Ampeg SVT AV

[catstrat]

This Ampeg SVT AV came to my shop 2 weeks ago and the bias leds don't work. I found a couple of burned /open screen resistors. I replaced all of them with 22 ohm 3 watt. The leds still don't work. I can adjust the grid bias using a regular voltmeter and i can set it to the required bias voltage. The leds are still good because they light when voltage is applied to them directly. GRid resistors are good. All the voltage at the ic chips are good. Am i overlooking something? Any and all suggestion are welcomed.

[Mars Amp Repair]

Hello catstrat,
First of all on the Ampeg's with the bias LED's, the power tubes need to be matched within about 8ma for the bias led's to work properly and they won't light properly with any less that ALL the 6 tubes in place.

Also the driver 12AU7's need to be good, too.

That being said, I have found the 10ohm 2watt 1% resistors in the bias detect circuitry being fried causing the same problem...shorted power tubes take these out as they are essentially in the cathode ckt for current detection. R35-40 on main pcb.

You can probably determine this by merely measuring the voltage on the cathodes of the power tubes to see if you're getting anywhere near 240mv. I think you could also use your Diode checking scale on your DVM to measure from the cathode (pin 8) on the power tubes to ground WITH POWER OFF & B+ DISCHARGED.

You'd be looking for the forward bias value of the diode that is in series with the cathode resistors.\

anyway, that should be a good place to start. glen

[catstrat]

Ampeg SVT AV --- again

Thanks everyone for your help up to this point. I guess my question is what schematic should i be using to trace this down. Loud Technoligies said the SVT 2 pro is the same as an SVT AV. And i think someone suggested the SVT VR. The SVT 2 pro is close but it has 22 ohm screen resistors and the resistors in this SVT AV are 220 ohm. The SVT VR is also 220 ohm screen resistors but it looks like it has a different biasing circuit altogether. I did check R35 -R40 and they're good. Voltages are good at the TL074 chip as well as the other chips. All diodes in the circuit check good as well as all of the grid resistors. I can still adjust the bias the old fashioned way but the LEDs don't work for crap. I wonder if this amp is just a hodge podge of left over parts. I guess it would make a dandy boat anchor.

[catstrat]

SVT AV

Well Glen i just looked closely at the schematic you posted which clearly says AV on it and it has 22 ohm screen resistors. The ones i removed when measured with an ohmmeter showed around 200 ohms. By the way the reason for the screen resistor change is one was burned causing damage to the board. The remaining 5 were doing the same thing. That's how this mess got started. I've used 22 ohm resistors per schematic in it but the leds won't work. I've tried the 220 ohm resistors in and it didn't work either and it started hissing and humming. I'm getting a signal through the amp with the 22 ohm resistors but this guys wants his LEDs working. Nothing i've tried thus far has worked. Is it really going to make that big of a difference whether it's 22 or 220 ohm screen resistors? Thanks everyone.

[Enzo]

I think you need to shift your focus away from the screen current limiter resistors and work on the biad circuit.

You need to VERIFY the 12AU7s driver tubes are indeed 12AU7 and not 12AX7 out of place. Further verify that they are GOOD 12AU7s. Bad or wrong 12AU7s in this circuit result in the red light coming on before the green and other odd bias adjust circumstances.

This whole circuit operates by dropping a voltage across some resistance in the cathode circuit at each tube. This is then run into a couple voltage comparator ICs. The comparators run off two reference voltages set by R71,73 or R62,64. Those provide a window, divided off the +15v rail. If the dropped voltage at a tube is higher than the upper reference - inferring higher current - then it turns on the red light. The green light normally wants to turn on. If the dropped voltage is lower than the lower reference, then the comparator shunts across the green LED turning it off.

[Mars Amp Repair]

Also to reitterate, all tubes must be matched within about 8ma in order for the bias ckt to work properly. Also are you certain you measured all the cathode resistors R35 to 40?

You can determine that by checking the cathode voltage on all the power tubes or each triplet. IOW V-1,2,3 need to be within 8mv of each other & V4,5,6 also need to be within 8mv of each other...those readings need to be within 8mv or so of the target 240mv that schemo indicates. that sounds like a lot, but remember you're measuring across 10ohm cathode resistors, not the usual 1 ohm.

In that complicated looking bias ckt comparator bias ckt, I'd take voltages on the inputs & outputs of the opamps & look for any inconsistencies. At least a few of these have to be working correctly.

Looks like the reference (neg) inputs of opamps should have a constant voltage on them. I'd guess for balanced operation the plus inputs should have the same voltage on them as the neg inputs as you adust the bias pot for that half of tubes. The output should then be reflective of the difference in the plus & minus inputs to determine if the opamp is good or bad.

for example: For the green LED of D48 to come on, the output of the opamps connected to it should probably be around '0'. For the Red led of D48 to come on, the outputs of the opamps connected to it should be around '0', too. So that gives some idea of what should happen...Weird little ckt to figure out.

I guess I'd just look for any obvious discrepencies. g

[Mars Amp Repair]

oops, sorry. the resultant of the inputs of the comparators would not be the difference of the inputs. It would swing positive & negative with respect to which input was the higher value (provided the opamp Vcc & Vdd rails are supplied with - & + on pin4 & pin 7. here I believe they are.

Been a while since I had to deal with anything but a closed loop opamp ;-]...g

[Enzo]

I tried to explain, I really did.

[Mars Amp Repair]

Yeah Enzo,
This is a tough one. Looking at it now, I'm not sure that I wasn't actually speaking in electronic tounges! g

[Enzo]

Here boys, this shuold clear things up.

[Mars Amp Repair]

good to know. I wondered how they were getting away with such low value screen resistors. g