Help Ampeg B-15NC B+ voltages low when pluging 6L6 Power Tubes

[Dom65]

Measure AC voltage across the cap when loaded, while the amp is at idle.

Ok, with a multimeter, I can read 2.5VAC across the first and second section. 1-3, 1-4, 2-3, 3-4 = 0V

 

[agedhorse]

Ok, with a multimeter, I can read 2.5VAC across the first and second section. 1-3, 1-4, 2-3, 3-4 = 0V

What is it unloaded?

 

[Dom65]

What is it unloaded?

1-2 = 0.4V

 

[agedhorse]

That's a little bit high but not out of the question. This is where comparing with a benchmark unit is helpful (and why most manufacturers keep a benchmark sample in their service departments for models that they still support.

 

[byacey]

Ok, with a multimeter, I can read 2.5VAC across the first and second section. 1-3, 1-4, 2-3, 3-4 = 0V

For ripple under load, I would have expected much more, considering how low the B+ is dropping. This is measured between ground and the first capacitor section?

 

[agedhorse]

For ripple under load, I would have expected much more, considering how low the B+ is dropping. This is measured between ground and the first capacitor section?

Agreed, but there may be something changed that we can’t see.

 

[beans-on-toast]

1 First power supply node being the capacitor tab connecting to ground bus (blue wire in 1rst picture)?

2 That spot on 5th photo?

3 Here, I can see that the washers were previously replaced and the contact to the chassis is very good. Actually, there was a noise problem due to an oxidized shunt. I fixed it by deoxidizing it, but knowing, now, the importance of it, I will change it.

4 I put a photo of the output jacks. Right now, if I plug to the main speaker out jack (same with nothing plugged) both tips (main and EXT) are conducting to ground bus. If I'm plugged in EXT Speaker, both tips stop conducting to ground bus.

5 See 1rst picture. 1 tab is connected to the ground bus.

6 Sure, I'm having a hard time finding the capacitor can here in Canada. I have to see if the owner is willing to pay the extra charges for ordering from the US.

7 Already did.

8 I connected the white power cord wire directly to one of the primary. I will revise that section since the wall ac hot (black) doesn't pass by the fuse first.

9 Right now, it is under a PT mounting screw but it doesn't have an eyelet. So I will revise that.

10 OK, would that help reducing the loud pop it has when using standby switch?

Thanks for your time.

1 The cap can ground tab is the first bus connection. Do not ground the can cap to the chassis at this point. The cap can requires that the insulator be in place.

2 Yes close to that point or towards the board fastener.Use a dental mirror if necessary to find the black wire coming from the OPT and find where it attacked to the eyelet.

3 Good. Oxidization of the bus ground point is a source of hum. The oxidization acts like a resistor. This grounding scheme is common with vintage ampeg amps and includes the v4B and the svt..

4 With a jack in place for the mail speaker out in place ofthe cable. I like to use the switchraft washers to isolate the jack. I prefer to not allow the speaker return [-] to flow through the chassis to the ground bus attachment at the input jack. The insulating washers prevent this. Use a wire to go from the jack sleeve to close to the PI ground point. Check the washers behind the pots as well. Be very careful with the grey shielded wires, when moved they can break.

This should make it clearer. The white wire from the speaker cable is goint to the isolated EXT SPEAKER jack. Study the schematic (good copies are available from Vintage Blue ) to see how the speaker out should be wired.

5 Good.

6 I found these but the price is nute. Next Gen Canada - Parts for Amps, Guitars, Pedals, and Pickups

7 Check

8 The black power wire should connect at the ring end of the fuse holder, not the side.

9 Check

10 The standby switch circuit is not uncommon. The switch connects to the center tap of the power transformer to ground. It’s hard on the switch. Some put a (approx) 47K 2W resistor across the standby switch. The amp never goes completely into standby when the switch is open. When the switch is closed, the resistor is bypassed. I’ve done this with other amps but not B-15’s.

