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

[Wasnex]

With. It is always there.

A a lightbulb limiter functions as a variable resistor. The resistance is proportional to current. So as the amp pulls more current, the limiter will drop more voltage.

 

[Dom65]

A a lightbulb limiter functions as a variable resistor. The resistance is proportional to current. So as the amp pulls more current, the limiter will drop more voltage.

That was THE problem. I'm reading 450VDC on first tab of capacitor can (the one receiving HT from pin 8 of rectifier).
So, the current limiter would only be used in case I have to plug an amp that I don't know what to expect from it the first time I plug it to protect it from damages? So, the lack of punch (sag?) that the owner reported to me could be to: OLD Tubes, Capacitor can, OT and PT I guess. I will see when I replace the Capacitor can that I'm waiting for.
Thank you so much and Sorry for throwing you all off with my lightbulb current limiter.

 

[Wasnex]

That was THE problem. I'm reading 450VDC on first tab of capacitor can (the one receiving HT from pin 8 of rectifier).
So, the current limiter would only be used in case I have to plug an amp that I don't know what to expect from it the first time I plug it to protect it from damages? So, the lack of punch (sag?) that the owner reported to me could be to: OLD Tubes, Capacitor can, OT and PT I guess. I will see when I replace the Capacitor can that I'm waiting for.
Thank you so much and Sorry for throwing you all off with my lightbulb current limiter.

 

[beans-on-toast]

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.

the 6404 code indicates the 4th week of 1964. This aligns with the B-15N NC revision. So the cap can is original and 60 years old. My experience tells me that these caps, due to their construction, don’t have a service life that extends that long.

 

[Dom65]

the 6404 code indicates the 4th week of 1964. This aligns with the B-15N NC revision. So the cap can is original and 60 years old. My experience tells me that these caps, due to their construction, don’t have a service life that extends that long.

Ah, I see,
Thanks.
It will be replaced by a CE manufacturing 40/40/40/40MFD 525V.

 

[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.

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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.

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Do not replace those green coupling caps unless necessary.



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

Hi there,
I finally found time to get back to that project after receiving the new electrolytic multi section capacitor can.
As you can see on the first picture, I redid the whole electricity thing. New ac cable with proper ring terminal for the ground. The hot is now going to the fuse first. The wires, which were properly identify before removing them from the old capacitor can, were soldered back in place in same order.
On second photo, you can see the can in place with its insulated base, as well as a cable lock for the new AC cable.
The 3rd picture shows the main output jack ground connected right where the black OT wire connect on the ground buss.
I also had to change a coupling capacitor that was starting to burst (the yellow one on first picture).

Now, I fired it "On" on Friday and everything was fine until 5 or 10 min when the sound went away for a brief period and came back. I kept playing for a half hour and everything was fine. Came back on Monday, and again it went away a couple of time over a 2h period. Yesterday, I kept it on the whole afternoon and everything was fine. Today again. I turned it off and when I turned it back on it blew the fuse. Put a new fuse and it's working again.
The problem is that it is intermittent and it comes back fast enough that I don't have much time to diagnose it.
The sound fades out quickly and comes back like a gate opening (in a sudden manner).
I already tried: Changing the rectifier tube, connecting to another input while signal is out (same), changing the new capacitor that I installed...
Could it be the new Capacitor can?

Thanks in advance for anybody who would be willing to help.

Dominic

 

[agedhorse]

What happens to the power supply voltages when this happens?

 

[Dom65]

What happens to the power supply voltages when this happens?

For what I could see so far, the DC seems to stay stable. Reading on the first tab of the can. It cuts for short period of time so it doesn't give me time to take readings.
Right now it's working fine and all the plates for all the tubes are pretty much On, as well as the filaments.

 

[agedhorse]

Then you will need a scope to monitor what's happening at various stages and I would also monitor the supply voltages when the symptom occurs.

 

[beans-on-toast]

The blown fuse could be an intermittent short.

Check the power plug, you may have broken wire. Either at the plug or at the chassis. Note when it goes out, see if the 6.3 VAC (heater circuit) lamps go out as well. Check in standby. Wiggle the power cord while testing. That would be a big clue.

I’d recheck every joint that you soldered. The yellow wire at the top of the cap can looks close to the ground tab. Nove it away if it is, check the sheathing.

Turn the amp on in the dark and look for any arcing.

 

[Dom65]

The blown fuse could be an intermittent short.

