Ampeg SVT-VR Regulated 12V DC Heater Supply Fault - Low Voltage?

[Horse57]

Hey folks, I've got a real baffler here! I've spent quite some time on it already and thought I'd put it to the hive mind! I have already trawled the forum and Google and haven't found anything relating to this issue.

The issue I have is with the regulated 12V DC heater supply for the preamp valves (V1 to V5) in this Ampeg SVT-VR. Pin 4 of the 12A*7's is fed 12V DC from a 7812 regulator and pin 5 is grounded. When all 5 preamp valves are inserted, the voltage drops to about 2.2V DC and obviously the valves fail to heat. Sequentially removing V1 to V5 the DC voltage creeps back up (see notes below with the Input and Output V of the 7812 regulator providing the 12V DC supply).

7812 INPUT/OUTPUT:

# Valves Inserted 7812 V IN 7812 V OUT
5 Valves 18.6V 2.2V
4 Valves 18.5V 3.5V
3 Valves 18.2V 11.8V
2 Valves 18.5V 12V
1 Valve 18.5V 12V
0 Valves 19V 12V

I will add, it is not valve socket specific so it’s not a short somewhere. It doesn’t matter what valves are inserted where, only the number of them drawing current from that heater supply. All valves have been replaced and test good so it's not a short in one of the heaters.

So, I assumed it would be the 7812 regulator as they do have over current protection and this could have gone wacko. I’ve changed that, no bueno.

Stranger still, is that the voltage to the power indicator LED also drops below where it should be when not in standby (B+ applied) and all preamp valves inserted - so the LED is very dim - but is fine when 2 or fewer of the 5 preamp valves are inserted. From what I can see this is a separate circuit, with the LED being driven by the NE5532 op-amp which is fed from the B+ and a -15V DC supply - both of which are fine. Depending on that circuits DC output to the LED (determined by B+ being applied or not) the LED lights either red (standby) or green (on). The only thing I can see that is shared by the LED circuit and the 12V DC heater supply is the ground (which is separate from the other ground connections in the amp) - but this checks out just fine.

The power LED should light red in standby and green in on (B+ applied). Please see the below chart of the voltages I'm getting at pins 1 & 7 of the NE5532 as well as the target voltage.

With 2 valves inserted:

Pins: 1 & 7 Schematic Specified (Target)
V Standby: -12.5V -13V
V ON: +9.6V +11V

With 5 valves inserted:

Pins: 1 & 7 Schematic Specified (Target)
V Standby: -12.2V -13V
V ON: +1.95V +11


I should add, the 12V DC supply is low in both standby and on states.

What I’ve tried:
- New preamp valves
- Cleaning valve sockets
- Replacing 7812 regulator that feeds the 12V DC supply (these do have over current protection so I suspected it may be this components but alas, the same issue persists)
- Replacing NE5532 op-amp as this is in the power indicator LED circuit which shares the ground with the 12V DC heater supply
- Checked for shorts/cold solder joints

What I think is happening is that the 7812 regulator is dropping voltage when the higher current is drawn by the 5 ECC83/12AX7’s. When only 2/3 are in circuit, less heater current is drawn and therefore the voltage remains at the desired 12V DC. But for the life of me, other than replacing the 7812 AGAIN I’m lost as to why this is happening! It’s rated for 1.5 Amps and the heaters of those 5 12AX7’s/12AU7’s only draw 750ma between them when run at the 12V DC. And this wouldn’t explain the funky LED issues. The two faults seems to be connected somehow and I am stumped!

Any help would be greatly appreciated!

I've attached annotated schematics with the relevant portions of the circuit highlighted.

If you need any other info give me a shout!

Thanks in advance,
Andy

[Jon Snell]

Check the supply to the 7812 and the two tank capacitors, C4 & C5.

[Helmholtz]

Check C2.

[nevetslab]

Welcome to the Forum!

