79 Ampeg SVT Volume Drop

[numberfourthree]

Spent time yesterday and today running more tests.

Issue is certainly in the Power Amp as I ran a Pre-Amp directly in and had the same issue. With amp on, inserting each tube does not cause any issue or blow fuses.

Measured Plate, Grid and Cathode of each tube. Everything read consistent (High, Low and In Btwn).

I did blow another fuse while the amp was on for a bit longer. A few diodes looked bad and I changed any out of spec resistors and the old .1 caps on the board which were original. Touched up any suspect solder joints.

Checked all the test points on the board. Results were off but not completely crazy. Suggest Filter Caps might be drifting. What I can't understand is how 660v is headed to the Blue and Blue/White of the OT primary. Perhaps my reading is off.

Schematic values in ()

Test Points
A = 663v (600v)
B = 523v (462v)
to C12 = 408v (362v)
D = -175v (-150v)
E = 331v (350v)
H = 525v

To Primary Blue = 660v (372v)
To Primary Blue/White = 660v (372v)

12DW7
Pin 1 = 304v (280v)
Pin 3 = 103v (95v)
Pin 6 = 204v (207v)
Pin 8 = 2.0v (1.7v)

12BH7
Pin 1 = 147v (159v)
Pin 3 = 6.5v (7v)
Pin 6 = 331v (207v)
Pin 8 = -41.8v (-47v)

12BH7
Pin 1 = 147v
Pin 3 = 6.6v
Pin 6 = 333v
Pin 8 = -40v

Issues persists. Likely can't go much further without a better equipment. Will reach out to the tech I work with. He isn't so excited as he hit a wall with a previous SVT but hopefully we can find the issue.

 

[agedhorse]

Of course the issue persists, you haven’t identified the cause of the problem.

How do diodes “look bad”? I’ve looked as several million diodes in my career and have never been able to identify a bad diode visually. You would need x-ray vision to identify lead bond failures, and you still wouldn’t be ample to identify most failures.

(assuming the diode isn’t mechanical broken or blown to smitherines)

Its also not going to be the .1uf caps, they rarely fail, but you know this now.

The test point voltages are nominal, nothing exact about them. Your readings do not indicate a failing caps, you don’t know this yet, but after you replace them with no change, you will.

 

[numberfourthree]

I’m aware you have a ton of experience and knowledge on the subject but have I done something to offend you? I don’t understand the condescending tone and need for you to belittle my posts. It’s borderline bullying and not at all necessary.
One of the reasons I posted here was the many long helpful threads on SVTs over the years. These are complicated amps and sharing experience seems to have been commonplace in the past.

 

[agedhorse]

Had you taken my suggestions you would have discovered how helpful the information I provided was.

Are you really asking for help? I ask this because you haven’t appeared to appreciate any of the information I have provided.

 

[Wasnex]

Spent time yesterday and today running more tests.

Issue is certainly in the Power Amp as I ran a Pre-Amp directly in and had the same issue. With amp on, inserting each tube does not cause any issue or blow fuses.

Measured Plate, Grid and Cathode of each tube. Everything read consistent (High, Low and In Btwn).

I did blow another fuse while the amp was on for a bit longer. A few diodes looked bad and I changed any out of spec resistors and the old .1 caps on the board which were original. Touched up any suspect solder joints.

Checked all the test points on the board. Results were off but not completely crazy. Suggest Filter Caps might be drifting. What I can't understand is how 660v is headed to the Blue and Blue/White of the OT primary. Perhaps my reading is off.

Schematic values in ()

Test Points
A = 663v (600v)
B = 523v (462v)
to C12 = 408v (362v)
D = -175v (-150v)
E = 331v (350v)
H = 525v

To Primary Blue = 660v (372v)
To Primary Blue/White = 660v (372v)

12DW7
Pin 1 = 304v (280v)
Pin 3 = 103v (95v)
Pin 6 = 204v (207v)
Pin 8 = 2.0v (1.7v)

12BH7
Pin 1 = 147v (159v)
Pin 3 = 6.5v (7v)
Pin 6 = 331v (207v)
Pin 8 = -41.8v (-47v)

12BH7
Pin 1 = 147v
Pin 3 = 6.6v
Pin 6 = 333v
Pin 8 = -40v

Issues persists. Likely can't go much further without a better equipment. Will reach out to the tech I work with. He isn't so excited as he hit a wall with a previous SVT but hopefully we can find the issue.

Sorry...I see some things that catch my interest, but no silver bullet.

