Help with Sunn 200S

[ineedhelp!]

OK so I'm not really too good with the whole electronics thing..but I have a Sunn 200S that was given to me, and whenever I take the amp off stand by it blows the 3A fuse in the back. I took it apart and nothing looks blatantly wrong...so any ideas for the not so smart me? haha thanks for the help!

[txstrat]

Welcome to the forum.
Try the amp with the power tubes pulled. Still blows fuses? If so, pull the rectifier tube too. What happens?

[ineedhelp!]

ok o I tried pulling every combination of the tubes, and the fuse blows every time..very annoying...so I thought maybe the stand by switch was bad...? is that a possibility? and if so, then how would I go about checking that out? or is their something else I should be trying?

Any and all suggestions are appreciated!

Thanks

[txstrat]

What gives you the idea of the stand by switch being the culprit?
Does the fuse blow when you switch the amp into "play" mode? Or does the fuse also blow when the switch is in "stand by" mode, while you power it up?
Anyway, when the tubes are pulled and the fuse pops it's not a simple thing to solve either way (definitely not the stand by switch). You should bring the amp to a qualified amp tech, since the voltages inside a tube amp can kill you.

[ineedhelp!]

Well, everything seems fine when the amp is turned on. The tubes warm up and all, but it is only when the amp is switched into "play" mode when the fuse pops. I probably take it to a service center but I'd like to have an idea of what the problem might be beforehand. Any help is appreciated! thanks!

[booj]

The standby switch connects the high voltage to the circuit. If the mains fuse blows you could have a problem with the power supply. If it blows with all the tubes removed it may have a wiring error from a previous fix.

[melvin]

Do you have a 3 amp slo-blo fuse in it?

I would suspect a bad filter cap. The rest of the power supply should be alright... It's also not the power tubes if it's going out with 'em pulled...

[soundmasterg]

Pull all of the tubes out of the amp and turn it on. If the fuse blows when you flip the standby to ON then the power transformer could be bad, but it could also be a problem in the bias supply...a bad cap, diode, resistor, etc. It also might be a problem with the heater wiring.

If the fuse does not blow, then turn it off, put the rectifier tube back in, and repeat the process. If the fuse blows here, then it could be a bad rectifier tube, or a bad cap, or a bad choke.

If the fuse does not blow, then put the power tubes in and try again. If the fuse blows here then it could be a bad power tube, bad caps, bad choke, etc.

Quite a few things it could be, but it requires some testing to find out. If you don't know how to work on high voltage electrical equipment, then you should take it to a tech as tube amps are considered high voltage equipment, and they do have lethal voltages inside them.

If you do find that the power transformer is bad, then you can get a replacement at Triode Electronics among other places.

Greg

[Dave Curtis, dB AudioTech]

Better yet, rig up a light bulb current limiter, and stop feeding it fuses...

(just clipping a lamp across the empty fuseholder would work)

[Enzo]

If the power transformer were bad, it wouldn;t care what the standby switch was doing. It would already be blowing the fuse.

heater circuits are live during standby, so we can rule them out.

The rectifier tube is live always as well, so if it were shorted, we'd know it even in standby.

the bias supply is always live too, so again, it must be OK.

Your output transformer could be shorted to frame or to secondary from the primary. Your choke could be shorted to frame. Your filter caps could be shorted or installed backwards. You could have an arcing tube socket.

[John!]

The rectifier tube is live always as well, so if it were shorted, we'd know it even in standby.

While it would seem that the rectifier is good, it might not be. I have a 200s that did the same thing. Everything looked good until the standby was flipped on, then the fuse blew. Replaced the recifier tube and it worked fine.

[Enzo]

Fair enough.