What's with the new SVT's?

[clyde1]

I've an ongoing issue with a newer (reissue) SVT. It just shuts down by itself when in use, apparently the protection circuit in the AC line does this. I haven't had it do it on the bench yet because it hasn't decided to cooperate.
A sound reinforcement company in town has had the same trouble with the two they've purchased. I can see a circuit board on the ac line in and assume
it's a protection circuit that's overprotective (like the Patriot Act?)-Oops! and not allowing any joy. Anyone have an experience with this? I'm thinking I'll just yank it out when I get the amp or would that be rash?

[Rastus_1]

I recently bought a SVT-CL & had problems with distortion at higher volumes & occasionall cuttout's. I also had trouble reproducing the fault untill I left it with the shop turned on & let them have a play with it. A tone generator is not enough, we needed a hot amp & actuall bass playing to reproduce the fault. As it was under warranty I just kept sending it back untill the problems were resolved. Turned out to be under-rated components in & around the protection circuit for the 240Vac (Australia). I didnt do anything myself so cannot give details, just going by the repair docket reports.

Yes it was a headache, having bought a very expensive amp just to keep sending it back to the shop, but after that got sorted, all is well. Two years (and a lot of gigs) later, it has never let me down and one of the best investments I have made for pure clean tone at volume. Maybe not such a good investment for my back (very heavy) but hey, you can't have everything.

The fan in the CL is really weak (bad design) which may have added to my cutting out events, so whenever I play now there is a small desktop fan I wedge behind the head. (I live in the tropics where heat, humidity & sweat are part of life). One day I will install a decent fan in the back grill.

Cheers
Geoff

[Enzo]

I just posted a service bulletin concerning this matter on www.ampix.org in the Enzo gallery. See if it helps.

Seems to me you could disable the thing completely by removing R51. Not saying I'd want to do that.

[Steve Conner]

As an EE, I'm always interested to see what people are doing with these protection circuits.

On the one hand, a big amp like the SVT can make a heck of an expensive mess if it blows up.

On the other hand, an amp that turns itself off when you crank it goes against all the principles of rock and roll.

I've experimented with these circuits in the past in my own amps, but always ended up yanking them out and throwing them away. I suspect Ampeg went for an electronic protection circuit because it worked out cheaper than a fuse! To fuse the 700V B+ supply in a way that met all the international safety standards, you'd probably need some kind of heavy-duty industrial fuse, in a holder buried inside the chassis to make it completely touchproof. That would mean a warranty service call at Ampeg's expense every time the fuse popped.

Also, I agree about that puny fan in the SVT-CL. Well, maybe not so much the fan as the grill cloth over it, that must cut down airflow. I'd bet just cutting a hole in the cloth would boost airflow quite a lot, at the expense of ruining the amp's looks of course

[clyde1]

Thanks Enzo, I'll pull that up again when the amp comes back in. I'm thinking
I'll yank the whole unit out like Steve suggests and have a look at the fan situation. The original didn't have any circuit at all, just AC in to power transformer, it seemed to work fine for most people. Nice site BTW Enzo.

[Rick Erickson]

Yanking the protection circuits out seems like swatting a fly with a sledgehammer. I would start by re-soldering anyrhing that even LOOKS like it might come loose. The newer amps are a pain because of all the pc boards involved - but you have to go through them all if you want the amp to function correctly. I didn't look at Enzo's service bulletin yet but there are issues with the diodes used in the overload sense circuit. Another issue is the cheesy stacked plastic jacks on the rear panel. If the switch contacts become intermittent they can cause all kinds of trouble - including, iirc, triggering the mute circuit. I have worked on SVT's that had the 5 Ohm plate resistors removed and replaced with jumpers. It wasn't pretty - in fact the power transformer was smoked as a result.

RE

[Enzo]

Look at the bulletin, it is not over long nor very technical. Just some tweaking to the circuit. With enough gripes about the issue, I'd be willing to believe that is what we have here.

[clyde1]

Enzo,
Could you use a sub for the 1N3070 diodes required?

[Enzo]

Look at the schematic. Those diodes are across R8, which is the input to the power amp. All they do is clamp the input signal. Same as clipping diodes in some guitar amp. The 1N3070 has a forward voltage of 1v. And a reverse rating of 200v. Current isn't an issue, but it is rated 500ma.

For all the world, it is a beefier 1N4148 (forward 1v, reverse 100v, forward cirrent 200ma) Both have a 1 second surge rating of 1A. Plenty for signal. And you are not likely to encounter in excess of 100v on a signal jack. Unless somethign with a hot chassis is connected there. SO a 1N4148 would likely work fine. However, Mouser sells real 1N3070 for 5 cents each...

[mykey]

I've an ongoing issue with a newer (reissue) SVT. It just shuts down by itself when in use, apparently the protection circuit in the AC line does this. I haven't had it do it on the bench yet because it hasn't decided to cooperate.
A sound reinforcement company in town has had the same trouble with the two they've purchased. I can see a circuit board on the ac line in and assume
it's a protection circuit that's overprotective (like the Patriot Act?)-Oops! and not allowing any joy. Anyone have an experience with this? I'm thinking I'll just yank it out when I get the amp or would that be rash?

this is a crate amp with an ampeg logo stuck on the front, there is no cure except the scrap heap. the protection circuit is going off because there's a short circuit. It's not anything like a real SVT. The bias is set all wrong just like on all crate amps, and it overheats and damages the transistors. give up.
diodes? shmyodes! diodes won't fix this.
The factory has been moved, they invented it, let THEM fix it! after they fix it, might work for an hour or two.

