Ampeg SVT-6PRO death & resurrection

[MarcusB]

Fellow techs - I have an SVT-6pro on the bench and with a little help from an old thread http://music-electronics-forum.com/t10621/ I was able to nail down the majority of the issues without even powering her up. Thank you Glen & Enzo! This unit has 15 blown power mosfets and many of the supporting bias resistors. I'm swapping out all 20 mosfets and the associated 60 resistors. After reading that thread, I find I have more to check before powering up the beast and frankly, after spending list pricing with Loud on the transistors I am most timid about applying power afterward. The referenced thread speaks mostly to an SVT5 which is a similar design but very obviously different when it comes to the PA section having one output rather than 2 separate channels. I do not have an SVT6 schematic. I would be most grateful to anyone that can post a PDF of the PA section.

The historical thread eludes to a method of firing up the amp via variac and bulb limiter for the first time. I fear the low voltage could be a setup to create an issue as well. If anyone can break down the steps for me on a typical or prescribed method... once again I be sincerely grateful.

thanks in advance!
Mark

[Phostenix]

I'll save R.G. a post:

Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.


Oh, and did I mention that a light bulb limiter is really a help with this kind of thing?

[Phostenix]

How to build said light bulb limiter:
Light Bulb Current Limiter

[Phostenix]

Related info about variacs & light bulb limiters in amongst some other chatter:

http://music-electronics-forum.com/t16400/

[MarcusB]

Thank you. So everything I've read indicates the limiter is extremely useful for tube amps and I can see why when it comes to loads and transformer use and more but with a solid state amp am I all washed up in thinking that supplies with any form of regulation are designed as such by intent to setup quiescent operating states and voltages in a guaranteed manner? Wouldn't bringing up a solid state amp defeat that and potentially cause havoc? I guess if I think in terms of Ohms law the current would be proportionally low assuming the load is constant. Still, I be skiddish.

[Phostenix]

I'm not sure about all of that, but if the amp still has the potential to draw excessive current & re-destroy itself due to failed parts you haven't found yet, it should show up using the light bulb limiter or variac.

[Phostenix]

Hopefully, you'll get some advice from the more experienced techs here, but in a similar situation (transistors, not mosfets), I have started with only 1 set of outputs installed for the initial power up - so if it blows, I only lose 1 pair. Were there any resistors that burned? Another thing I've done is monitor those resistors while bringing up the voltage on the variac to make sure it doesn't go over the power rating of the resistor. Even if you have no idea what the voltage across a particular resistor should be, you can calulate the max voltage across it for its power rating & have a number that can't be exceeded.

[MarkusBass]

I'm not very experienced but I also think that with solid state amps low voltage can cause some issues. I usually either install 1 set of output transistors, or put small value resistors between the power supply and the power amp (something like 47 Ohms/0.25W). Maybe it's only luck but it (the second solution) worked always for me.

Mark

[MarcusB]

Thanks guys! I may try some of those recommendations. Could still use an SVT6pro schematic to have accurate bias adjustment info. Can someone please post a PDF?

[Phostenix]

From Ampeg's website:




For amplifier support or any questions concerning owner’s manuals or schematics please contact the LOUD Technologies Inc, please contact technicalsupport@loudtechinc.com.

Technical Support, Service and Parts departments are available Monday-Friday, 10am-Noon & 1pm-3pm Pacific Time


At this time Ampeg does not display schematics online. However, if you wish to obtain a physical copy of product schematics, please contact the LOUD Technologies Inc. Service Center at 1-800-898-3211.

[52 Bill]

In our shop, we have a light bulb limiter bolted down to the bench, so that anyone including non-techs can quickly check to see if an amp is safe to plug in.

In 95% of the cases it will work with tube or solid state amps with no problem at all. But, there are some solid state amps that don't like to be limited and will not power up correctly when checked this way. I don't recall all of the models that don't react well, but SWRs and some Ampegs come to mind.

I too have used Phostenix's method of testing with only one set of outputs installed to see if all was well before committing to a full set of new outputs.

[cowpoke]

I just finished doing a complete rebuild of a svt6-pro. Does anyone know the best way to set the bias on the lower amp. This is channel A (+) half.

Do I need to use jumpers and measure it upside down outside of the case? Is it safe to measure the bias without the heat sink? This seems risky.

The top section or (-) half is not a problem as the .47 ohm 5W resistors are accessible.

Any advise on testing the bottom section would be greatly appreciated.