The rectifier plate diodes help protect the tube rectifier from flashover when the current rushed in when the amp is first powered up and the cap canis discharged. The diodes increase what’s called the peak inverse voltage. This helps protect the tube. On a scope, capacitor charging looks like spikes which can be hard on the rectifier. Because yow will be using a 40-40-40-40, the 40uF cap, I’d sure the two diodes as outlined. This is a technique that Fender and others use. See the Fender Tweed Deluxe schematic. I came across this in an old Bell Labs publication that discusses flashover in rectifier tubes. It was a problem with telephone equipment.

————

The bottom of the cap can has a rubber pressure relief valve on the bottom. When it blows open, liquid leaks out. The cap needs to be replaced. The rubber can crack with age and this lets the liquid electrolyte leak out. It can look fluffy. Sometimes you see goop (a technical term) applied around the base of a cap or other component. It’s a silicone material, not the same as leaking dried electrolyte.

————

The chassis interior look pretty dirty. Circuit washes are available. You can use isoproply alcohol (99% from a pharmacy, ask for it) with q-tips and cotton makeup removing pads for a more cost effective solution. Be careful on the resistor color bands.

————-

Do not replace those green coupling caps unless necessary.



————

It’s such a basic amp but bringing it up to spec can take a lot of work.

 

[Dom65]

For ripple under load, I would have expected much more, considering how low the B+ is dropping. This is measured between ground and the first capacitor section?

No, like specified, I took the readings between the different sections of the capacitor can.
Wow! Now taking a measure between ground and first Capacitor can section (the tab that receives HT from pin 8 of rectifier), the multimeter flickers from overload (OL) to 1499. I had to use hold on the multimeter to be able to read it. Ouch! I guess I will wait for the new capacitor can to arrive before I turn it on again.

 

[byacey]

No, like specified, I took the readings between the different sections of the capacitor can.
Wow! Now taking a measure between ground and first Capacitor can section (the tab that receives HT from pin 8 of rectifier), the multimeter flickers from overload (OL) to 1499. I had to use hold on the multimeter to be able to read it. Ouch! I guess I will wait for the new capacitor can to arrive before I turn it on again.

There is something wrong with your meter then. A reading of 1500VAC is clearly an erroneous measurement.

Measuring ripple between the capacitor sections doesn't tell you much of anything.

 

[agedhorse]

Like I mentioned earlier, there’s more going on here that we aren’t understanding…

IMO, the OP does not have the necessary education, background and understanding to safely work on this equipment.

Ripple voltage is always measured across the capacitor (in this case each section terminal to ground) and not section to section which produces meaningless results.

I’m backing away because of the possibility of the OP getting injured, or injuring the user of the amp.

 

[Dom65]

1 The cap can ground tab is the first bus connection. Do not ground the can cap to the chassis at this point. The cap can requires that the insulator be in place.

2 Yes close to that point or towards the board fastener.Use a dental mirror if necessary to find the black wire coming from the OPT and find where it attacked to the eyelet.

3 Good. Oxidization of the bus ground point is a source of hum. The oxidization acts like a resistor. This grounding scheme is common with vintage ampeg amps and includes the v4B and the svt..

4 With a jack in place for the mail speaker out in place ofthe cable. I like to use the switchraft washers to isolate the jack. I prefer to not allow the speaker return [-] to flow through the chassis to the ground bus attachment at the input jack. The insulating washers prevent this. Use a wire to go from the jack sleeve to close to the PI ground point. Check the washers behind the pots as well. Be very careful with the grey shielded wires, when moved they can break.

This should make it clearer. The white wire from the speaker cable is goint to the isolated EXT SPEAKER jack. Study the schematic (good copies are available from Vintage Blue ) to see how the speaker out should be wired.

View attachment 7092222

5 Good.

6 I found these but the price is nute. Next Gen Canada - Parts for Amps, Guitars, Pedals, and Pickups

7 Check

8 The black power wire should connect at the ring end of the fuse holder, not the side.

9 Check

10 The standby switch circuit is not uncommon. The switch connects to the center tap of the power transformer to ground. It’s hard on the switch. Some put a (approx) 47K 2W resistor across the standby switch. The amp never goes completely into standby when the switch is open. When the switch is closed, the resistor is bypassed. I’ve done this with other amps but not B-15’s.

