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

[catstrat]

confounded SVT.

I made sure D55 and D56 were in the right direction and installed the right 1/4 watt 200 ohm resistor in the R14 location. Powered it up and turned the standby to play mode and got a hum,then a screech and smoke. Quickly turned it off and R14 was fried again. Why does this keep happening. If R14 is a cathode resistor as the schematic shows it to be, am i pulling so much current through that tube that it's burning R 14? I've never seen this before. I subbed another known tube and it still did the same thing. This is why i tend to question Ampeg schematics. I can pull the 12ax7 out and turn on the amp with no apparent problem. What gives?

[g1]

Maybe there is DC coming into the grid? You could try inserting a cord or dummy plug into the PA IN jack to disconnect the preamp from the V1 grid, then measure either side of R7 for DC.
Also check that pin2 circuit is grounding via R9.
Another thing is that there is still a path from the top of R14 to the spkr. out board via J10.

[bob p]

It's hard to imagine what's going on without having some actual data to work with. I like g-one's recommendation. Try tracing signal and posting some measurements taken with a scope, or at least with a VOM.

[catstrat]

confounded SVT.

I just replaced R14 again and made sure the tracings aren't shorted somewhere and double checked the values of everything. On the subject of the clamp circuit, i used 1N 914s to replace the 1N3070s. I looked in the Mouser catalog and i couldn't find 1N 3070s. Is it some kind of special diode? I don't see how the clamping circuit could cause this problem , or could it? I'm going to power it up without the 12ax7 in it and check voltages at the tube pins and see what i get. There has got to be dc voltage getting in there somewhere. Does this amp have both signal ground as well as common earth ground? Thanks.

[Jazz P Bass]

There should not be anything coming in J10 (feedback from OT) but I would disconnect it at first.

[catstrat]

I replaced R 14 replaced the board. pulled all the O/P tubes, left out the 12ax7 and checked the voltages. There was no voltage at all on pin 3 cathode, but there was about 102 volts at pin 8 cathode. I haven't checked the schematic yet on that, but i will. I checked the voltage with both the speaker board jumper connected and disconnected with no problems either way and no extreme amperage draw. I'm beginning to think now it's somehow still a bias issue. I'll test the O/P tubes again and see if one is not shorted. But how would that cause R14 to fry like this, unless the current pull is so massive through the circuit that it would cause everything before it to overheat. If it is a shorted o/p tube i'll be real upset.But it won't be the first time. If i still drank, i'd have a whiskey about right now.

[catstrat]

All tubes test good on my tester which is a B@K model 700. But testers don't always show you everything happening with the tube. I'm going to leave out the O/P tubes and i'm going to set the bias voltage at pin #5 to the most negative voltage i can. Then install the 6550s and give it a whirl. That will tell me if it's the bias or not. It'll either work or it won't.

[Jazz P Bass]

102 volts on that cathode connection without a tube installed is impossible.

Unless: the output transformer is shorted primary to secondary & it's sending that through J10.

[Enzo]

Mouser has 52,000 of the 1N3070 in stock right now

1N3070TR Fairchild Semiconductor | Mouser

[catstrat]

SVT

I checked all the voltages again this morning and the voltage at V1 12ax7 pin 8 is 92 volts with tube in. 0 volts with tube out. I also had 31 volts at pin 7 with the tube in and 0 volts with tube out. Shorted tube, i've subbed 4 12ax7s already with the same results. I cleaned the sockets as well as the tubes themselves and that didn't help. I changed out c 9 and that made no difference, not that it would. I checked the values of pin 8 cathode circuit resistors and they all check good. Whatever is happening is when i plug in the tube so that tells me it's not a problem with the support circuits. But a problem with either the socket or, it just can't be much else. I checked for voltage at the NFB circuit and had 0 volts. It figures, i have a tendency to get problems that no one else has ever run into before. I'll look at the schematic again but i don't see how this is happening to tell you the truth. With the 12ax7 out i can power up the amp with the 6550s in it with no problem at all. It's only when i plug the 12 ax7 in the socket.

[g1]

Why have you jumped from V1a circuit to V1b? Pin 8 has 68K in the cathode circuit so it will have a high cathode voltage. There is no DC voltage listed, but 2ma current through the tube would give over 100V, so it doesn't sound wrong to have the voltage you have there. Is there around 250V on the plate?
What were the results from the suggestions in post #32 and #35?

[catstrat]

SVT

Thanks G-one for steering me in the right direction. Sometimes in pursuit of one problem i tend to create problems that really don't exist. I hope to get better at that one day. Man do i feel stupid! I still have some issues with it and i don't need to muddy up the water with created problems. It's test time. Wish me luck.

[catstrat]

SVT

I looked at the bias control pots again and decided to resolder the connection as a precaution and cleaned the daylights out of them. That had to be parts of the problem. I set the bias to -85 volts at the O/P tube pin 5 as a starting point. This time i installed the tubes and left the j 10 jumper off. I was able to adjust the bias according to the leds. Back to the J 10. Someone else serviced it in the past and they marked j10 improperly. When i pulled the driver board i had to disconnect that j 10. When i re installed the board i assumed the markings on the j 10 connectors were right. Wrong. This amp didn't work at all when i got it. When i realized i was installing j10 wrong, i corrected it and that fixed the r14 burning issue. Now the amp is up and working fine

[catstrat]

As i said , the SVT is now fixed. I'd like to give a big ole' thanks to everyone who helped out on this. Thanks to everyone, i got a major knowledge boost on these. The next one i get i won't be so intimidated by it. Again, thanks everyone.

[bob p]

I hate having to fix someone else's mistakes. Glad to hear that you got it running.