Engl Screamer 50 screen resistors

[100watt]

I'm looking at an Engl screamer 50 for a mate. I was told the problem is loss of volume/no output. Doesn't matter which channel you use you need the channel and master volume on max just to get a crackly sound that dies away quickly. Checked o/p tubes etc then discovered when I removed the power amp pcb that one of the power tube screen resistors is missing and the other only had one leg attached.Looks like someone has poked around in here before! Is this likely to be the cause of the problem?engl-e330-ii-amplifier-schematic (3).pdf

[pdf64]

Yup. The tubes can't conduct without some screen voltage.
Excessive screen grid current can be caused by a load that's open circuit or a high impedance.
But the tubes may now be toasted if they weren't before, so proceed with caution after replacing the resistors, eg power up via a light bulb limiter.

[Chuck H]

Pretty goofy that someone would reinstall the board with the screen resistors in such a state. How did you "check" the output tubes? Tube testers are no good for establishing suitability for many guitar amp applications. Plugging the tubes into a similar amp is the only way to know if the tubes are ok or not. And as mentioned, no screen grid resistors, no (or very little) sound. I'm only seconding pdf64's post so that I can warn against the possibility of bad power tubes. Bad power tubes often blow up screen grid resistors. A novice may attempt a repair by simply replacing the blown up resistors. Another thing to look out for would be, if it was a power tube failure that put the amp out of service, why did the tube/s fail? This could indicate problems with the bias supply. Perhaps the amp just didn't get plugged into a proper load. It happens. Then some chowder head started a repair, didn't finish that repair, but put the amp back together anyway.?. Do check for proper bias voltage before firing the amp up with tubes installed.

[100watt]

Thanks guys. Checked the tubes in another amp before taking the engl apart. Couldn't understand why the screen resistor was missing and the other unsoldered. I'll replace resistors and give amp thorough check over. Sent the preamp signal to another amp via effects send to check the preamp which was fine.

[drewl]

The missing one may have been bad, bad output tubes can fry the screen grid resistors.

Who knows why people try to "fix" some things.

I have A Marshall that had the two one ohm cathode resistors replaced with a 1.5 Meg and a 10 Meg ohm!

No wonder it doesn't work!

[Chuck H]

Thanks guys. Checked the tubes in another amp before taking the engl apart. Couldn't understand why the screen resistor was missing and the other unsoldered. I'll replace resistors and give amp thorough check over. Sent the preamp signal to another amp via effects send to check the preamp which was fine.

So it's possible that someone replaced the tubes, but never finished repairing the amp. Definitely check for proper bias voltage at both sockets in this case before plugging the tubes in. If a failure was caused by a bias circuit failure the new tubes will be at risk as well once the circuit can conduct.

[pdf64]

Resistors can get so hot that they unsolder themselves and fall out, if that gravity and pcb orientation makes that feasible!

[Chuck H]

Resistors can get so hot that they unsolder themselves and fall out, if that gravity and pcb orientation makes that feasible!

Also pointing to a potential bias circuit failure since both screen resistors were affected.