Ampeg SVT II Pro protect circuit

[jvm]

I have a SVT II Pro that came in with a broken 6550 output tube. I also found a burnt circuit trace between pin 11 of J11 and R66. I repaired the circuit trace and powered up the amp with a replacement tube and the other 5 original tubes (all tested good, no shorts and very close on current draw). The amp stays in protect mode( red led) and the bias led's do not light at all. I measured -89 VDC at pin 13 and 15 of the tube board which are the cathodes of the 12au7 biasing tubes. Schematic calls for approximately -45 VDC. No high voltage at the power tubes as the protect circuit has cut off B+, I presume. My first experience with this Ampeg protect circuit. Any suggestions?

ampeg_svt-2_pro service manual.pdf

[Jazz P Bass]

"pin 11 of J11 and R66".
Could you kindly indicate which page & the coordinates that this is on?

[jvm]

Sorry, Jazz. Page three, J11 is at G1 and G2. R66 is at G5.

[MarkusBass]

I have a SVT II Pro that came in with a broken 6550 output tube. I also found a burnt circuit trace between pin 11 of J11 and R66. I repaired the circuit trace and powered up the amp with a replacement tube and the other 5 original tubes (all tested good, no shorts and very close on current draw).

The first question you should ask yourself (even before fixing the trace) is: what was possible reason for the failure and what else could have failed? The trace was burnt not because it was badly designed but because of some other reason (maybe shorted tube?). When you discover this reason, you will know how to fix the amp.
This trace is part of a current sensing circuit. Apart from R66, there is also R40 - 10 Ohms resistor that allows for the measurements of the tube current (by the following comparator - IC5B). Since the trace was burnt, I would start from checking the R40 resistor. Is it OK? Also, since R66 was burnt, there must have been high voltage and high current flowing into the IC5 opamp. I would replace it (after R40 verification). Then, you have D34 and D37 diodes that you should also take a look at. Maybe they are shorted.

My first experience with this Ampeg protect circuit.

There is nothing special about the circuit. IC5 works as a typical comparator: it compares tube voltage (from the R40 resistor) with reference voltage on the R62 resistor (from R67, R64 and R62 voltage divider).

Mark

[jvm]

Thanks for your input, MarkuBass. R66 is not burnt, though. Only the circuit trace was burnt. I tested IC5 against IC3 and got exactly the same readings pin for pin. R40, R66, D34 and D37 all test good as well. I checked the low voltage power supply and found D27 shorted. I replaced it and now the protect LED is green instead of red and I have 720 VDC at the output tube plates( no tubes installed). The bias LED's still do not come on though. With or without tubes installed.

[MarkusBass]

My mistake. I thought that you mention that R66 was burnt but now I see it was not.
So the D27 diode was shorted and there was no +15V power supply. Bias diodes are supplied from +/-15V rails. I hope they are present/correct now. If this is the case, I would start measuring voltages provided to the comparators. I think you need to put tubes in, since the circuit measures the quiescent current through tubes. I would start with voltages on 10 Ohms resistors. Are they correct?
If they are correct, I would measure voltages that are compared by comparators: voltages on R67 and R62, R76 , R71. With comparators you have an easy task: you check voltages on both inputs and verify whether the voltage on the output is as expected, or not. If not, either the comparator is bad, or the voltages on the inputs are incorrect. If the input voltages are incorrect, you have to find out why.

Mark

[jvm]

D27 failed again. Protect LED is red again. Low voltage power supply is at 4 volts, both negative and positive side. I have +17v at C24, -17V at C25. Any tips on the quickest way to find what is pulling down this power supply?