70s SVT blows house fuses

[Hoss]

Hello Tb-ers!

Yesterday at a show my beloved SVT was killing the Ground Fault Circuit Interrupter.

Power switch in OFF-position - as soon as I plugged the power chord the GFCI of the stage fell. The SVT was not even turned on!
Then after 3 tries it would allow the plug to put in without blowing the GFCI. As soon as I switched the power switch the GFCI stopped electricity.

I had no backup amp and I had to use a 100Watt Roland Bass Combo

Back home I tried it at my house. Amp plugged - turned on - standby - plays absolutely flawless.
I was so happy to believe that the stage had bad current supply.
Then I switched the amp into standby and as soon as I switched the power OFF - the GFCI of my house turned off everything.
Unplugged the amp - reset the GFCI - Plugged the turned off amp and as soon as the plug was inserted the GFCI fell again.

I called my neighbour - he knows a little about electric stuff. Wanted to show him the problem. As he arrived everything worked and after 4 tries of - plug unplug switch on off - and no GFCI fell I felt a little dumb calling him up but everything works...

What do you think?

Thanks for helping me out - theres a big open air festival coming up next weekend and I need an amp I can rely on.

[Floyd Eye]

Take it to your favorite amp tech. I wouldn't trust it.

[MAMMOTHvolume]

power transformer going short.

[Hoss]

Thanks for your reply Floyd.
It was just checked recently at my favorite tech because of a hum problem. No big deal - only a slight hum coming from the preamp that can be eliminated with the hum-control knob but occurs again when the plug is twisted 180 degrees. I either adjust the hum control to the opposite position or I unplug - twist and plug. Same result - hum (nearly) gone.

After checking the whole amp my tech reported that he cant find a single fault in that amp. The hum of the preamp is inherent because the toroidial transformer has a strong field that the preamp amplifies. I shall check what position the plug should be for best result or adjust the hum control and life with it.
The only thing he did is, that he desoldered the polarity switch from the .047 cap. "Dont need it and it could cause noise."

The amp plays loud and clear and sounds great! It never blew fuses or GFCI before!

For sure I will bring it again to the shop on monday but I just wanted to know some opinions. It is a strange feeling when you receive your amp back from service and at the first show it will fail and nerves everyone.

[dmusic148]

Originally Posted by Hoss

Power switch in OFF-position - as soon as I plugged the power chord the GFCI of the stage fell. The SVT was not even turned on!

This suggests the power cord, or plug, is defective- I'd start by replacing the cord/plug. Or, the power switch is defective.

[Hoss]

Originally Posted by MAMMOTHvolume

power transformer going short.

Theres no connection to the power transformer when power switch is turned off I belive???
And even then its only the filament transformer before the Amp goes out of standby.
I hope so much it is just a faulty power switch...

[B-string]

Blowing a GFI is a clue there is a potential for shock. Amp switched off , no connection to the transformer. The leakage needs to be found in your amp, switch maybe, fray in the cord occasionally shorting to ground (chassis).....

[craig.p]

Another vote for the cord set. Especially if you're still on the original. Age, wear & tear, etc. etc. etc. Only a minuscule amount of leakage current needs to be present to trip a GFI.

[Rick Auricchio]

My vote is for the switch. I had a kitchen hand mixer that consistently blew the GFCI whenever I switched it on. If I left its switch on, I could plug it in and use it.

A GFCI senses unequal current flow on the hot and neutral and shuts off. If the switch doesn't make good contact simultaneously on both lines, the GFCI blows.

[rbonner]

OK look, this is probably a good time to do a couple things. The first is to figure out the leakage to ground. The second is to have the amp changed over to a three wire cord.

A good amp tech knows how to make this modification. What bothers me is the problem seems very intermittent. But this also says the path to ground is intermittant.

What I want to know are you calling a circuit breaker a GFCI? A GFI breaker is located in a bathroom and near water... Unless you have a super modern house with ARCFAULT breakers in the bedrooms...