Schematic says 660V at A, rather than 600V. Most of the readings I have seen from various amps have been in the 680 to 695V range, so 663V could be low, but it may be normal for your amp.

Actually many of the PS voltage readings I have seen for SVTs have been considerably higher than the schematic. So 523V on node B is not necessarily a surprise, except for the fact that Node A is almost down to the schematic voltage. I believe B only feeds the preamp. So if the voltage is this high you may be cooking the preamp tubes. Different eras of SVTs used different values for R49...so the voltage at this Node varies from amp to amp. Also the voltage depends on how much current is being drown by the preamp and phase inverter.

The voltage midway across the main filter caps should be 1/2 the voltage at Node A. I am talking about the junction between C10 and C12A. Since you have 663V at A, this should be very close to 331.5V. If you have 408V there, I would probably expect a problem. One or both of the capacitors may have too much leakage or the bleeder resistors may be out of spec. Current bleeding through this filter section can pull Node A down. From what I have seen, twist locks tend to have a shorter lifespan than axial and radial caps.

Node D looks normal, but E is lower than what I have typically seen. Many amps seem to fall right at 350V.

Node H is way high. It should be lower than node E. Node E feeds node H; not the other way around. If the voltage at Node H is higher than Node E it seems very odd. The solder eyelets between node H and Node B are super close, so it's easy to form a solder bridged. But you indicate Node H is running between the voltage at the Node A and Node B. My best guess is this not an accurate reading....either that or it got connected to the Node A somehow. Check the wiring to/around R49 so see if any unintended/accidental connections are made on various terminal strips. If this reading is actually 525V, the voltage on Pin 1 of the 12BH7s would probably be higher...so I think the reading is probably inaccurate.

To Primary Blue = 660v (372v)
To Primary Blue/White = 660v (372v) ​

The centertap should read the same as Node A, so your reading should be 663V. The DC voltage should only drop off slightly from the center tap to the outer windings Blue & Blue/White. So 660 at Blue and Blue/White would probably be about right. I believe the voltage readings in rectangles are AC measurements. So the schematic indicates 372V AC at Blue and Blue/White. This would be with the amp pushing 300W.

The readings on the 12DW7 and 12BH7s don't look all that odd. Off the top of my head I don't know how close the cathodes of the 12BH7s typically run to the schematic voltage of -47V. This voltage is the bias that is applied to the output tubes. Since your amp is running closer to -40V, I am a bit suspicious.

Do you have the bias TP's set to 72mV and is the voltage drop across each screen resistor about the same? The low bias may be a response to the low plate and low screen voltage...or it may be the cause of it. So we have sort of a chicken and egg scenario. The low plate voltage suggests the tubes may be pulling more current than spec. Which is what you would expect with low bias around -40V. Another possibility is your transformer is actually running to spec.

Checking the voltage drop across the screen resistors is sort of a crude way to see if all 6 tubes are conducting and properly sharing current. If one tube is dead on each side of the transformer and the bias is reduced significantly to get the total current back up to 72mV I would expect the output to be weak.

Since you don't have test equipment, you can improvise. Use an online tone generator to create a test signal. Feed it to the preamp and adjust for 0.257VAC at pin 7 of V1 (in the power amp). This should push the amp to full power, so it needs to be connected to a robust speaker.

Trace the AC through the phase inverter and driver tubes. With 0.257VAC at pin 7 of V1, the schematic says the cathodes (pin 8) of V2 and V3 should have 33.2VAC. These AC reading and others are posted in rectangles on the schematic.

 

[agedhorse]

I would recommend re-checking those measurements before assuming anything.

If the balance across the main filter caps is way off, and the amp has been "re-capped", I would suggest that there was something wrong with the replacement parts or the equalizer resistors. This is the first thing that needs to be correctly identified and corrected. Be SURE that your measurements are correct and you are actually measuring what you think you are measuring.

 

[beans-on-toast]

Spent time yesterday and today running more tests.

Issue is certainly in the Power Amp as I ran a Pre-Amp directly in and had the same issue. With amp on, inserting each tube does not cause any issue or blow fuses.

Measured Plate, Grid and Cathode of each tube. Everything read consistent (High, Low and In Btwn).

I did blow another fuse while the amp was on for a bit longer. A few diodes looked bad and I changed any out of spec resistors and the old .1 caps on the board which were original. Touched up any suspect solder joints.

