Engl Fireball Trouble

[chunkitup]

At practice tonight I heard a small pop as the Engl V1 power tube monitor came on indicating the tube was off or not working correctly. The sound greatly diminished and I tried several times turning the stand by on and off (several seconds apart) only to get the small pop and the monitor light illuminating. Most power tubes go slowly and sound bad in the end but this amp sounded good til the last song then pop. I am not familiar with the Engl tube monitor. What brings the light on and shuts it off?
Just so I get this right from the beginning, what should my first step be? I have built several amps and I feel confident in doing anything needed to be done. Try a different tube? Retube and rebias the whole amp? Open up and start metering? Fire up the o-scope?

[Amp Kat]

Change the power tubes first.

[Enzo]

Exactly. WHo says power tubes fail slowly? They wear out and lose their tone slowly, yes, but that is not failing. In my experience they fail about as slowly as light bulbs. SOme have the decency to pop and crackle a bit before they go altogether.

On any amp, when the power tube failure indicator comes on, replacing the power tube should be the first step.

[Robert M. Martinelli]

Engl "tube monitor" operating mode

I am not familiar with the Engl tube monitor. What brings the light on and shuts it off?
Just so I get this right from the beginning, what should my first step be? I have built several amps and I feel confident in doing anything needed to be done. Try a different tube? Retube and rebias the whole amp? Open up and start metering? Fire up the o-scope?

Hi,

My first advice is exactly the same Enzo and AmpKat gave, start replacing the power tubes;

To answer your question about "what brings the light on and shuts the tube(s) off" - The fireball tube monitor works as follows ( if memory serves me well ) - first of all, the system monitors the output tubes only after the st-by switch has been switched to the "on" position - the 4 cathodes are connected to GND via a 4.7 Ohm resistor, so normally a voltage drop is present across these resistors - this voltage is brought to voltage comparators ( LM324s ) and these voltage comparators operate both LEDs and relays, the relays lift the cathodes' potential to 150 VDC by means of zener diodes, inhibiting current through the failed tube - it' s important to note that if a single tube fails, the system shuts it off together with its "companion"
( remember we' re dealing with a push-pull ), and this explains the audible volume loss you experimented - that' s because you only have two tubes working so you' re at half power. If this was not the case, the "companion tube" could have been affected by its failing mate.

Hope I' ve managed to be clear enough.

Best regards

Bob

[chunkitup]

Ok I finally got to work the amp a bit after work today. I thought I would just try firing it up with the same two tubes in. The amp worked fine. Now the situation in which the tube failed was under loud conditions. I bought the amp and had not gotten the MV past 10oclock(3-4). Last night at band practice I had the MV at 12 high(5) for about 10-15 minutes before pop. Now what would this tell me? I did only turn the amp on today at about (3) for a short while.