If it is blowing a regular breaker that is a bunch different than popping a GFI.

BOB

[Jim C]

Originally Posted by rbonner

What I want to know are you calling a circuit breaker a GFCI? A GFI breaker is located in a bathroom and near water... Unless you have a super modern house with ARCFAULT breakers in the bedrooms...

If it is blowing a regular breaker that is a bunch different than popping a GFI.
BOB

Well said, not so sure that many tube amps (with unpolarized plugs) would work repeatadly with a modern, highly sensitive GFI, and no reason why they should as they weren't designed for this application as it was not code back in the day

[Jerrold Tiers]

Note that the OP is in Vienna, in Austria.

Possibility #1

It really IS a GFCI problem....... meaning an "earth fault" is present. That suggests a considerable amount of leakage current.

BUT, if in Vienna they use Schuko plugs as in Germany, the line and neutral can be reversed, the plug goes in either way. So the transformer might be leaky, because the power switch does not open both sides of the mains. It is at least possible that a leaky transformer might open the GFCI when plugged in one way, but not when plug is turned over, because the switch does not open both mains wires.

A leaky or otherwise defective mains cord could cause the problem also.

Possibility #2

It is an intermittent problem that causes the ordinary circuit breaker to open. That is most likely to be a problem with the mains cord. if the cord is in a certain position, maybe it can short the mains. In otehr positions it may not.

Because the amp works fine, I think there is nothing much wrong with it in terms of a short inside the amp. Most likely the internal fuse would open before the house breaker opened.
.
.

If neither of those is the problem, then maybe it needs to be passed through the shadow of St Steven's a few times...... I understand that cures many problems

One thing...... What is this about a toroidal transformer? The SVT should not HAVE a toroidal transformer...... it has a standard "square" one. Actually it has THREE standard square ones.

[Jim C]

Damn Hoss,
You never told us you lived far away in some strange land; you all probably got some strange voltage as well with all that fancy Uropian power.

What Jerrold said

[M0ses]

Originally Posted by Hoss

Then after 3 tries it would allow the plug to put in without blowing the GFCI. As soon as I switched the power switch the GFCI stopped electricity.

No offense, but it really would have been a very good idea to stop trying to plug in and turn on your expensive but finicky tube amp after something clearly indicated a problem three times. If you had managed to get it to turn on, something awful may have happened to it.

[Hoss]

Thanks a lot for all your answers!

Yes I am located in central Europe, Austria! I am 300 Miles from where your Californian Gouvernor is from, but thats another story.

Here we have what we call an "FI" or "Fehlerstromschutzschalter":
And heres the link to the translation programm I used:
http://dict.leo.org/?lp=ende&lang=de...schutzschalter

And here is a diagram of it: http://de.wikipedia.org/w/index.php?...20091011181450

We have normal AC with 50Hz but 230 Volt instead of 115. The Schuko Plug that Jerrold mentioned is infact to be used in both directions - always connected to the ground.

Of course my amp has a three prong chord. As I mentioned the amp just came from service, it was checked and since the last 2 Years regular use it never ever blew GFCI before! When I got it 2 Years ago the taps of the trannies were resoldered for European Voltage, the Plug was changed to a Schuko, it got new Tubes, new power-capacitor set from fliptops, new 5 Watt anode resistors and since then works FLAWLESS and sounds GREAT. Greater than any SVT I played before!

@Moses - The GFCI is a very sensitive and trusty thing - you could repeat this procedure 100 Times with no risk of getting a shock or damaging something.

The power cord indeed has a small tear in the very outer insulation. But the inside "padding" as obviously the insulated cables inside are intact. I tried to shake and tear on the cable when the amp was running to identify some tiny leaks there - nothing happened. The Schuko plug is wired properly as its nearly new and was installed by a pro tech.

The amp never got dropped and is always treated with care.

