Ampeg SVT-ii Pro No output w/EQ on

[ca7922303]

No, the diode itself could not do that. Make sure it was put in the right way and no solder bridges were introduced in replacement. Something related to the repair procedure caused your new problem, like a connector issue or broken wire or something.

2-12au7s & 1-12ax7 on main board not lighting up. All other tubes lit.

[ca7922303]

2-12au7s & 1-12ax7 on main board not lighting up. All other tubes lit.

Have all tubes lit now and output was good then output farted out, now flashing red/green standby/power/fault lights.

[ca7922303]

D30, not R30.
Also check all the 2W 10R resistors on the power amp board (R35 thru R40).
On the power tube board, check all the 22R resistors and all the diodes.
These are all parts that can fail with power tube problems and I believe you did replace a power tube.

All 22R resistors and diodes still good along with all the previous measurements are the same as before.

[ca7922303]

No, the diode itself could not do that. Make sure it was put in the right way and no solder bridges were introduced in replacement. Something related to the repair procedure caused your new problem, like a connector issue or broken wire or something.

Tube board to main board connector was loose from wrestling main board back in I guess.

[nevetslab]

The Screen resistors need to be changed to 220 ohm 1W Metal Oxide (MO1S size). If there are diodes still installed across those 22 ohm screen resistors, remove them. Ampeg issues a Tech Note regarding these changes.

If the amp still isn't powering up, and the Fault lamp is lit, remove all six power tubes and see if it still remains in that state. If so, then there may be an issue on either the main PCB, the AC Mains PCB (which has the relay control circuit that engages the HT/Screen Supply Power Xfmr, or the DC control that runs the relay on the AC Mains PCB. That comes from J6 on the preamp PCB, where the 6.3VAC (from the Power tube PCB) feeds the Heaters on the preamp PCB via J5). Sometimes the filter cap C36 becomes loose. Also the the square terminals on the Preamp PCB, as well as on the power amp PCB develop solder joint fractures over time, and cause intermittent operation or put it into protect. There are several of these square-terminal headers on the power amp PCB...and all need to be solid solder joints.

Solder joint fractures is the number one long-term ailment to all of the Ampeg SVT2-Pro, SVT-CL, SVT-VR amps. Also on the preamp PCB, the solder joints of all the panel controls fracture. On the Main PCB, the two Bias Pots which are part of the rear support method the brilliant Ampeg designers used..those fracture as well, as do the Preamp Out/Power Amp Input jacks & XLR connector.

I service SVT amps on a regular basis here at CenterStaging, as we have a large rental fleet of them.

Power Tube matching is another task I go thru on a regular basis. I first set up the bias voltage of the two driver tubes...with no power tubes installed. Usually start with the bias voltage around -55VDC as read on pin 8 of the two driver tubes' cathodes. I'll start with power tubes V3 & V4 and read the cathode current thru their resistors R36 and R38....looking for 220mV (22mA). If the tubes aren't already marked per their position, I'll do that so I can keep track of them. I'll then adjust the two bias pots Bias 1 & Bias 2 for that value. Then, I'll set those tubes aside, and using the same tube sockets V3 & V4, I'll go thru the other two pairs, one pair at a time, and write down the cathode voltage across those 10 ohm resistors. 220mV = 22mA. I record all the readings, then, assuming there's a range of readings that might vary as much as 15mA thru 26mA, I'll group the tube into two best-fit sets, plug the six tubes in and re-bias to try and get near that 22mA range nominal bias level. This assumes you don't have one or more tubes that have failed or aren't responding.

The Bias LED circuit has a range of 0.147V (IC4 Pin 2, IC5 Pin 6) thru 0.294V (IC4 pin 6, IC5 Pin 9) at the point of GRN LED turning on, then at where the RED LED turns on. For the GRN LED to light on both triad sets of power tubes, all three tubes must be installed, though the RED LED can light up from any tube conducting more than 30mA. Normally, the Bias LED circuit (window comparators) are a reliable circuit and don't normally fail. What does fail is the poor quality of the PCB traces and pads, and the PCB's don't take to re-soldering/de-soldering very well.