The rectifier plate diodes help protect the tube rectifier from flashover when the current rushed in when the amp is first powered up and the cap canis discharged. The diodes increase what’s called the peak inverse voltage. This helps protect the tube. On a scope, capacitor charging looks like spikes which can be hard on the rectifier. Because yow will be using a 40-40-40-40, the 40uF cap, I’d sure the two diodes as outlined. This is a technique that Fender and others use. See the Fender Tweed Deluxe schematic. I came across this in an old Bell Labs publication that discusses flashover in rectifier tubes. It was a problem with telephone equipment.

————

The bottom of the cap can has a rubber pressure relief valve on the bottom. When it blows open, liquid leaks out. The cap needs to be replaced. The rubber can crack with age and this lets the liquid electrolyte leak out. It can look fluffy. Sometimes you see goop (a technical term) applied around the base of a cap or other component. It’s a silicone material, not the same as leaking dried electrolyte.

————

The chassis interior look pretty dirty. Circuit washes are available. You can use isoproply alcohol (99% from a pharmacy, ask for it) with q-tips and cotton makeup removing pads for a more cost effective solution. Be careful on the resistor color bands.

————-

Do not replace those green coupling caps unless necessary.



————

It’s such a basic amp but bringing it up to spec can take a lot of work.

Thank you so much for your help and patience.
It sure helps me a lot.

 

[Dom65]

Like I mentioned earlier, there’s more going on here that we aren’t understanding…

IMO, the OP does not have the necessary education, background and understanding to safely work on this equipment

Ripple voltage is always measured across the capacitor (in this case each section terminal to ground) and not section to section which produces meaningless results.

I’m backing away because of the possibility of the OP getting injured, or injuring the user of the amp.

I sure don't have the experience and knowledge of so many here. But I'm ready to learn.
As far as safety, I am very cautious with every measurement and move I do there. I take the time power off, disconnect ac cable and discharge filter cap can between every move. I am not gonna give back this amp until I feel it is back in shape and safe. So far, apart from removing death cap in the circuit, I have not modify anything in the circuit. Since the amp was working perfectly for more than 20 years (touring and in the studio work, I know from the owner), I am pretty sure that once I put my finger on the problem (and it seems like the filter cap can is at fault here), it will be back to where it was before. I will see when I change it (I'm waiting for it late December).
Thanks for your expertise.

 

[beans-on-toast]

Often the cap cans have date codes on them. Out of interest what is the make and codes on the can.

 

[Dom65]

There is something wrong with your meter then. A reading of 1500VAC is clearly an erroneous measurement.

Measuring ripple between the capacitor sections doesn't tell you much of anything.

Ok
I went back to it and my multimeter just goes crazy between the first tab and ground. It flickers between crazy numbers and OL.
Between tab 2 and ground I have 35VAC, 3 to ground 28VAC 4 to ground 26.5VAC.

 

[Dom65]

Often the cap cans have date codes on them. Out of interest what is the make and codes on the can.

It's hard to read because the can was installed with the writings facing the power transformer, but I can read UPN 10376 then the 40MFD 500V 4X, then Cornel Dubilier and 6404.

 

[Dom65]

Also, I use a current limiter light. Built from a video of uncle Doug.

 

[Wasnex]

Also, I use a current limiter light. Built from a video of uncle Doug.

Are the low plate voltages with or without the lightbulb limiter?

 

[Dom65]

Are the low plate voltages with or without the lightbulb limiter?

With. It is always there.

 

[agedhorse]

With. It is always there.

This will give a false reading as well.

 

[agedhorse]

I sure don't have the experience and knowledge of so many here. But I'm ready to learn.
As far as safety, I am very cautious with every measurement and move I do there. I take the time power off, disconnect ac cable and discharge filter cap can between every move. I am not gonna give back this amp until I feel it is back in shape and safe. So far, apart from removing death cap in the circuit, I have not modify anything in the circuit. Since the amp was working perfectly for more than 20 years (touring and in the studio work, I know from the owner), I am pretty sure that once I put my finger on the problem (and it seems like the filter cap can is at fault here), it will be back to where it was before. I will see when I change it (I'm waiting for it late December).
Thanks for your expertise.

But you DON'T know that it was working perfectly, you just THINK you know.

You can't POSSIBLY disconnect the power cable and make the measurements of voltage which is why I am concerned.