DBT and the PLWO's...

[jbeckva]

Ok, so this is somethin' botherin me about doing a DBT with the PL's the way they are. I think there's a lil bit of truth here.. I observed it the first time around...

Is the DBT really REALLY going to save ya from a good zap or two?? Sure.. it'll limit the current coming in the way it's been determined to do, but as those two big ole cans (caps) charge up, they are still going to reach higher and higher voltages, right? So.. say your DCP (for example) on the other end is firing into a dead short, but you don't see it until you've brought up the input VAC to a sufficient level to kick it over. BUT.. by that time, those caps have what.. about 60 volts or so stored up... whatever is going to cross that line? So your DCP kicks in, and ya fire the outputs right into a short.. whack.. boom... pow. There is plenty of energy already stored up in those caps to do damage, right?

Yeah, I guess it'll still save you from shorted bridge rectifiers and caps.. so I do plan on continuing use of a DBT here.. heheh

(just some musings here as I too get close to the "point of no return" i.e. plugging me badboy PLWO into a DBT .. AND Variac... )

 

[laatsch55]

No Jer probably not. But it will tell you in that antsy first moment of the new creation that at least it's not gonna blow up from being plugged in.

 

[mlucitt]

Jer,
I have used the DBT with a shorted output. The bulb glowed brightly, brighter than normal. I use a 100 Watt bulb and it got lit up sooner than normal too, I would say a glow at 20-30 volts and bright at 50-60 volts.

Also, the bulb limits the current that is available to the amp so in theory, you will do less damage when powering up through the DBT. The power supplies in these amps are robust. I have not seen a transformer blow yet, maybe Lee has seen one. He deals in Kilowatts of power and AMPS in the double digits that would leave tracks in my carpet where the power cords were laying.

Here is the drill. No inputs connected, no outputs connected. DBT between the Variac output and the amplifier. VARIAC set to 0 Volts. Pull the DC power fuses (two on the PL 400 and four on the PL 700). Plug in the amp and switch it ON (if it has a switch). Power up the VARIAC and bring the VARIAC voltage up very slowly. The bulb should stay off or very dim until the PS/amplifier "wakes up" at about 30 Volts, the DBT will suddenly glow and then quickly dim because of the power surge. As the caps begin to charge, the lamp will glow a bit more. Continue to bring the voltage up and notice that the amp is full "on" at about 80 Volts and the DBT should be steady and dim as the current flow to charge the PS caps is pretty constant. Continue to increase the voltage to 120 Volts. The BDT should be very dim or even out once the caps fully charge (depending on the wattage of your bulb). Note that the amplifier will not be at the full amperage or voltage coming from your power panel because of the DBT, which is OK at this point.

If this test is OK then proceed.

Reduce the VARIAC power to 0, switch off the amplifier, and unplug it from the DBT. Let the amplifier rest for 30 minutes or short out each of the PS caps with a resistor and clip leads across the caps AND to ground. Reinstall the proper DC power fuses and repeat the test above.

If your DBT comes on brightly before the amp "wakes up" there is a short somewhere. Shut it off and look at the bridge rectifier wiring, fuse wiring, or transformer wiring. If the DBT suddenly decides to get bright between 30 and 80 Volts there is a problem on the main board. If the DBT glows bright above 80 Volts, I would look for rail voltage on the outputs (normally Left, I think) or check the output devices / output clamp diodes (1N4004).

Good Luck.

 

[laatsch55]

I don't think my breaker panel would support a transformer blowup. No Mark , never have. Ed Blackwood tells me this is a very RARE occurance.

 

[mlucitt]

That makes it just about universal. I think we could weld cold steel with one of the PL transformers, if we could get the voltage down to about 12VDC.

 

[laatsch55]

Anything less than 40, probably.