Checked all the test points on the board. Results were off but not completely crazy. Suggest Filter Caps might be drifting. What I can't understand is how 660v is headed to the Blue and Blue/White of the OT primary. Perhaps my reading is off.

Schematic values in ()

Test Points
A = 663v (600v)
B = 523v (462v)
to C12 = 408v (362v)
D = -175v (-150v)
E = 331v (350v)
H = 525v

To Primary Blue = 660v (372v)
To Primary Blue/White = 660v (372v)

12DW7
Pin 1 = 304v (280v)
Pin 3 = 103v (95v)
Pin 6 = 204v (207v)
Pin 8 = 2.0v (1.7v)

12BH7
Pin 1 = 147v (159v)
Pin 3 = 6.5v (7v)
Pin 6 = 331v (207v)
Pin 8 = -41.8v (-47v)

12BH7
Pin 1 = 147v
Pin 3 = 6.6v
Pin 6 = 333v
Pin 8 = -40v

Issues persists. Likely can't go much further without a better equipment. Will reach out to the tech I work with. He isn't so excited as he hit a wall with a previous SVT but hopefully we can find the issue.

Alway note the AC line voltage when taking readings. The schematic references are at 120VAC or 240VAC.
If your line voltage is off, it will be reflected in the readings.

Those small brown ceramic capacitors on the power board tend to fail and cause problems. C2 (120pF) in particular. Lift one leg and see if it helps with the loss of volume and fizz.

You need the correct tube at the phase inverter, 12BH7 or 12AU7 whatever your amp requires. A 12AX7 has insufficient drive. Also pay attention to the PI voltages. An issue there will cause a loss of volume and a fizzy sound.

When resoldering, used leaded solder. That’s what is in there originally.

I would pay close attention to the tube socket solder joints on the PCB. Then check that the tube socket contacts are clean and tight. When removing and inserting a tube, it shouldn’t feel too loose. You cleaned the sockets but inspect them for oxidation. Sometimes it takes several rounds. Do not get the cleaner on the socket base. Use a small plastic inter-dental brush to scrub the metal.

The wire solder joints on the PCB next to the capacitors look to be a mess. Be careful to not use too much heat as the pads can lift. Apply a little flux, solder will require less heat.

I’d recheck the wiring to the capacitors against the schematic. Also check the solder joints on the capacitors. If you are using a cap can, an isolation wafer (washer) is required. The can should not be grounded to the chassis.

 

[numberfourthree]

Sorry...I see some things that catch my interest, but no silver bullet.

Schematic says 660V at A, rather than 600V. Most of the readings I have seen from various amps have been in the 680 to 695V range, so 663V could be low, but it may be normal for your amp.

Actually many of the PS voltage readings I have seen for SVTs have been considerably higher than the schematic. So 523V on node B is not necessarily a surprise, except for the fact that Node A is almost down to the schematic voltage. I believe B only feeds the preamp. So if the voltage is this high you may be cooking the preamp tubes. Different eras of SVTs used different values for R49...so the voltage at this Node varies from amp to amp. Also the voltage depends on how much current is being drown by the preamp and phase inverter.

The voltage midway across the main filter caps should be 1/2 the voltage at Node A. I am talking about the junction between C10 and C12A. Since you have 663V at A, this should be very close to 331.5V. If you have 408V there, I would probably expect a problem. One or both of the capacitors may have too much leakage or the bleeder resistors may be out of spec. Current bleeding through this filter section can pull Node A down. From what I have seen, twist locks tend to have a shorter lifespan than axial and radial caps.

Node D looks normal, but E is lower than what I have typically seen. Many amps seem to fall right at 350V.

Node H is way high. It should be lower than node E. Node E feeds node H; not the other way around. If the voltage at Node H is higher than Node E it seems very odd. The solder eyelets between node H and Node B are super close, so it's easy to form a solder bridged. But you indicate Node H is running between the voltage at the Node A and Node B. My best guess is this not an accurate reading....either that or it got connected to the Node A somehow. Check the wiring to/around R49 so see if any unintended/accidental connections are made on various terminal strips. If this reading is actually 525V, the voltage on Pin 1 of the 12BH7s would probably be higher...so I think the reading is probably inaccurate.

To Primary Blue = 660v (372v)
To Primary Blue/White = 660v (372v) ​

The centertap should read the same as Node A, so your reading should be 663V. The DC voltage should only drop off slightly from the center tap to the outer windings Blue & Blue/White. So 660 at Blue and Blue/White would probably be about right. I believe the voltage readings in rectangles are AC measurements. So the schematic indicates 372V AC at Blue and Blue/White. This would be with the amp pushing 300W.

