to me that the simplest thing to do is just make a habit of using the standby switch.

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[crusty philtrum]

Re: Amp blowing fuses when turned off, only when not set to standby first?

4 x 6L6 with a single 5AR4 ? Well that's gonna be a problem, I find it hard to believe they did that. Most rectifier tubes are only suitable for use with one pair, i.e. 40-50 Watt amps. The only reliable way to rectify 100 Watt (four power-tube) amps is to use either solid state rectification or TWO tube rectifiers.

 

[Low_fidelity2100]

Re: Amp blowing fuses when turned off, only when not set to standby first?

Seems to me that the simplest thing to do is just make a habit of using the standby switch.

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Yep. An that's exactly what I'm gonna do.

 

[Low_fidelity2100]

Re: Amp blowing fuses when turned off, only when not set to standby first?

4 x 6L6 with a single 5AR4 ? Well that's gonna be a problem, I find it hard to believe they did that. Most rectifier tubes are only suitable for use with one pair, i.e. 40-50 Watt amps. The only reliable way to rectify 100 Watt (four power-tube) amps is to use either solid state rectification or TWO tube rectifiers.

Yeah, definitely only has one 5AR4. I've only ever owned one amp prior to this that had a rectifier tube (vibro champ), so I really have no experience with them.