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

[Dom65]

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

I got (4) what looks like a pretty close replacement (same specs and all).

 

[Dom65]

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

Yeah,
I know, but unfortunately, it's been working perfectly since yesterday afternoon.

 

[agedhorse]

I got (4) what looks like a pretty close replacement (same specs and all).

That doesn't look like an LED bulb replacement

Yeah,
I know, but unfortunately, it's been working perfectly since yesterday afternoon.

It sometimes happens this way.

 

[Dom65]

That doesn't look like an LED bulb replacement

I had to buffer the base (just like the original) for them to fit, but they are working up to now.

It sometimes happens this way.

 

[Dom65]

I got (4) what looks like a pretty close replacement (same specs and all).

Or did you mean that the burnt one was not original?

 

[Dom65]

After 3 days of working perfectly, the problem is back.
After swapping the 6SL7, I thought that one of the those could have been the problem but No.
This morning, I tested the amp one more time and everything was alright. I reassembled it and gave it a try again and
No sound again... I flipped the standby switch on and off and it was back.
I disassembled it again, now leaving the standby on, I turned it back on and as I was testing it the sound cut again.
B+ was good on last tab of electrolytic can and the lights were still working (so 6.3V ok). Now taping on the tip of incoming 1/4 inch cable I could hear a very weak sound in the speakers. Then it came back like a gate opening.
Could a coupling capacitor act that way?
Otherwise, what could it be? A power tube behaving? Can I run with only 1 to eliminate that possibility?

 

[agedhorse]

This is the time to use a scope and proper troubleshooting stage by stage. It could be ANYTHING.

 

[beans-on-toast]

Now taping on the tip of incoming 1/4 inch cable I could hear a very weak sound in the speakers. Then it came back like a gate opening.

The gray shielded wires can develop intermittent problems. They become brittle and can break whenever they are moved. The braided wire is also prone to rusting under the gray sheath.

Without having a scope…
Trace the signal path. First, try all three input jacks. Does the intermittent issue occur with the Channel 1 pair and the Channel 2 jack?

Plug an instrument cable in, tap the tip at other end of the cable and listen for the buzz. Do this for all three input jacks. Find the wires that run from the input jacks to pin-1 of the two 6SL7 input tubes (V1 and V2). Check those solder joints. Carefully move the wires with an insulated probe. Some use a chopstick while testing continuity from the jacks to the pin-1’s.

In a similar way the circuits that follow should be traced.

The bus chassis ground must be good and free of corrosion. Check the jack to chassis contacts.

 

[Dom65]

The gray shielded wires can develop intermittent problems. They become brittle and can break whenever they are moved. The braided wire is also prone to rusting under the gray sheath.

Without having a scope…
Trace the signal path. First, try all three input jacks. Does the intermittent issue occur with the Channel 1 pair and the Channel 2 jack?

Yes, I was able to quickly switch to channel 2 while having the signal cut problem and I it didn't come back.

Plug an instrument cable in, tap the tip at other end of the cable and listen for the buzz. Do this for all three input jacks. Find the wires that run from the input jacks to pin-1 of the two 6SL7 input tubes (V1 and V2). Check those solder joints. Carefully move the wires with an insulated probe. Some use a chopstick while testing continuity from the jacks to the pin-1’s.

In a similar way the circuits that follow should be traced.

I've been using a test probe that I built from Billm () a while ago with good success (when the problem is not intermittent). I feed the input with a sine tone and I trace it (follow along listening on a small speaker) from the input to the last output of the power tubes and I can't find the fault yet.

The bus chassis ground must be good and free of corrosion. Check the jack to chassis contacts.

I'll double check these.
I've noticed that my new coupling capacitor (that goes to the second 6L6) is passing much more DC than the one that goes to the first 6L6.
First (on the left side of schematic) stabilize at around 18mVDC while the second one is at 240mVDC and keeps going up. It is the Yellow one.
Is that normal?

View attachment 7136809

 

[beans-on-toast]

If the component is in doubt, you can always try switching the position of the two coupling capacitors and see if the reading follows. A capacitor tester would come in handy here. But first check that the nominal value is correct.

 

[Dom65]

If the component is in doubt, you can always try switching the position of the two coupling capacitors and see if the reading follows. A capacitor tester would come in handy here. But first check that the nominal value is correct.

Ok, I've got a problem with one of the 6L6. After changing the capacitor and getting same results, I swapped the power amp tubes and the problem switched side. The problematic tube was also a lot hotter.
I need:
Tube tester
ESR Capacitor tester.
A oscilloscope...

 

[agedhorse]

Ok, I've got a problem with one of the 6L6. After changing the capacitor and getting same results, I swapped the power amp tubes and the problem switched side. The problematic tube was also a lot hotter.
I need:
Tube tester
ESR Capacitor tester.
A oscilloscope...

Sure makes the job a whole lot easier and more productive.

If the output of the PI stage is good and the symptom exists, you can eliminate 95% of the amp as possible causes.

 

[beans-on-toast]

Ok, I've got a problem with one of the 6L6. After changing the capacitor and getting same results, I swapped the power amp tubes and the problem switched side. The problematic tube was also a lot hotter.
I need:
Tube tester
ESR Capacitor tester.
A oscilloscope...

Nice to keep a matched pair of gold standard power tubes on hand for testing.

 

[Dom65]

Nice to keep a matched pair of gold standard power tubes on hand for testing.

For sure...
What would you recommend in the current production?

 

[Dom65]

Sure makes the job a whole lot easier and more productive.

If the output of the PI stage is good and the symptom exists, you can eliminate 95% of the amp as possible causes.

I totally agree.
I always got by repairing and modifying amps of mine or from friends and acquaintances with minimum tools, but now I realize that if I want to be efficient, I need the proper tools.

 

[beans-on-toast]

For sure...
What would you recommend in the current production?

It doesn’t matter as long as the tubes are burnt in and tested/matched well. JJ are often the least expensive (some have issues), I prefer tung-sol and mullard reissues.

 

[Dom65]

It doesn’t matter as long as the tubes are burnt in and tested/matched well. JJ are often the least expensive (some have issues), I prefer tung-sol and mullard reissues.

Great thanks!

 

[Dom65]

Anybody's got recommendations for an ESR capacitance tester?

 

[Dom65]

OK,
I changed the power tubes and the problem came back.
I was feeding a 1khz tone, tracing the signal with my audio probe and the signal came back when I touched the coupling capacitor between the 510k (actual value 533k) and 470k (actual value 494k) resistor of the PI. I changed it and it's been ok till now.
I noticed that the signal going to the 2nd 6L6 is stronger (by 4db to 5db), putting my probe at the exit of their respective coupling caps.
Shouldn't it be more balanced?
All the resistors in PI circuit are otherwise within specs.

 

[agedhorse]

Resistors values are plenty fine.

ESR isn’t necessarily an issue, what is the measures capacitance value, BUT if intermittent a measurement isn’t going to be terrible helpful.

It’s going to come down to analysis and in circuit observation.