Ampeg SVT CL amp stays in fault mode after screen resistor replacement.

[bob p]

All that does is move the problem to some other component.

To a more expensive component.

Those resistors are being used as fusible links to protect those other components that Enzo is talking about.

[bob p]

Maybe I was too brief. I don't want to come across as a wiseguy who drops hit and run answers.

Expanding a bit further on my previous post, this isn't a case where bigger is better. When screen resistors burn out it's not a good idea to just put bigger resistors in their place as a knee-jerk response. What's important to consider is why the screen resistors burn out, and what happens if you put screen resistors in their place that are incapable of burning out. I've seen lots of guys who respond to screen resistor failures by putting 5W wirewound vitreous enamel resistors in their place, like the resistors you'll find in a Komet. (What a stupid design.)

Doing that results in parts that look good, but that never results in a good outcome, as it removes the power limitation safety factor that's designed into a well-conceived amp. Ultimately, you need to consider that if the resistor fails, in doing so it acts as a sacrificial part (fuse) that stops a bad situation from getting worse; in contrast if you just increase the dissipation rating then you're just facilitating the opportunity for a bad situation to continue to evolve toward a catastrophic outcome. At best doing that will in result in melting some screen grids. At worst doing that will result in catastrophic failure mode that takes out more than just the tubes.

There's a reason that those screen resistors failed. It's not because they had an insufficient dissipation rating. The problem that caused them to fail still needs to be addressed.

I've not seen the factory change myself but i'm sure it's out there and what Ted says makes sense to me.

Unless I'm mistaken Jazz posted the factory service bulletin in the first response to this thread. It would be helpful to read it and think about what it says, and then come back and ask questions if you don't understand what the engineering change order is trying to accomplish. If you haven't seen the ECO... well, it's not because the information hasn't been offered to you, it's because you're not taking the time to read it. That doesn't foretell a good outcome. So take some time at your leisure to assimilate the information and let us know if you need more help sorting through it.

[Ted]

Having looked at the ECOs, the one concerned with changing the screen grid resistors and removing the diodes does not seem to be there.
This is it:

[bob p]

Hmmm. It seems that I was wrong about that TIB being posted in Jazz' links. Sorry about that.

I do remember it being in one of the SVT service doc PDFs that somebody posted here a couple of weeks ago. Looking on my hard drive, I found it in a file named SVT2PRO.PDF that I downloaded from this site on 11-17-13, but now I can't find that file on the board with the search feature. (I continue to have lots of problems with this site's search feature not turning up results for files and threads that I know to exist.)

Thanks to Ted for fixing the problem by re-posting the TIB as a separate file.

Gotta run -- it's getting to be dinnertime. Happy Thanksgiving everybody!

[g1]

I found it in a file named SVT2PRO.PDF that I downloaded from this site on 11-17-13, but now I can't find that file on the board with the search feature. (I continue to have lots of problems with this site's search feature not turning up results for files and threads that I know to exist.)

I also have that file and downloaded it the same day. If you're anything like me, you're pulling out imaginary hair trying to think where it came from (like spending an hour searching for that screw you dropped that disappeared )
Well, google advanced search of this website shows that it was in the thread about using micro's for bias control:
http://music-electronics-forum.com/t34840/#post324725.
That was quick, if only google could help me find those lost screws!

[catstrat]

These diodes are not in place on this o/p tube board. Maybe the factory left them out. However there are diodes D8 and D9 on the driver board. What do these do?

[Jazz P Bass]

D8 & D9 are a part of the bridge rectifier which comprises D6- D13.

[catstrat]

The amp now comes out of fault mode and the bias is adjusted according to the leds. So i'm sure that's fixed. I still have a very loud noise, kind of like a hum, squeal all in one. I'm have problems trusting these schematics i have. Diodes d55 and d56 are shown to be almost a clipping circuit of some kind. they're 1N 3070s. Can i replace those with 1 N914s? I actually clipped those thinking maybe those were the cause of the noise and nope. I tested them in circuit and one showed it was bad, but you can't always get a reading in circuit. Thanks.

[catstrat]

I just learned a valueble lesson today which i should have already known. Always make sure you're looking at the right schematic. R 14 which is actually a cathode resistor so it ties to ground, it's also a 220 ohm resistor not that 220K the other schematic shows. Dummy me replaced it with a 220K thinking it was a grid bleed. Well, it was on the other schematic, but not on this one. I was starting to blame Ampeg for not updating schematics when it was my fault all along. Sorry Mr. Ampeg.

[catstrat]

Well it turned out that i was looking at the right schematic. Apparently the SVT 2 pro driver board is the same as the SVT CL. So the value of R14 according to this is 220 ohms and it's a cathode resistor. I've installed the proper 1/4 watt replacement. The question i still have are diodes D55 and D56. The schematic shows them being parralel but in opposite directions. The board is silk screened showing them in the same direction I installed 2 1N914s in the same direction as the 1N 3070s. I thought diodes were diodes except Zeners, but maybe not. Should the diodes be opposite each other or in the same direction? Now when i turn it off standby into play mode i get a hum and about a 4 or 5 amp draw through my Variac. I turn it off quickly. So it's still not fixed. Thanks .

[g1]

The silk screening does not show you what the traces are doing. You have to look at the traces to know if they are opposite or not.

[Jazz P Bass]

Here is a link to the full schematic set for the SVT CL.
Download Ampeg SVT CL.zip from Sendspace.com - send big files the easy way
(Make sure you click on "Click here to start download from Sendspace" Anything else is an ad.)

[catstrat]

yep G-0ne. I'm kind of like you and i do tend to obsess over stuff. It turns out that i was looking the right chematics and they're 2 r 14s. One on the driver board and one in the pre amp circuit. The two diodes D55 and D56 look like a clipping circuit to me and they're right before the tube that has r14, according to the schematic. I'll have to look at the tracings and as i remember they're joined together at one end of the diodes and then the tracings at the other end split and go to different parts of the circuit. I'll change the diodes to the schematic which is reverse from one another.I'm hoping the driver circuit on this amp is the same as the SVT pro 2. That's what they tell me. Thanks everybody for your help. SVTs have always been a mystery to me. Not the old ones but the new ones. Plus i hardly ever work on any of them.

[Jazz P Bass]

Those diodes are being used as a clamp, a signal amplitude limiter.

Easy enough to check if they are in correct, one anode & one cathode go together to ground.

[Enzo]

Yes^^^^

They are not clipping diodes, they are clamps. They prevent the signal at that point from exceeding a certain level. Yes, they clip any signal that large, but they are not there to introduce a clipped sound, they are there to limit the maximum signal amplitude.

The 1N3070 is a fast diode, so it won;t miss any narrow spikes.

The schematic is correct. and as g-one said, just because the two diodes face the same way on the board does not mean they are not corss connected.


The power amp board and preamp board are separate circuits, and so each has its own series of part numbers, R14 included. Both have other numbers as well.