trying to bias laney protube50

[seaker]

I've aquired a Laney AOR Protube 50 that I've been tuning up. I'm trying to set the bias on the new power tubes and either there is a circuit problem or I must be doing something wrong. Probably the later ;-]. 2 el34 class AB, basic 50 watt marshall style power stage. When I read up on how to bias the thing I read that I can use the “OT Shunt Method, put an ammeter across ½ output and measure current, adjust bias as needed.” So I look at the schematic... and where do I hook up the meter? Across the Plate to the Cathode? I tried this and got no current... well I got 1 microamp. hmmmmm... So I also read that I can install 1ohm resistors between cathode and ground and measure the current that way ( volts across resistor).Well that is a quick solder job away... let me try that, dag no resistor in my bin, well I got the thing open so I put my meter between the cathode and ground, now it should carry all idle current right? That is, all the current that would have generated the volts across the resistor that I would have measured to calculate the idle current....still nothing... weird. Tubes are warm and I can hear preamp hiss from speaker so power stage is working somehow....I measure the bias voltage. I can adjust it from -35 to about -40 volts by twiddling the bias pot on the board. Plate voltage is up about +400v. I monitor cathode current while I adjust bias and nothing changes. Now the kids are yelling upstairs and I'm flummoxed, better power down and think about this before I blow something up. And yes the damn thing was working before started messing with it ;-]

so the possibilities are
1: I am an idiot
2: On my first pass trying to use 'OT shunt method' i hooked things up wrong.
3: but even so when i lift the cathode ground and put an milli-ammeter in between, I should get something right? A series circuit is a series circuit. Idle current HAS to pass from cathode to plate, so I should see something with my meters. Unless...
4: I'm working with a combination of brain farts and bad meters. I tried both a DMM and a VOM.

When I go home tonight I'll test all the meters and try again. Any ideas?

TIA
Justin Harrison

[Tom Phillips]

So I look at the schematic... and where do I hook up the meter? Across the Plate to the Cathode? I tried this and got no current... well I got 1 microamp. hmmmmm...

When you connected the current meter as you described you shorted the high voltage to ground. I suspect that you:

• Blew a fuse in the amp
• Blew a fuse in the meter
• Toasted the meter
• Damaged something in the amp like the OT (I think this is less likely than 1,2 or 3)

So now it’s time to check the amp and the meter and determine what still works.

To measure current with the shunt method the current meter is put across each half of the OT primary winding one half at a time. That’s between the center tap and the plate lead. Convenient places to connect depend on the amp model being tested and I don’t recommend this method unless you really know what you are doing. Adding the 1 Ohm resistors should work well for you but you still need to be comfortable and careful working around live high voltage circuits.

Regards,
Tom

[MWJB]

The OT shunt method of determining bias is relatively dangerous! It should ONLY be attempted if you are going to use clips/wire grabbers to attach the meter and if you are going to be VERY, VERY careful (otherwise you can easily get a short...plate to heater shorts are quite spectacular...flames, smoke & all that, and can be costly!). Make all connections/disconnections with the amp unplugged from the wall.

Yes, you hooked things up wrong. Your red lead goes to the main B+ supply (OT CT/1st filter cap), black to the power tube plate that you want to measure.

Disconnecting your power tube cathode from ground turns the tube off. Your meter may have an impedance of 10's of megaohms which the tube may see as an open circuit.

I'd wait until you get some 1ohm cathode resistors and install them as this is a far safer way to read your bias. Even better, buy some bias probes then you can bias any amp with an externally accessible bias pot without dropping the chassis.

[MWJB]

As Tom says, by trying to measure the current from plate to ground, with your meter set to mA, you will most likely have blown the mA fuse in the meter.

[seaker]

[QUOTE=Tom Phillips;11983]When you connected the current meter as you described you shorted the high voltage to ground.

duuhhhhh.... of course I see that now

probably blew the fuse in the meter, I still had approx +400volts on the plate after my futile attempt at 'OT shunt method'


To measure current with the shunt method the current meter is put across each half of the OT primary winding one half at a time. That’s between the center tap and the plate lead..


somehow that's what I thought Iwas doing.... as I said brain farts

you still need to be comfortable and careful working around live high voltage circuits.

maybe not comfortable yet, but yes very carefull, soemtimes you just gotta try it. There's always a first time. I have soem electronics backgound but mostly small signal analogue and digital. Been trying absorb the tube stuff as well as I can

thanks
Justin

[seaker]

The OT shunt method of determining bias is relatively dangerous! It should ONLY be attempted if you are going to use clips/wire grabbers to attach the meter and if you are going to be VERY, VERY careful (otherwise you can easily get a short...plate to heater shorts are quite spectacular...flames, smoke & all that, and can be costly!). Make all connections/disconnections with the amp unplugged from the wall.

Yes, you hooked things up wrong. Your red lead goes to the main B+ supply (OT CT/1st filter cap), black to the power tube plate that you want to measure.

Thanks that clears things up.

And yes I was very carefull about the hookups as you wrote, thanks for the warning

/justin