Engl Powerball 1 problem, power amp tubes defective or not?

[bassmarth]

Hello,

I've got a problem with my Engl Powerball MK1 amp.

When I turned it on yesterday, the V1 and the V3 Tube monitor LEDs lit indicating a problem with the corresponding tubes.

OK, wanted to check this and changed V1 into V2 position and vice versa, did the same with V3 and V4. But still the V1 and V3 monitor LEDs light up.

Where would you look after next?

Does anyone know if there are fuses inside the amp? Or is this perhaps a common failure?

First I thougt I could solve this by simply replace the 6L6 tubes, but now I am unsure if this would do it.

I run this amp with a 4x 12 Engl cabinet which is connected properly. In standby mode, the amp behaves like always.

Any suggestions welcome.

Martin

 

I hope this is the right place for my question.

[RudysBlues]

Hi,

Could be a contact problem on the tube socket. Maybe contact cleaner would help. I would think about servicing the Amp. If you give it to a professional repair shop, make sure you explain the behavior.

 

Cheers

Rudy

 

[WRGKMC]

It looks like a B+ or inverter tube issue.

 

If I have the right schematic Its a fairly simple amp design. I'm not sure about these LED's you're talking about. I see some for channel switching. The one you have may be an enhanced design. The LED's might indicate an over current condition. Since you swapped the tubes, the issue may or may not be tube related.

 

For example, Old tubes can arc when they vibrate and cause components to blow. They may read fine when you moved them but they may still have intermittent faults.

 

What I'd suggest first is inspecting all solder contacts first. If these tube sockets are PC mounted, the tubes heat up the board and weaken solder joints. You may have cracked a solder joint and all it needs is re-soldering. Be careful if you do re-solder. You want to use fresh flux core solder and get it to flow. Its best left to a pro if you've never soldered before.

 

#2. Would be to check those inverter tubes. It looks like they use a pair. if one went out that supplies inversion for two of the tubes, you wont see a change swapping the two sets of power tubes around. This would fit your symptom. I don't suggest you just swap them around. Preamp tubes are either good or bad and when inverters go bad they usually short. Just moving the tube may move the short from one position to another and double the damage. Instead, buy a new 12AX7 and move it from position.

 

The amp only seems to have one Mains fuse, if it blows the entire amp goes dead.

 

I highly doubt cleaning the tube sockets would make a difference either. Most tube sockets use stainless steel clips which dig into the softer metal tube pins. Stainless steel doesn't oxidize, that's why they use it. When you wiggle or remove a tube it scrapes the pins restoring an electrical contact. Since you've already pulled the tubes out, if the pins weren't making a good contact they would have started working just reseating them.

 

If you clean them, you must only use zero residue cleaner, not lubricating pot cleaner. The lubricants put a film on the metal and when the tubes heat up that lubricant cooks making a non conductive film on the metal. The lubricant also attracts dust and can become a path for the Bias voltage to short out. After that your board cooks, changing it from a non conductor to a charcoal conductor. I've done a couple of those repairs and I can assure you, you don't want to go there.

 

Even with the zero residue alcohol cleaners, what might happen is the fluid gets down in a cold solder joint removing oxidation from the lead and making an electrical contact. The amp may work for a short period but it wouldn't be fixed. As soon as the metal oxidizes, it looses contact and you'll be right back were you started. This is if you in fact have a cold solder joint.

 

You could have a blown screen resistors too. This would prevent a pair of tubes working and would not move when you swapped tubes. The resistors connect to the tube socket and should be at least 1W so they are larger then you're typical 1/4 Watt. If they look cooked there's a good chance they are open. you'd have measure or replace them to be sure.

 

I'll also add, many of these faults come from an improper speaker load. I had to find that one out first hand long ago. I tried running a 16 ohm cab on a 4 ohm head. Took a month but those tube would turn cherry red and blow the screen resistors. I went through a half dozen sets of power tubes before I could connect the dots. Lucky it was a Fender amp. If it was a marshall I would have been buying transformers instead of $1 resistors and $10 power tubes.

[onelife]

...

 

If I have the right schematic Its a fairly simple amp design. I'm not sure about these LED's you're talking about. I see some for channel switching. The one you have may be an enhanced design. The LED's might indicate an over current condition. Since you swapped the tubes, the issue may or may not be tube related...

 

The LED's do indeed indicate an over current condition and the activation of a protective circuit that turns off the problematic tube.

 

[ATTACH=CONFIG]n31937943[/ATTACH]

 

Notice on page 1, the circuitry between the cathodes of the power tubes and ground (via Standby switch). There is a relay for each circuit that controls a shunt across a Zener diode.

 

On the lower portion of page 2, near the Standby switch, notice the rest of the circuit including the LEDs in question.

 

I'm not sure about the part of the circuit that does the actual sensing of the current but a closer look at this drawing should be able to provide some clues.

 

 

 

[WRGKMC]

^^^^ The schematic I found was very similar to this but it lacked the protective circuitry.

 

The LED's kick on when he turns on the B+ so the problem is likely high voltage related.

 

The only other test he can make is to pull both the power tubes and inverter, then see if the light comes on when B+ is turned on. If it stay out the problem might be a bad inverter tube. If it still lights he has a bad component that's supplying the B+ or supporting the tubes. Unless the problem is an obvious one like the items I've mentioned, this amp would need to be taken to a tech, properly tested to see what's going on with the high voltage.

[onelife]

Like a Fender amp, the inverter is a dual triode with one side feeding one side of the push pull output stage. The pairing of the output tubes is V1/V2 and V3/V4.

 

Since the V1 and V3 LEDs come on, I don't think the inverter is causing the problem. I haven't looked closely enough at it yet to determine if it could be Bias related but, at this point, that is where my suspicions lie.

 

 

 

 

[WRGKMC]

I wasn't giving much hope on it actually being the inverter. The idea of removing it then applying B+ might help isolate how far back the problem goes. If B+ is applied with the power tubes and inverter removed and he still gets an fault indicator, then the problem is likely in the B+ circuitry. It can be anything from the power supply to power amp and he's probably be better off taking it to a tech for repair.