Engl Powerball II: what are these "mods"? Why can't I bias it?

[J M Fahey]

Me too, since we canīt match PCB to Schematic

[g1]

What strikes me the most in that amp is the function of the bridge rectifier and these resistors.

I think you are correct about it being for DC heaters. One leg of the bridge is going to pins 4&5 of preamp tube.

[J M Fahey]

Sharp eyes, huh?

In which case resistors might be the artificial balanced ground .

Well, in that case, it does NOT have any influence on sound; except, hopefully, less hum.

It should have a, say, 4700uF x 16 or 25V capacitor somewhere ... where is it?

[Roberto]

Hi, I confirm you that the heaters of (the first two only, IIRC) preamp tubes are supplied in DC.

[xtian]

I have an ENGL Powerball II on the bench—one power tube has a cathode-to-heater short, so I have pulled all power tubes out for testing. Inside, I see four screen resistors with value 150 ohms. All four measure shorted (~0.1 ohms). The only schematic I can find online shows 470 ohm screen resistors. I am trying to contact ENGL for a schematic.

Also, where is a convenient point to check power tube current? All I can see is the solder-side of the PCB and no component labels are visible. Measuring resistance from pin 8 to ground gives ~10M--maybe this is because of the relays shown on the cathodes, and this can only be measured during operation?



I can resort to measuring resistance and then voltage drop across the OT primaries.

[xtian]

Oh, crap. The 470R screen resistors are hidden UNDER the PCB. Those 150R resistors are something else--can't find them on the schematic.

[xtian]

Measuring resistance from pin 8 to ground gives ~10M--maybe this is because of the relays shown on the cathodes, and this can only be measured during operation?

Confirmed. Only when powered up and when Standby is in "run" position do the cathode relays engage, and only then do I see ~4R7 resistance between cathode and ground.

[xtian]

OK, I'll wrap up my little session. New power tubes, biased by observing ~130mV from cathode to ground (divide by 4R7 resistor = ~27.7mA per tube). Amp working and sounding good.