Amp Tripping Circuit Breaker in Practice Studio (but nowhere else!)

[harry]

Hi all,

I have built an amp based upon an old Sound City. It's not the first amp I've built, and all the others work just fine. This one works fine for me at home, and at one gig I've played - however when I take it to the local practice studio it trips the house circuit breaker and all the lights go out.

Any idea what could be doing this? The amp has 6xEL34s and 5x12AX7/AT7s. It does not have a standby switch for the HT. The only other thing I can think of is that at home and at the gig I played I was plugging into a 4-way extension and at the practice studio I was going straight into a wall socket.

A little confused and worried! I'd intended to take this amp on tour, but not if it trips circuit breakers everywhere.

Thanks,

Harry

[Enzo]

So at home where it works plugged into a 4-way thingie, try plugging it straight into a wall outlet. Now does it blow the breaker?


At this point we either have a problem in the amp or a problem in the practice studio wiring.

I'd go to the hardware or Home Depot, and in the electrical area they will have these little outlet testers. It looks like an electrical plug with three little lights on it. You plug it into the wall outlet or the outlet strip or whatever, and the lights tell you if it is wired right. You should carry one of these around anyway, always check the wiring whenever you go to a new gig venue. I have to imagine they have similar products in the UK.

If the practice place wiring is wrong, tell them, show them.

Or the amp you build could have a miswired power cord. Make sure the green wire from the power cord is connected to the amp chassis. See that the two hot wires go where they should. And check that neither is connected somehow to chassis. Unplug from the mains, and check from either wire to chassis, there should NOT be any continuity.

[g1]

Sounds like the circuit breaker at the studio may be overloaded or defective. How much stuff is running off that particular breaker? Can you unplug some other stuff or turn off some of the lights?
The amp itself should have it's own fuse or breaker. If it is the correct type and not blowing then the amp should not be the problem.

[bkahuna]

Sounds like a breaker weakened by people plugging in too much crap for too many years.
If you have an appropriate fuse in the amp, and it's wired correctly, it almost has to be the breaker.

[R.G.]

Sounds like a breaker weakened by people plugging in too much crap for too many years.
If you have an appropriate fuse in the amp, and it's wired correctly, it almost has to be the breaker.

Add my vote for what all of enzo, g-one, and bkahuna posted.

Get one of the plug-in wall socket testers. They're under $10 most places, sometimes under $5. Very cheap insurance for what they do.

If the power outlet is not wired correctly, some of the faulty conditions **endanger your life** if you play, because you're connected to what is supposed to be the safety ground by your guitar strings. Everyone is familiar with getting shocked on microphones. Some people have died from the issue.

If this happens only at the studio, the *studio* has a problem.

Breakers do get old and wear out. They cost under $20 on average at Home Depot if they are a standard type. It's easy to plug in a different breaker.

[harry]

Thanks for the replies everyone.

The thing is, after we tried mine, my bandmates turned on four other amps between them - some 100w tube amps. In fact I borrowed the house JCM900. Granted, we didn't turn them all on at the same time, but it still suggests to me that something might be at fault with my amp. In terms of what could make a difference to the initial power surge, the only difference between my amp and the JCM900 is two extra EL34s and an extra ECC83.

I just tried the amp at home straight into the wall. Works fine (and sounds good!). I then unplugged the power socket from the wall and measured resistance with a meter across all three pins. My cheap meter only goes up to 2M, but nothing registered on it between any of the pins.

Looking at the schematic of the amp's power section, there is a 1.8R resistor that I don't remember installing: http://www.soundcitysite.com/sc_webpages/maxscmod.jpg - could this be to limit inrush current?`

[Enzo]

The resistance test on the mains plug - try it with the amp power switch set to ON. Obviously the amp will not be plugged into the wall or energized, but the ON switch wil complete the circuit through the transformer. That way you SHOULD get a reading between the two hot pins of the plug - the reading will be the resistance of the transformer winding. It will lilely be a low resistance. However, we still do NOT want to see a low resistance to that ground pin.

[MWJB]

"The thing is, after we tried mine, my bandmates turned on four other amps between them - some 100w tube amps. In fact I borrowed the house JCM900. Granted, we didn't turn them all on at the same time, but it still suggests to me that something might be at fault with my amp. In terms of what could make a difference to the initial power surge, the only difference between my amp and the JCM900 is two extra EL34s and an extra ECC83." No, the difference between your amp & the 100W Marshalls isn't just the tubes but a greatly increased VA consumption, should have 50% more heater current available compared to the 100W amps at least. If it only does this in the studio outlet, probability is that it is the outlet, not the amp (best check to be sure though) as Bkahuna says.

In the UK we have fused AC plugs, my amp with a 200W PT blows anything smaller than a 13A mains fuse.

[gbono]

In the US we sell a product called the Killawatt
Newegg.com - P3 Kill A Watt Electricity Load Meter and Monitor
Though I never checked to see if this company makes a version for EU customers. Maybe something similar would be useful in diagnosing your issue with mains over current protection?

[MWJB]

How would this help?

[gbono]

Mmmm, the unit allows you to view your amplifier's, or any appliance under 1800W, real time current, voltage, power factor, etc. I assume you would find this useful since your unit appears to be drawing down the mains OCPD????

I use mine as a watt meter, to set output transistor bias for some amp designs, and many other tasks.

[MWJB]

Sure, it's going to let you view the consumption in the wall sockets where the amp works OK, but it's not going to stop the circuit breaker in the studio from tripping.

[gbono]

Yes the only thing that will stop the studio's OCPD from tripping is - a correct understanding of the actual load(s) on the circuit (I suggest the killawatt) and an analysis of the performance of the OCPD under the known loading condition(s). Outcomes could be: actual loads are less than the rating of OCPD, actual loads are greater than the rating of the OCPD. Next you conduct tests to see if OCPD is faulty, conduct tests to verify integrity of wiring system, etc. I believe this process is known as the scientific principle -

[MWJB]

"I believe this process is known as the scientific principle - " Smartarse. If the OCPD is fine & the studio owner doesn't see the need to spend money on account of 1 guy's amp, then you're back at square one, spent $35 & time wasted for nothing.

Most folks would either - not plug into the underrated wall socket/ask the studio owner whether the breaker could be uprated, or for an alternative outlet/use a different amp with a smaller current inrush...I believe that's called being practical.

[gbono]

Sorry the facts of life upset you - you have your answer now don't you?