I've posted numerous threads pertaining to all of the common ailments these amps go thru.

Happy hunting!

[ca7922303]

The Screen resistors need to be changed to 220 ohm 1W Metal Oxide (MO1S size). If there are diodes still installed across those 22 ohm screen resistors, remove them. Ampeg issues a Tech Note regarding these changes.

If the amp still isn't powering up, and the Fault lamp is lit, remove all six power tubes and see if it still remains in that state. If so, then there may be an issue on either the main PCB, the AC Mains PCB (which has the relay control circuit that engages the HT/Screen Supply Power Xfmr, or the DC control that runs the relay on the AC Mains PCB. That comes from J6 on the preamp PCB, where the 6.3VAC (from the Power tube PCB) feeds the Heaters on the preamp PCB via J5). Sometimes the filter cap C36 becomes loose. Also the the square terminals on the Preamp PCB, as well as on the power amp PCB develop solder joint fractures over time, and cause intermittent operation or put it into protect. There are several of these square-terminal headers on the power amp PCB...and all need to be solid solder joints.

Solder joint fractures is the number one long-term ailment to all of the Ampeg SVT2-Pro, SVT-CL, SVT-VR amps. Also on the preamp PCB, the solder joints of all the panel controls fracture. On the Main PCB, the two Bias Pots which are part of the rear support method the brilliant Ampeg designers used..those fracture as well, as do the Preamp Out/Power Amp Input jacks & XLR connector.

I service SVT amps on a regular basis here at CenterStaging, as we have a large rental fleet of them.

Power Tube matching is another task I go thru on a regular basis. I first set up the bias voltage of the two driver tubes...with no power tubes installed. Usually start with the bias voltage around -55VDC as read on pin 8 of the two driver tubes' cathodes. I'll start with power tubes V3 & V4 and read the cathode current thru their resistors R36 and R38....looking for 220mV (22mA). If the tubes aren't already marked per their position, I'll do that so I can keep track of them. I'll then adjust the two bias pots Bias 1 & Bias 2 for that value. Then, I'll set those tubes aside, and using the same tube sockets V3 & V4, I'll go thru the other two pairs, one pair at a time, and write down the cathode voltage across those 10 ohm resistors. 220mV = 22mA. I record all the readings, then, assuming there's a range of readings that might vary as much as 15mA thru 26mA, I'll group the tube into two best-fit sets, plug the six tubes in and re-bias to try and get near that 22mA range nominal bias level. This assumes you don't have one or more tubes that have failed or aren't responding.

The Bias LED circuit has a range of 0.147V (IC4 Pin 2, IC5 Pin 6) thru 0.294V (IC4 pin 6, IC5 Pin 9) at the point of GRN LED turning on, then at where the RED LED turns on. For the GRN LED to light on both triad sets of power tubes, all three tubes must be installed, though the RED LED can light up from any tube conducting more than 30mA. Normally, the Bias LED circuit (window comparators) are a reliable circuit and don't normally fail. What does fail is the poor quality of the PCB traces and pads, and the PCB's don't take to re-soldering/de-soldering very well.

I've posted numerous threads pertaining to all of the common ailments these amps go thru.

Happy hunting!

Thanks for all the great info. I'll start on it this evening when off work.

[ca7922303]

The Screen resistors need to be changed to 220 ohm 1W Metal Oxide (MO1S size). If there are diodes still installed across those 22 ohm screen resistors, remove them. Ampeg issues a Tech Note regarding these changes.