[Enzo]

Mykey, how on earth is that remark going to be helpful? THOUSANDS of people tour with SVTs and are quite happy with them. Your automatic anything-from-SLM-sucks remarks don't get people through gigs.

Perhaps Mykey, you would be kind enough to tell us exactly WHICH transistors are overheating and getting damaged in this TUBE amp.

The protection circuit is not detecting short circuits, it is overly sensitive to peaks. The 1N3070 diodes are added to limit peaks. The protection threshold can be raised with a few component changes. This protection tripping problem only occurs under certain conditions.

[Rastus_1]

As a professional musician, I love my SVT-CL. After initial faults (cutting out etc) were repaired, the amp has proved to be a reliable source of awesome valve tone at any volume. I along with most other pro & amateur musicians love them.

As a user I am glad I have the protection circuitry, if something does go wrong, the amp will shut down before too much damage is done either to myself or the amp.

As a tech, I am really glad about the circuitry as I understand & have seen what happens when that something does go wrong. Also it will be a lot cheaper & easier to fix.

These new SVT’s are not as good as the old ones, no problem with that. Point to point wiring is much better & the old amps do have that certain something in the sound. The new amps use circuit boards, so in my mind something is more likely to stuff up. So I am keeping the protection circuitry firmly in place. There has been a few times where I forgot to plug in the speaker lead, the amp just goes straight into fault mode once standby button is closed. This protects my investment from myself, & that can’t be bad.

As mentioned before by me & Enzo, the “Threshold” does require adjustment to suit local differences in power supply. Once over that hurdle, you (or your customer) will end up with a great amp.
I don’t think there has been a mention of which SVT you are trying to repair (there are several). Transistors would suggest an SVT-3.

Cheers
Geoff

[mykey]

Mykey, how on earth is that remark going to be helpful? THOUSANDS of people tour with SVTs and are quite happy with them. Your automatic anything-from-SLM-sucks remarks don't get people through gigs.

Perhaps Mykey, you would be kind enough to tell us exactly WHICH transistors are overheating and getting damaged in this TUBE amp.

The protection circuit is not detecting short circuits, it is overly sensitive to peaks. The 1N3070 diodes are added to limit peaks. The protection threshold can be raised with a few component changes. This protection tripping problem only occurs under certain conditions.

It's helpful because people want to know the difference between reliable equipment and junk. Crate amps, no matter what misleading brand name has been attached are the same design in a different box. They all suffer from the same problems and the same design defects.
OK when you tell someone to change the protection circuit design, you are admitting that the amp was designed wrong, thanks. You helped prove the point.
As I said many times, it is better to let the factory take responsibility for the failures rather than being blamed yourself. And after the factory fixes it, it WILL break again. Let the customer be pissed at the factory, not the tech or the music store; that is if the store was foolish enough to sell it in the first place.
HOT biasing of the output transistors to reduce distortion and very poorly designed heat sinks are one of the most common problems. (will someone tell the factory that heat RISES?) a thermostatic control (which fails) compounds it. Arching and burning through the circuit board is just another one of many fun features. After the transistors are overheated again and again, they fall on their butts. CLUE: heat the transistors with a hair dryer BEFORE testing them.
THOUSANDS(?) of people tour with crap? I rather doubt that. The only reason some (few) tour with these heaps is that the equipment has been given to them at dealer cost, or for FREE. surprise!
Professionals tour with RELIABLE amplifiers, and that rules out the (fake) Ampegs. However they do make great boat anchors.

[mykey]

[QUOTE=Rastus_1;53462]As a professional musician, I love my SVT-CL. After initial faults (cutting out etc) were repaired, the amp has proved to be a reliable source of awesome valve tone at any volume. I along with most other pro & amateur musicians love them.

"most other" includes all republicans?

As a user I am glad I have the protection circuitry, if something does go wrong, the amp will shut down before too much damage is done either to myself or the amp.

As a tech, I am really glad about the circuitry as I understand & have seen what happens when that something does go wrong. Also it will be a lot cheaper & easier to fix.

"After initial faults (cutting out etc) were repaired,"
Thanks, you just proved my point AGAIN.

just curious why the factory doesn't test the amps and correct the "initial faults" before they sell it to you?

"These new SVT’s are not as good as the old ones, no problem with that."

I think three times is the charm here. Even the techs at the factory admit that SLM amps are "for occasional use only" The amps are not designed to be used continuously. Professional amps ARE.
Source of information: Technicians in the SLM factory. I have talked with them many times.

[Rastus_1]

I dont know what amp you are looking at, it is obviously not a CL or VR (the true professional amps). You must be looking at one of the cheaper transistor or FET amps.

I have an SVT-3 (transistor amp) open at the moment & the heatsink/fan works great, even in the tropics. Maybe you DO have a crate with an ampeg badge on it?

Either fix the problem or give the amp back to your customer so they can get on with playing music.

Oh yeah, get a music DVD or watch a large concert, you WILL see a SVT-CL or VR onstage. These are the amps of choice & are either hired or owned, they are never free.

Cheers