One area that periodically causes grief is the AC Secondary wiring from the separate Heater/Bias Supply xfmr. Those two heavy WHT wires that attach to the Tube PCB assy with right-angle 1/4" female insulated fast-on terminals often become oxidized and start causing loss of voltage. Usual fix on those is cutting off the terminals and any of the overheated wiring, then hard-soldering the wires to the male terminals of the PCB. I've attached the PCB layout drawing below, as well as the Power Xfmr and Output Xfmr schematics that Ampeg never provides.

PWA Tube PCB Layout (365PWA_0).pdf
Ampeg SVT-CL, -AV, -VR Output Transformer.pdf
Ampeg SVT-CL Power & Heater Xfmrs.pdf

I just checked my files and the SVT-VR Parts list, and see I haven't yet drawn up the SVT-VR Power Xfmr schematic #94-279-40 nor verified the Heater/Bias xfmr schematic that's on my SVT-CL Power & Heater xfmr drawing is like that of their 94-412-40 xfmr diagram. Another task yet to complete.

Over time, the most common fault I find on the SVT-CL and SVT-VR amps, as well as the older SVT's is solder joint fractures. Cause is from the sheer weight & mass of these amp in transit, whether in a road case or not....loose hardware on the power and output xfmrs is the biggest offender, and once either/both are loose, they become a vibration generator to the chassis, and begin shaking everything, and eventually succeed in causing solder joint fractures on parts that are used as mounting brackets (front panel pots, rear panel phone jacks, bias pots, XLR, IEC AC Mains connector, Fuse holder, square pin harness connectors to/from the main PCB, tube pins, etc. This applies to the right angle 7-pin header on the Power Tube PCB. Those solder joints are known to fracture over time, and THAT is the source of your Heater Voltage in the preamp!

[Horse57]

Welcome to the Forum!

One area that periodically causes grief is the AC Secondary wiring from the separate Heater/Bias Supply xfmr. Those two heavy WHT wires that attach to the Tube PCB assy with right-angle 1/4" female insulated fast-on terminals often become oxidized and start causing loss of voltage. Usual fix on those is cutting off the terminals and any of the overheated wiring, then hard-soldering the wires to the male terminals of the PCB. I've attached the PCB layout drawing below, as well as the Power Xfmr and Output Xfmr schematics that Ampeg never provides.

PWA Tube PCB Layout (365PWA_0).pdf
Ampeg SVT-CL, -AV, -VR Output Transformer.pdf
Ampeg SVT-CL Power & Heater Xfmrs.pdf

I just checked my files and the SVT-VR Parts list, and see I haven't yet drawn up the SVT-VR Power Xfmr schematic #94-279-40 nor verified the Heater/Bias xfmr schematic that's on my SVT-CL Power & Heater xfmr drawing is like that of their 94-412-40 xfmr diagram. Another task yet to complete.

Over time, the most common fault I find on the SVT-CL and SVT-VR amps, as well as the older SVT's is solder joint fractures. Cause is from the sheer weight & mass of these amp in transit, whether in a road case or not....loose hardware on the power and output xfmrs is the biggest offender, and once either/both are loose, they become a vibration generator to the chassis, and begin shaking everything, and eventually succeed in causing solder joint fractures on parts that are used as mounting brackets (front panel pots, rear panel phone jacks, bias pots, XLR, IEC AC Mains connector, Fuse holder, square pin harness connectors to/from the main PCB, tube pins, etc. This applies to the right angle 7-pin header on the Power Tube PCB. Those solder joints are known to fracture over time, and THAT is the source of your Heater Voltage in the preamp!

Thanks for the helpful pointers folks!

Regarding "This applies to the right angle 7-pin header on the Power Tube PCB. Those solder joints are known to fracture over time, and THAT is the source of your Heater Voltage in the preamp!"