The readings on the 12DW7 and 12BH7s don't look all that odd. Off the top of my head I don't know how close the cathodes of the 12BH7s typically run to the schematic voltage of -47V. This voltage is the bias that is applied to the output tubes. Since your amp is running closer to -40V, I am a bit suspicious.

Do you have the bias TP's set to 72mV and is the voltage drop across each screen resistor about the same? The low bias may be a response to the low plate and low screen voltage...or it may be the cause of it. So we have sort of a chicken and egg scenario. The low plate voltage suggests the tubes may be pulling more current than spec. Which is what you would expect with low bias around -40V. Another possibility is your transformer is actually running to spec.

Checking the voltage drop across the screen resistors is sort of a crude way to see if all 6 tubes are conducting and properly sharing current. If one tube is dead on each side of the transformer and the bias is reduced significantly to get the total current back up to 72mV I would expect the output to be weak.

Since you don't have test equipment, you can improvise. Use an online tone generator to create a test signal. Feed it to the preamp and adjust for 0.257VAC at pin 7 of V1 (in the power amp). This should push the amp to full power, so it needs to be connected to a robust speaker.

Trace the AC through the phase inverter and driver tubes. With 0.257VAC at pin 7 of V1, the schematic says the cathodes (pin 8) of V2 and V3 should have 33.2VAC. These AC reading and others are posted in rectangles on the schematic.

Wow. All very helpful will dig in further this weekend.

 

[numberfourthree]

I would recommend re-checking those measurements before assuming anything.

If the balance across the main filter caps is way off, and the amp has been "re-capped", I would suggest that there was something wrong with the replacement parts or the equalizer resistors. This is the first thing that needs to be correctly identified and corrected. Be SURE that your measurements are correct and you are actually measuring what you think you are measuring.

Thank you this is very helpful. I’ll recheck all test points.

I do appreciate your advice and know it comes from a long career designing amps. I simply don’t have a scope so am doing what I can before I take to another tech. I also generally wouldn’t start by swapping out parts (esp caps). I hate when techs suggest that in my old Fenders. But I am seeing a few leaking caps, one diode that looked smoked and tested weak and resistors that are out of spec. I did the work carefully and kept all the old parts in case things need to be swapped in. I also am not a fan of the quick suggestion of buy a new tube set. I am trying to diagnose if I can as much on my own.
Trust me, I’ll be shopping for a scope!

 

[numberfourthree]

Alway note the AC line voltage when taking readings. The schematic references are at 120VAC or 240VAC.
If your line voltage is off, it will be reflected in the readings.

Those small brown ceramic capacitors on the power board tend to fail and cause problems. C2 (120pF) in particular. Lift one leg and see if it helps with the loss of volume and fizz.

You need the correct tube at the phase inverter, 12BH7 or 12AU7 whatever your amp requires. A 12AX7 has insufficient drive. Also pay attention to the PI voltages. An issue there will cause a loss of volume and a fizzy sound.

When resoldering, used leaded solder. That’s what is in there originally.

I would pay close attention to the tube socket solder joints on the PCB. Then check that the tube socket contacts are clean and tight. When removing and inserting a tube, it shouldn’t feel too loose. You cleaned the sockets but inspect them for oxidation. Sometimes it takes several rounds. Do not get the cleaner on the socket base. Use a small plastic inter-dental brush to scrub the metal.

The wire solder joints on the PCB next to the capacitors look to be a mess. Be careful to not use too much heat as the pads can lift. Apply a little flux, solder will require less heat.

I’d recheck the wiring to the capacitors against the schematic. Also check the solder joints on the capacitors. If you are using a cap can, an isolation wafer (washer) is required. The can should not be grounded to the chassis.

Also great suggestions. I agree the soldering is a mess at points. Really not a too tricky to clean all that up. The little 120pfd is also worth a look.

 

[Wasnex]

You need the correct tube at the phase inverter, 12BH7 or 12AU7 whatever your amp requires. A 12AX7 has insufficient drive. Also pay attention to the PI voltages. An issue there will cause a loss of volume and a fizzy sound.

AFAIK the period correct tubes are 12DW7 for the PI and 12BH7 for the drivers.

The MTI era mod to convert all of the 12DW7s to 12AX7 does not change the circuit supporting the PI tube. AFAIK, simply plug in a 12AX7.