Check the power plug, you may have broken wire. Either at the plug or at the chassis. Note when it goes out, see if the 6.3 VAC (heater circuit) lamps go out as well. Check in standby. Wiggle the power cord while testing. That would be a big clue.

I’d recheck every joint that you soldered. The yellow wire at the top of the cap can looks close to the ground tab. Nove it away if it is, check the sheathing.

Turn the amp on in the dark and look for any arcing.

I would pretty much rule out the power plug, as I had same problems with the old power cable. I did the change over today, since I was missing some pieces to complete that part. Did a test of continuity on common and hot tab and ground tab to end of ground buss in amp on power plug and everything was alright.
I already redid one of the soldering (just to make sure) and the rest are conducting as they should. I moved the cables further form the ground tab.
There is no arcing in the dark. Temperature of tubes seems ok.
It has been working all the afternoon. I will try to see in the dark if the tubes are all glowing when the problems comes back...

Thanks you so much for your help.

Dominic

 

[beans-on-toast]

I would pretty much rule out the power plug, as I had same problems with the old power cable. I did the change over today, since I was missing some pieces to complete that part. Did a test of continuity on common and hot tab and ground tab to end of ground buss in amp on power plug and everything was alright.
I already redid one of the soldering (just to make sure) and the rest are conducting as they should. I moved the cables further form the ground tab.
There is no arcing in the dark. Temperature of tubes seems ok.
It has been working all the afternoon. I will try to see in the dark if the tubes are all glowing when the problems comes back...

Thanks you so much for your help.

Dominic

By lamps I meant the two #1847 miniature bulbs. The heater coils have persistence and may not fade fast enough when the intermittent issue occurs.

 

[Dom65]

By lamps I meant the two #1847 miniature bulbs. The heater coils have persistence and may not fade fast enough when the intermittent issue occurs.

By lamps I meant the two #1847 miniature bulbs. The heater coils have persistence and may not fade fast enough when the intermittent issue occurs.

Oh, I see.
The lights are burnt. The 6.3V is getting there ok.
The owner said that when he bought the amp in 1998, they where already burnt.
He doesn't care for them.
Although, now I see that it could help me quickly identify a filament problem.
So basically any 6.3 #1847 miniature light would do?

 

[coreyfyfe]

Oh, I see.
The lights are burnt. The 6.3V is getting there ok.
The owner said that when he bought the amp in 1998, they where already burnt.
He doesn't care for them.
Although, now I see that it could help me quickly identify a filament problem.
So basically any 6.3 #1847 miniature light would do?

#47 bayonet base bulbs

Invalid Link Removed

 

[Dom65]

#47 bayonet base bulbs

Invalid Link Removed

Great!
Thank you so much!

 

[beans-on-toast]

Oh, I see.
The lights are burnt. The 6.3V is getting there ok.
The owner said that when he bought the amp in 1998, they where already burnt.
He doesn't care for them.
Although, now I see that it could help me quickly identify a filament problem.
So basically any 6.3 #1847 miniature light would do?

As Corey said. Many like the LED lamps because they come in different colors.

Without the lights, you. can connect a voltmeter and you’ll see if you loose heater power with the intermittent issue. If yes, it tells you that good chance the issue is on the primary side of the power transformer (intermittent wiring connections perhaps) or the power cable. A simple test.

 

[coreyfyfe]

Entirely an aside from actually fixing your issue here, but I put the led style in my B15 to get the green back in the plexi logo. I think AES sells the individual LEDs cheaper than the 10 pack of bulbs, though obviously the 10 pack is a cheaper value if you need more than the two lights. I believe I got my 47 base LEDs from comet pinball though, many more options from them.

 

[Dom65]

As Corey said. Many like the LED lamps because they come in different colors.

Without the lights, you. can connect a voltmeter and you’ll see if you loose heater power with the intermittent issue. If yes, it tells you that good chance the issue is on the primary side of the power transformer (intermittent wiring connections perhaps) or the power cable. A simple test.

Yes, I had one of my multimeters connected there since 8h30 this morning and the voltages were good.
Amp is working normal...
I just came back from buying lights for it. If there is a problem with the 6.3V, I will know right away.

 

[agedhorse]

Be aware that some LED replacements fail quickly on an AC source because they do not have the necessary reverse blocking within the lamp.

 

[agedhorse]

Yes, I had one of my multimeters connected there since 8h30 this morning and the voltages were good.
Amp is working normal...
I just came back from buying lights for it. If there is a problem with the 6.3V, I will know right away.

It's when the amp ISN'T working normal that is of interest.