Ampeg SVT-CL

[The Dude]

Sorry for helping to steer your thread so far off course. But, since it's your baby,.....

The Lab Series amps were designed and built by Moog and distributed by Gibson. I theorize Billy used them to emulate the Legend amp he used on the Eliminator album. Both had primarily tube preamps and solid state power amps. I'm often surprised by his tendency to use solid state power amps. You would think, being a blues rock player, that he would be a purist tube guy- not always the case.

Another one that gets me is when people ask how to get the "Billy Gibbons tone". IMO there isn't A Billy Gibbons tone. If you listen from album to album, his tone is vastly different from one to the other. All good, but all different. I don't think his live tone these days is anything like his live tone was when he played through the modified plexi's.

[bsco]

Hey man..don't worry about steering the thread off course...that seems to have been done quite a few posts back...and besides, I am learning something here that I did not know before.....
Cheers,
Bernie

[bob p]

getting back to the original intent of the thread:

I avoid using oversized/over-rated resistors. I like to use metal oxides because the package is reasonably sized for it's power rating and doesn't add unnecessary mechanical stress.

Although some people are inclined to increase the wattage spec on plate or screen resistors to decrease the incidence of failures, I avoid putting too large of a wattage spec resistor in these locations; there are amps like Komets that will use vitreous enamel resistors on the screen grids so that the resistors will never burn up. I think that's a bad idea because if the resistor is made bulletproof and it isn't allowed to fail when it needs to fail, then the result is that you shift the point of failure to a more expensive component. My approach is that it's OK for a resistor to fail -- even if it takes the amp out of service -- because that failure of an inexpensive part protects the amp from enduring a catastrophic failure somewhere else.

Just my $0.02.

[Jazz P Bass]

I have made this point myself only to be rebuffed by the eminent SGM thusly: " Ah, so you Want the resistor to act like a fuse".
Duh, yes.

[Leo_Gnardo]

I have made this point myself only to be rebuffed by the eminent SGM thusly: " Ah, so you Want the resistor to act like a fuse".
Duh, yes.

I'm with ya JPB!

When the "real" Ampegs were made in the "real" USA they put resistors in series with the plates (!!!) on their "real" SVT's.

To. act. as. fuses.

Currently (look out SGM-talk coming up) the ersatz Cratepeg made in China so-called "SVT" is the item, they have switched their focus to having "fusing" and "standby" functions to the screen grids instead of plates. Yes those tiny screen grid resistors are supposed to burn up in case an output tube goes shorty-short and that's the game. What a pain in the neck to replace them too, also knowing they're in a position to be sacrificed in case of output tube failure.

Of course those who are too smart for their britches can go ahead and put in any resistor they want or why not just bypass with a sturdy piece of wire, then when the tubes fail they can replace SVT transformers instead. (Caution I DO NOT RECOMMEND this last "solution.")

[Mick Bailey]

I avoid using oversized/over-rated resistors.

I'm with you on that. I use fusible flameproof MO (the spec that Fender fits) with a sensible rating for the amp. You can't guarantee that a blown HT fuse will get replaced with the correct value, particularly if it blows at a`gig. So a fusible resistor is a second line of protection against damage from a shorted tube.

[soundguruman]

I have made this point myself only to be rebuffed by the eminent SGM thusly: " Ah, so you Want the resistor to act like a fuse".
Duh, yes.

Yes on one hand the resistor acts as a fuse.
on the other hand if I use bigger resistor , less noise, even over longtime heating cooling..
But if I use a big resistor, I know there is no circuit fuse...

And I see that any component could short at any time, but.............. it doesn't "usually."
so there is a ying / yang to this.

and I see fender marshall use 1/4 watt to act as fuse too....
but I also realize that short / long-term noise from heating cooling is compromised.

and there is several other amps that use 1 watt, not just me.

yeah there is 2 sides to it.

The classic struggle between good and evil.

[bsco]

I totally agree with NOT INCREASING RESISTOR WATTAGE by very much ..I would rather tear the unit apart to replace a burnt out resistor rather than having to replace transformers, burnt up boards, etc.....the resistor is much much cheaper in the long run..

[bob p]

> Duh, yes.

Well, that WAS the intent of the original design, right?

I've heard someone say all too often that the SVT can blow a pair of power tubes and keep on going, just like the Energizer Bunny, as if that's a GOOD thing. I don't agree that it is.

The problem is that if you blow out a pair of tubes due to something that's seriously awry in the amp, then you're supposed to shut the amp off and switch over to your back-up. If you're a numbskull who insists on driving an amp into the ground in a blaze of glory after it has malfunctioned by blowing out a pair of tubes, then you're likely to take out more hardware that's more expensive. If you're not smart enough to pay attention to the blown-tube warning signs then you're just asking for the catastrophic failure. Why tempt fate?

I've stated previously that I think the design of the safety circuits on the new "Crate-Peg" SVT is actually a good feature. They represent a step forward in the design that wasn't availabe in 1969. Back in 1969 Ampeg used fusible link plate resistors to protect the amp because that was the technology that was available to them. Fast forward to the 21st Century and now we have IC comparator arrays that monitor the voltage drop across current sensing resistors in the 6 power tube cathodes. The comparators are smart enough to sense a seriously out of spec condition and force the amp into protection mode so that incinerating a fusible link resistor isn't even necessary.

In both cases we have responsible engineers who used the current state of the art technology to protect the amp from a catastrophic failure. I would argue that the 21st-Century comparator circuit is a superior design when it comes to protecting the amp, because it renders it impossible for some idiot to continue to run an amp into the ground after a pair of tubes have already failed. (Of course, if you don't understand how the circuit works you're likely to badmouth it as a "Crate-Peg" design that cannot be found in a "real Ampeg.")

In the past, the end user had to be smart enough to recognize a problem and shut off the amp so that he didn't sail his ship onto the rocks. Today we've got amps that are smart enough to do this for you, so that even a drunk or stoned musician will get the message: time to switch to your backup amp to finish the gig. I think that this is a good thing.

Just my $0.02.