Jerrold -do you think that the behaviour of the hum balance in connection to the direction of how the Schuko is plugged is normal or does it indicate a fault? As discribed above - The "hum" only occurs "in one direction" - hum balance twisted 180 degrees or Schuko twisted it is nearly gone - and the hum is from the preamp ONLY. Shorting the pre-power connection and plugging my instrument into the Power-amp-in, the hum is totally gone - the power amp is dead silent (great filtering of the new set of caps from fliptops).

The toroidial transformer is located inside the preamp and is for the mid-EQ. Its a tiny 1.5 inches thing. I may send a picture?

Jerrold - if the transformer is leaky - wouldnt it then open the GFCI ALWAYS when the Schuko is in the specific direction? As written - when my neighbour arrived it worked with no problem in both directions - one with hum from the preamp - the other without hum.

The Power Switch is what I hope is the bad thing.
But: It looks normal from the outside, it clicks properly without wobbling, always puts the amp on when enganged. Obviously it still is the original from 1974. As so many switches are in this amp.

Infact the only thing that got changed since it happened the first time on Friday is that the Polarity switch got deconnected from the .047 capacitor. All other wires are still on the Polarity switch.

The really big problem is, that my bandmates gonna kill me when I am using an amp that occasionally blows this kind of protection circuits. Imagine this happens on a big open air festival stage when we open for a big band? They will never let us play again If that kind of problem occurs. A band coming with a defectife amp and already knew that.

By the way if anyone cares - heres a video of my band:
http://www.youtube.com/watch?v=sEmGMGIxWIY

Thanks again for all your answers to this problem. Thumbs up for the TB community!

[Floyd Eye]

Nice. I liked the ZZTop show vids.

[Hoss]

Thanks floyd!
It was fun to play that show - a big honour for all of us to open up for a legendary Band like ZZ-Top!!! Man - they fill the large halls since the 70s!

Back to my "patient" - I took out both the preamp and poweramp to pack them ready for the tech. I always dismount them so the tech saves time and can start right on.
I connected the preamp and poweramp stage, plugged the amp, connected to a 4 Ohm Speaker, Standby and...

...it worked fine.

Tried to mechanically "stress" the components a bit. I pulled on the power cord - shook the plug, turned power off, on, twisted the plug. I even punched on the trannies and shook the preamp unit while operating. Everything worked fine - no GFCI falls again. If there is a loose contact, a leaking part or shorting laminations of a tranny - It would short wouldnt it?

Its hard for me to either bring it to my tech and theres NO detectable fault, or play the show on Friday whith the risk to shut everything down on stage...

I should take Jerrolds advice and pass it to the shadows of st.stevens a few times. Maybe that helps.

PS: There was no fan connected now but it cant be the fault because theres no connection when power off, right?

[craig.p]

Instruct your tech to replace the cord set, including the plug.

I can't see how the problem would involve the power switch but it certainly wouldn't hurt to replace that, too.

These are both relatively cheap, and once they're replaced, you'll KNOW they're no longer possible contributors. Consider it in the context of tech hourly rate and shipping costs.

I wonder if there's some insulated wire inside the amp that's touching the chassis (earth ground) or something electrically continuous with the chassis, such as a metal standoff. Yes, even if what's touching earth ground is insulated it still might be a problem. I don't know European power, but if it's single-phase, wouldn't this cause leakage current from hot to earth ground, which would trip the GFI?

Side note: The eventual fix may have to no relation whatsoever to whether or not you were able to induce the failure, even though everything you tried ought to have been able to induce it.

[B-string]

Other thoughts I came up with are intermittent or fretting at the ground connection and the hum balance is making me think of a grid to cathode or heater leak in one of the preamp tubes. No power on possible residual voltage from the power supply going to the ground circuit through the hum balance?
Won't hold Arnold against you, even though I left my birth state because of him

That is not a toroidial transformer by the way, that is a multi-tap air core inductor.

[B-string]

The more I think about this the more I suspect control grid sag in a preamp tube. No tube tester will find it, swap out the preamp tubes one at a time with the hum balance centered. If one tube swap reduces the hum you found the troubled valve.