On the SVT VR the preamp chassis has a separate 12V DC feed for the heaters. This comes from the 7812 regulator, which is fed from the filament TX via J4 on the main board. Definitely worth taking a look at J4, but given the stable voltage feeding the 7812 but the drop in it's output voltage depending on the number of valves inserted in the preamp (current draw) I'm fairly certain it's something to do with that regulator...

Do correct me if I've missed something there!

Thanks!

[Jon Snell]

7812s are cheap and do fail over time if there is at least 15 volts on pin 1, there will be sufficient voltage to produce 12 volts. The heatsink compound must not be dry and if the bolt/nut used to fix, it is not tight, it will fail due to over temperature. The official torque is 0.8lb/ft, I like to get things right and do not have jobs bouncing back through my errors or an oversight.
The humble 7812 is rated at 1.5Amp MAX, if cooled sufficiently, 500mA in free air.

[nevetslab]

Thanks for the helpful pointers folks!

Regarding "This applies to the right angle 7-pin header on the Power Tube PCB. Those solder joints are known to fracture over time, and THAT is the source of your Heater Voltage in the preamp!"

On the SVT VR the preamp chassis has a separate 12V DC feed for the heaters. This comes from the 7812 regulator, which is fed from the filament TX via J4 on the main board. Definitely worth taking a look at J4, but given the stable voltage feeding the 7812 but the drop in it's output voltage depending on the number of valves inserted in the preamp (current draw) I'm fairly certain it's something to do with that regulator...

Do correct me if I've missed something there!

Thanks!

Opps! I stand corrected. I spend so much time in the SVT-CL, I forgot that the five tubes in the SVT-VR ARE all running on 12VDC, and it comes in on the 7-pin connector to the right of V4, Pins 3 & 4. Sorry about that. But, you may still be dealing with solder joints. Or thru the Via's from the 12VDC and GND to J6 Pins 3 & 4. I believe that's the path from the main Power Amp PCB assy to get to the Preamp PCB.

I haven't been down this road on the SVT-VR yet, so I'm very interested in finding/following this problem. There are several SVT-VR's in our Rental Fleet. Do you have the PCB layout diagrams for the Power Supply/Driver board? Takes a lot of flipping between top and bottom of the layouts to see how the connections are being made, and possible locations where losses could occur. at the load (the five tube's heaters in the preamp.

[Horse57]

Fixed... Almost! So, I pulled the 7812 replacement and tested it with a bench power supply, (feeding the input of the 7812 18.5V DC as the amp would). I then gradually lowered the load until I was at the 700mA current draw mark (that's around the collective heater current draw of the 5 preamp valves). Above the 300mA the 7812 started rapidly dropping output voltage as it did in the amp!

I then tested another two 7812's I'd bought in the same batch in exactly the same way. All performed the same! Faulty.

Apparently there are a lot of counterfeit 7812's going around, and I'm guessing that's what I have!

So I've ordered some more from mouser (not my original supplier) and am hopeful that a replacement will solve the issue!

Will report back once they've arrived.

All the best!

[Jon Snell]

Fixed... Almost! So, I pulled the 7812 replacement and tested it with a bench power supply, (feeding the input of the 7812 18.5V DC as the amp would). I then gradually lowered the load until I was at the 700mA current draw mark (that's around the collective heater current draw of the 5 preamp valves). Above the 300mA the 7812 started rapidly dropping output voltage as it did in the amp!

I then tested another two 7812's I'd bought in the same batch in exactly the same way. All performed the same! Faulty.

Apparently there are a lot of counterfeit 7812's going around, and I'm guessing that's what I have!

So I've ordered some more from mouser (not my original supplier) and am hopeful that a replacement will solve the issue!

Will report back once they've arrived.

All the best!

Lesson learnt. Never buy components through fleabay or amazon.
Why they would counterfeit a cheap component, I don't know.

[The Dude]

Be sure to read specs, too. There are TO220 12V regulators rated at 1A and others rated at 1.5A. All are not the same.