I believe Ampeg started using 12AU7 for the drivers with the CL. I compared the circuits awhile back and a few resistor values are different. Probably not advisable to run 12AU7 without updating the circuit.

 

[beans-on-toast]

AFAIK the period correct tubes are 12DW7 for the PI and 12BH7 for the drivers.

The MTI era mod to convert all of the 12DW7s to 12AX7 does not change the circuit supporting the PI tube. AFAIK, simply plug in a 12AX7.

I believe Ampeg started using 12AU7 for the drivers with the CL. I compared the circuits awhile back and a few resistor values are different. Probably not advisable to run 12AU7 without updating the circuit.

Yes I meant drivers but ran out of editing time.

There was a time when 12BH7’s were in short supply and were expensive. Some did modify the circuit to use 12AU7’s. It’s important to verify the circuit and see what components are in there. But a bad driver will make the amp sound fizzy.

 

[numberfourthree]

Yes back in the day I combed electronics stores in Cleveland and have a decent selection of 6C4, 12DW7 and 12BH7. I originally bought a blue line in the mid 90s but then came across the 79 in a GC and it sounded tons better. It had all original GE tubes so I don’t believe any conversions for power tubes have occurred but I’ll double check.

 

[numberfourthree]

Been a busy few weeks and I've not wanted to post on this until I was able to be a lot more thorough. Needless I've slowly run a ton of tests and still little that stands out as any issue.

I retested the schematic test points and results seem a bit more consistent with schematic.

DC Voltage
A = 678V
B = 534V (462v) also a bit high here still
to C12 = 418V
D = - 178V
E = 339V
H = 332V

Plate voltage on V1 is still a bit high.
Pin 1 = 314v vs (280v)
Pin 6 = 233v vs (207v)

V2 and 3 all run in spec.

V4-9 all are consistent
Plate = 672-674
Grid 2 = 336-341
K = .077

Transformers aren't so easy to measure off the strips.

Power Transformer
Primaries are 120ac

Secondaries
Red / White = 296vac
Red = 270ac
Violet = 259ac
Blue Yellow = 135ac
Green Yellow = 135ac
Green = 259ac

I also disconnected the PT fully and tested resistance.
Primary
Black to Black White = .8ohms
Blue to Blue White = .8ohms

Secondaries
Violet to Blue Yellow = 13ohms
Violet to Green Yellow = 43ohms
Violet to Green = 57ohms
Blue Yellow to Green = 43ohms
Blue Yellow to Green Yellow = 30ohms
Green Yellow to Green = 14ohms
Red to Red / White = 6.4ohms

Did the same with the OT
Primaries
Red White to Blue & Blue White = 29ohms
Blue to Blue White = 57ohms

Secondaries that pair all read between .4ohms and .2ohms.

Continuity is clean across the board, I repaired a few of the messy traces and reflowed solder in a few places. So as far as I can see everything checks out more or less, I can't figure out what would cause such a volume drop.

Not a solution but a question as i've noticed the way the 47k resistors are wired across V4-V9 seems to have changed over the years. Earlier SVTs have the resistor go from pin 5 to a switch, some run the resistors across pins 4 and 5 (mine does both), MTI era seems to run them across pins 4 and 5. Is there a better preferred way?

 

[Wasnex]

There is sort of a chicken-or-egg argument with the power supply voltages. As current goes up it pulls the voltage down. If the power supply node feeding a tube section is reading high, then the plate voltage for that section generally reads high.

If node B is >462V the preamp plate supply will probably run high. Node C will also probably run high. Node C feeds V1, so the readings are not a surprise.

The only part of the voltage readings I find unusual is node A is so close to the schematic while Node B is significantly above the schematic. This may be a red hearing though.

Did you reconfirm your reading at the junction between C10 and C12A. This should be very close to 1/2 the voltage at node A. If I remember correctly, your reading was not 1/2 node A. This suggests a problem in node A. R47 and R48 pull a little bit of current. The voltage drop across these resistors is what centers the voltage. If C10 or C12A have excessive leakage that voltage will not be centered, and I would expect low voltage at node A.

It does not matter that the voltage at node A is still higher than the schematic. What matters is the voltage at the junction between the capacitors is not centered and nod A is lower than I expect based on what appears to be the actual real world working voltage. AFAIK most vintage SVTs read between 685V and 700V at node A. From what I have seen, all power supply nodes in vintage 6550 amps run over spec except E and H. Node E typically sits right at 350V AFAIK. In the older 6146 based amps, even nodes E and H tend to run about 10V over spec.

