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

[catstrat]

Happy Thanksgiving everyone. I have an SVT CL on my bench. I completely retubed it with new Output tubes, new 12au7s and new 12ax7 in the output section and new preamp tubes. It had some burned screen resistors which i replaced all the screen resistors with 1 watt 220 ohm resistors per Ampeg. It had 22 ohms to start. What are the diodes in series with the screen resistors there for? Should i have changed those to? Also, what is T1 and what does it do? I only have partial schematics on this thing. If i have the right schematic there is K 1 relay on the AC input board that is controlled by an MPS a18 transistor i believe. But where does the signal come from to trigger the fault mode? I know it depends on the condition of the output section but everything in the output section is new except those diodes. I guess i need someone who knows more about SVT CLs to explain how the fault mode circuit works in this. Thanks.

[Jazz P Bass]

I believe these are the SVT CL schematics: http://music-electronics-forum.com/t34501/

There is a delay on the relay engaging.
The schematic lists the expected voltage reading.

[mhuss]

Check the DC voltage on the output tube cathodes. If any of these is too high it will keep the amp in fault mode (this voltage is what feeds the fault relay amp).

In the schematic I see diodes in parallel, but not series. This still seems odd to me, they would steal current from the screen resistor as soon as the screen current gets above about 3 mA. Burn up the screens while protecting the screen protection resistors?

[catstrat]

SVT CL

You're right about the diodes, i meant to say parralel. Thanks for the description of what they do. I tested it this morning and it finally came out of fault mode. The led was red in standby mode. I let it warm up for maybe 20 minutes and clicked of the standby. I heard a hum and saw a flash and i turned off the variac and started the investigation. I removed the tubes and pulled the output tube board and everything on the OP tube board looked fine. But on the board that holds the 12ax7/12au7 tubes, resistor R 14 was burned. According to the schematic i have R 14 is a grid bleed resistor. It ties in with a couple of other resistors and goes to ground. I've never seen a grid bleed resistor burn like that before. So do i have the right schematic for this? This amp was built in the USA by SLM in the early 90s. The question i have, among many, how many different circuits did Ampeg use in these? The schematics i have don't even show the T1 i am curious about. I assume it's some kind of choke or transformer. I did test all the tubes, both output and phase invertor, My tester showed the 12ax7 tube being really week, the 12au7s test good. 2 of the 6550s test a lot stronger than the other four. These are brand new tubes. Could this problem be caused by mis matched output tubes? Thanks for all the help everyone.

[Jazz P Bass]

2 of the 6550s test a lot stronger than the other four. These are brand new tubes. Could this problem be caused by mis matched output tubes? Thanks for all the help everyone.

No way did mismatched tubes make that preamp tube puke.
To use those output tubes, what I would do is use a high & a low tube on each half of the output circuit.

Replace the resistor & install a new preamp tube.

This fault may have been envisioned by the designers as there is a 1/2 watt resistor below the one that failed (1/4w)

[catstrat]

H Jazz P. I've already replaced R14 with a 1 watt 220k. It was a quarter watt before. I installed a new 12ax7 tube for good measure and put in the OP tubes the way you suggested. It's in standby mode, the red LED is on. So i'm going to find out here in a few minutes. Wish me luck. SVTs aren't my favorite amps to work on but it's my job.

[catstrat]

Well, now it won't come out of standby.

[catstrat]

Hmmm. Dis regard the last post of mine. My variac was on about 55 volts so no wonder it stayed in standby mod. It was a brain Poot! I cranked up the voltage to 115 volts and turned the standby off and it made a gosh awful noise. I pulled the 12ax7 tube, i guess it's kind of like a pre driver, and turned the stanby switch off and the noise went away. I was able to set the bias with the 12ax7 out of it. The bias is now set according to the LEDs on the back. They're both green and the amp is now free of hum. Next step is to reinstall the 12ax7 tube and see what happens. I'm making headway!!!

[Ted]

When you change the screen grid resistors to 220R you should also remove the parallel diodes. There is apparently an Ampeg service note to this effect. If you think about it, the diodes will start to conduct when the voltage across the screen grid resistor exceeds one diode drop. For a 22R this will correspond to a screen grid current of ~27mA. For 220R the diode will start to conduct at about 2.7mA
So if the intended effect of changing the resistor was to limit screen grid current, it won't work. In fact without removing the diode it actually makes things worse.

You might also check the cathode resistors, which from memory are 10R.

You can also slug IC2B between with a 0.1uF between pins 6 and 7 to reduce the sensitivity of the fault circuit to noise.

I have never seen the Ampeg note (it's been referrred to on this forum a few times), so some kind person might perhaps post it here.

[bob p]

Doesn't the service bulletin for the CL require that you change the value of the screen resistor and get rid of the diode?

[Jazz P Bass]

I am not at all sure why you changed R14 to 1 watt.

That 1/4 watt value was chosen for a purpose I am sure.

[catstrat]

Hi Jazz p. It seemed like the thing to do. When i see a resistor that's overheated like that i put in a higher wattage. Should i have not done that? I don't work on enough SVTs to know the quirks they have. Anyway, the bias is holding but it's still making an awful noise, When i remove the 12ax7 in the driver section it quiets down. I reinstalled that 12ax7 and pulled the two pre amp 12ax7s and the noise returned. I'm thinking it's in the driver section 12ax7 circuit that's causing this. But i don't seem to have that part of the schematic. I have the output tube schematic and preamp but not the one dealing with the driver part of the circuit. How close are these to the vintage SVTs from the 70s? Also i changed the screen resistors per Ampeg but didn't know about the diode removal. Maybe i should do that as well. Thanks.

[Jazz P Bass]

Hi Jazz p. It seemed like the thing to do. When i see a resistor that's overheated like that i put in a higher wattage. Should i have not done that? .

I am not an engineer but I do hold high regard for them.

What worries me about upping component wattages is what happens when a failure occurs.

If that resistor was chosen as to open on certain faults, in the hopes of preventing further damage, well guess what?

The driver board schematic is attached.

[Enzo]

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

[catstrat]

It looks like the 12ax7 gets it's signal after it traveled through the power amp in jack. Maybe i should check that jack for problems. I have a sneaky suspicion it may be a ground problem. This amp was completely dead when it got here. Maybe i should change that resistor back to 1/4 watt just to be on the safe side. I've heard Ted mention that i should also remove the parallel diodes if i change the value of the screen resistors. I've not seen the factory change myself but i'm sure it's out there and what Ted says makes sense to me. I'll just clip those out. I've shut it down for the night. Between cooking turkey tomorrow and feeding like a killer whale, i'll try to work on it some more, If i don't sleep all day. Thanks everyone and anymore suggestions are always welcome.