If the amp still isn't powering up, and the Fault lamp is lit, remove all six power tubes and see if it still remains in that state. If so, then there may be an issue on either the main PCB, the AC Mains PCB (which has the relay control circuit that engages the HT/Screen Supply Power Xfmr, or the DC control that runs the relay on the AC Mains PCB. That comes from J6 on the preamp PCB, where the 6.3VAC (from the Power tube PCB) feeds the Heaters on the preamp PCB via J5). Sometimes the filter cap C36 becomes loose. Also the the square terminals on the Preamp PCB, as well as on the power amp PCB develop solder joint fractures over time, and cause intermittent operation or put it into protect. There are several of these square-terminal headers on the power amp PCB...and all need to be solid solder joints.

Solder joint fractures is the number one long-term ailment to all of the Ampeg SVT2-Pro, SVT-CL, SVT-VR amps. Also on the preamp PCB, the solder joints of all the panel controls fracture. On the Main PCB, the two Bias Pots which are part of the rear support method the brilliant Ampeg designers used..those fracture as well, as do the Preamp Out/Power Amp Input jacks & XLR connector.

I service SVT amps on a regular basis here at CenterStaging, as we have a large rental fleet of them.

Power Tube matching is another task I go thru on a regular basis. I first set up the bias voltage of the two driver tubes...with no power tubes installed. Usually start with the bias voltage around -55VDC as read on pin 8 of the two driver tubes' cathodes. I'll start with power tubes V3 & V4 and read the cathode current thru their resistors R36 and R38....looking for 220mV (22mA). If the tubes aren't already marked per their position, I'll do that so I can keep track of them. I'll then adjust the two bias pots Bias 1 & Bias 2 for that value. Then, I'll set those tubes aside, and using the same tube sockets V3 & V4, I'll go thru the other two pairs, one pair at a time, and write down the cathode voltage across those 10 ohm resistors. 220mV = 22mA. I record all the readings, then, assuming there's a range of readings that might vary as much as 15mA thru 26mA, I'll group the tube into two best-fit sets, plug the six tubes in and re-bias to try and get near that 22mA range nominal bias level. This assumes you don't have one or more tubes that have failed or aren't responding.

The Bias LED circuit has a range of 0.147V (IC4 Pin 2, IC5 Pin 6) thru 0.294V (IC4 pin 6, IC5 Pin 9) at the point of GRN LED turning on, then at where the RED LED turns on. For the GRN LED to light on both triad sets of power tubes, all three tubes must be installed, though the RED LED can light up from any tube conducting more than 30mA. Normally, the Bias LED circuit (window comparators) are a reliable circuit and don't normally fail. What does fail is the poor quality of the PCB traces and pads, and the PCB's don't take to re-soldering/de-soldering very well.

I've posted numerous threads pertaining to all of the common ailments these amps go thru.

Happy hunting!

Should I try removing the diodes installed across the 22 ohm resistors without changing to 220 ohm 1 watt resistors?

[The Dude]

Are they shorted?

[ca7922303]

Are they shorted?

Diodes next to the 22 ohm resistors on the tube board all read(.014/.014) diode test.

[Enzo]

Yes the diodes will all read low because they have 22 ohm resistors in parallel. Regardless, follow the suggestion. Remove the diodes. Just snip them off. And change the 22 ohm to 220 ohm. I am unsure why you would want to only do half the update.

[ca7922303]

Yes the diodes will all read low because they have 22 ohm resistors in parallel. Regardless, follow the suggestion. Remove the diodes. Just snip them off. And change the 22 ohm to 220 ohm. I am unsure why you would want to only do half the update.

Dont have the 220 ohm resistors on hand and didn't know if getting rid of diodes might correct the fault possibly?

[ca7922303]

Not finding anything at back of main board whether it be bias control, balanced out jack or any jacks back there that aren't connected to board. Checked with DMM.

[Enzo]

The 22 ohm to 220 ohm change and the removal of the diodes will not affect your problem. It is an engineering change recommended be done on any unit in for service. It is a reliability upgrade.

[ca7922303]

Have output now, bias lights are green, no fault light. Output started out strong then intermittent to low distorted output.

[ca7922303]

Ok, have strong normal output now, bias control 2 green light, bias control 1 red and green. Unable to get red to go out.