V2 and V3 are feed from the Node E and H.

The 47k resistors are most likely the grid resistors. Per the photo I have, 69 SVTs ran the resistors between pin 5 and a terminal strip. There are two terminal strips and three resistors are connected to each.

On the images of skunks and MTIs, resistor appears to be connected between pin 5 and 6 on the tube socket. Then a wire is run between pin 6 of three tubes.

I added the red connection for clarity. The pink wires in the amp make the actual connections.

An additional wire is connected to pin 6 on one of the tubes in each circuit. I think the advantage to connecting the resistors a across pin 5 and 6 is they are more resistant to physical shocks because the leads are short.

There are different ways to take the measurement on the PT windings.

Here is the open circuit spec for a MM PT clone.

Red to Red = 520V (Plate)
Orange to Orange Wht = 520V (Screen)
Yellow to Yellow Whit = 272V (Bias)

And here is the open circuit spec for a Heyboer clone:

Red to Black = 520V (Plate)
Violet to Green = 520V (Screen)
Blue/Yellow to Green/Yellow = 270V (Bias)


In an idling amp I believe the voltages will be pulled down about 3%

Your plate reading appears to be 566VAC. Which is higher than both MM and Heyboer (520V). If your reading is accurate then your plate supply (node A) should be over 700VDC. I believe the correct way to measure is from Red to Red/White. Notice on both images, the B+ is measured tap to tap. Since you measured 296V and 270V, you didn't measure the correct way, and my guess is the readings are not valid.

 

[numberfourthree]

Wasnex, thank you again for the detailed response.

I do think the voltages off the caps were a bit odd and and high in a few spots. The previous tech used an assortment of caps to get the proper values. One cap can is 50uf / 50 uf and the 2nd 100uf / 100 uf. so it's a bit confusing to find the junction btwn C10 and C12A but I'll give it another close look and measurement.

From C10 to the board measures 757vac also seeming high. You're correct that these high measurements could be off. My main meter is 600v, I have a cheaper meter that is 1000v but it's not as accurate.

I'll remeasure Red / Red White as it doesn't appear correct per your suggestion.

 

[Wasnex]

Wasnex, thank you again for the detailed response.

I do think the voltages off the caps were a bit odd and and high in a few spots. The previous tech used an assortment of caps to get the proper values. One cap can is 50uf / 50 uf and the 2nd 100uf / 100 uf. so it's a bit confusing to find the junction btwn C10 and C12A but I'll give it another close look and measurement.

From C10 to the board measures 757vac also seeming high. You're correct that these high measurements could be off. My main meter is 600v, I have a cheaper meter that is 1000v but it's not as accurate.

I'll remeasure Red / Red White as it doesn't appear correct per your suggestion.

The original capacitor scheme had a pair of 100/40 twist locks. I would consider a 100/100 and 50/50 pretty much equivalent, but you would probably relabel the schematic so it's easier to keep track of the connections.

The first thing I recommend is tracing out the connections and creating an updated schematic and wiring diagram.

Maybe the current setup is like this:

Red and Blue rectangles added to show the multi-section cans.

Or maybe the 50/50 is tied together in parallel in place of C10

Hard to figure out what's going on in you don't know where you are in the circuit.

This may be helpful:

This is the angle I typically view the card from while working on the amp. Find the junction between R48 and R47 in the lower right corner.

Here is the area you need to look at:

The orange circle goes to the case of C12. This is ground.

Green goes to the + of C10. This is node A.

Purple goes to the top (+) of C12A. This is the junction between C10 and C12 and also the junction between R48 and R47. Notice the label on the drawing says To + C12 (70 + 40 MFD). This is because C12 was originally a 70/40/40. The 70 and first 40 were tied together in parallel for 110uF.

This should be the same points (electrically) on the schematic:

You need to figure out how your amp is wired and create similar references.

 

[numberfourthree]

Absolutely. I never knew the original cans were 70/40/40. I do plan to revert the amp to 100/40 cans eventually but want to get it working first.
The trace between R47 and D5 is a bit of a mess, like many. I'm working to clean that up and wire from under the board to avoid the large piles of solder.

 

[agedhorse]

Replace those bastardized caps with the proper replacements.

This is why it's so frustrating working on an amp that has been worked on by an under-qualified person/tech. It generally doubles the amount of time necessary to properly repair the amp, often it can become much more.

 

[numberfourthree]

My thinking as well. I’d prefer to trace any issues with something that matches the schematic.