Ampeg SVT 6 Pro Blowing Power Supplies?

[V2factoryman]

Hi all,

New to the forum and only just got back into bass playing seriously over the last three years or so after a long lay off. I bought a used SVT 6 Pro and an Ampeg 1 x 15 and 2 x10 (separate deals on ebay), to upgrade from an Ashdown MAG300 + 1x15 + 2 x 10.

Cabs and amp are awesome once working, however I am experiencing an intermittent but serious problem when the amp is first switched on. I've been to three gigs with it, twice I've had the following problem and once it was fine.

The green power led and the red mute led illuminate briefly then die. This is because the power supply has been tripped. This has happened in three separate venues (including at home), so it's not the power supply per se. The fuse in the plug doesn't blow (it's 13A), neither does the fuse in the amp.

The first time it happened I was in a rehearsal facility, Blueprint in Manchester. I changed the ac lead in and it worked once we had reset the power supply

The last time this happened was Sunday when we were preparing to play a local festival and it knocked out the whole of the left hand side of the stage! As you can imagine I was pretty tense/anxious/annoyed! I changed the ac lead in, knocked the power out again. Eventually the techs from the PA outfit ran a direct power lead from the genny using a special ac adaptor and it powered up and ran fine.

However I can't have this sort of stress every time I go out and play, this was a big outlay to me in what I thought was an iconic name but now I'm worried to use it! I kept the Ashdown head as insurance and used that on the next gig Monday night.

Does anybody have any idea what might be causing this? Is there a big power surge when it's first switched on? What sort of ac lead in should I be using, does it have to be heavy gauge? (the amp didn't come with it's own lead, I'm using the one off the Ashdown). Hope someone can help, really disillusioned at the mo! :-(

Cheers,

Andy.

[PURPOLARIS]

I used to have an SVT 6 Pro and it had the same problem although not as bad as yours. It used to trip my house power occasionally when first powered up but was fine elsewhere. It does draw a lot of power when first switched on apparently. There is a fix for it, I remember reading something about it at the time.

[Musky]

[quote name='PURPOLARIS' timestamp='1346265873' post='1787258']
I used to have an SVT 6 Pro and it had the same problem although not as bad as yours. It used to trip my house power occasionally when first powered up but was fine elsewhere. It does draw a lot of power when first switched on apparently. There is a fix for it, I remember reading something about it at the time.
[/quote]
Yeah, I remember that as well, probably this thread - [url="http://basschat.co.uk/topic/33872-help-with-inrush-current"]http://basschat.co.u...-inrush-current[/url]

Oh, and welcome to the forum Andy.

Edited August 29, 2012 by Musky

[MarkBassChat]

The solution is very simply; you have to put a thermistor in series with primary winding of the power transformer (Ampeg also uses this solution). Similar to this one:
[url="http://uk.farnell.com/ametherm/sl22-5r012/thermistor-series-sl22/dp/1653457"]http://uk.farnell.com/ametherm/sl22-5r012/thermistor-series-sl22/dp/1653457[/url]
You just have to make sure that the maximum allowed current (for the thermistor) is equal or greater than the maximum current drawn by the amp. For me 5 Ohms and 10-15A should be OK.
And be sure that you know what you do - this part of the amp is connected directly to the mains power.

Mark

[V2factoryman]

Wow, thank you so much one and all for the prompt and amazingly helpful replies! I think I will go for the Canford solution as identified by Martin in the thread which Musky listed. Although it's expensive I know zilch about electronic components and the idea of amending the circuitry in the amp fills me with dread that I might zap myself on stage one night. Probably an irrational fear but I have no wish to become toast.

With a separate unit it can also be used with other equipment if the same problem arises. I'll get one and let you know how I get on.

Thank you all once again, what a nice bunch you are :-)

[paddy mcbride]

Hi Andy

The thermistor in a box has worked a treat and I have never had a problem with the amp since. Not bad for 4 years of use.

Paddy.

[V2factoryman]

Hi everybody,

I heard back from Paddy about his solution, which I was going to take up.

Then I met a tech who knew precisely the problem, and proposed the solution wiring in the thermistor to the circuit in the amp housing itself, taking care to keep everything sheathed and locating it in an area where the heat would dissipate. By the way, he calculated that the operating temperature of the thermistor would be around 60 degrees C.

Tried it out last night at a gig and it worked a treat, massive tone and power, amp never got more than vaguely lukewarm so heat is dissipating fine. finally got what I wanted after more than thirty years in and out of the business. Will now update my profile! Thanks for all your help fellas! Yee haa!