What is my bias probe actually measuring?...

rawk100

I've had a few marshalls over the years and always tend to solder a 1 ohm resistor from pin 8 to ground on the output valves so I can measure the cathode current for biasing. I've got a JCM900 SLX at the moment and decided to buy a bias probe from China via eBay rather than soldering in a resistor. It's your typical probe which fits between the valve base and the valve and has two flying leads which connect to the meter, it didn't come with any instructions. I always thought these probes had a built in 1 ohm resistor that allowed you to measure the cathode current. First of all I checked the voltage from pin 3 with my meter and it read 429Vdc. I then took a reading from the probe on the voltage scale and it read about 26vdc.....! I tried my meter on the 400mA current setting and it read zero. So what does this 26vdc represent cause I'm confused, I guess I was expecting to read about 40mV based on the 1 ohm resistor method?

ecc83

Cluck knows! What the Chinese probe is telling you. But if you have had the smarts in the past to fit 1R sense resistors, why bother with one now?

Then, a probe only measures one valve at a time. You have the hassle of changing it around, unless you buy two (four!) . Much better to fit a common sense R (and don't cheap out here, fir a 10W jobbie!). After all, you have to get the fecking chassis out to tweak the pre set!

Measure the resistance from pin 8 top of probe to pin 8 at the pin, should be 1R AFAIK?

Dave.

ICBM

No idea what the 26V is - it's too low to be the actual bias voltage, that should be in the high 30s to very low 40s for an EL34 Marshall.

40mA is too hot for the correct bias current by the way, you need more like 30-35mA depending on plate voltage.

rawk100

I've just posted this in another thread but....
it seems that the probe I got from China doesn't have a 1ohm resistor in it. One wire is connected to the socket side of the probe (where the valve goes) on pin 8 and the other wire is connected to the pin of the probe (that pushes into the valve base) on pin 8. The pin and socket on pin 8 of the probe aren't connected. So all it does is divert the volts off pin 8 through your meter. My meter doesn't have an internal resistor so the probe isn't much good.

ICBM

Ah, so the 26V is the residual voltage leaking past the valve.

It sounds like it should still measure the current directly. Not sure why your meter isn't reading that when set to mA - could the meter's fuse be blown?

It's not a good idea to do that anyway though, since feeding the cathode current out of the amp can cause instability which will at best give a spurious reading.

If you can open up the socket unit you could easily bridge the connections with a 1-ohm resistor.

rawk100

ICBM said:

Ah, so the 26V is the residual voltage leaking past the valve.

It sounds like it should still measure the current directly. Not sure why your meter isn't reading that when set to mA - could the meter's fuse be blown?

It's not a good idea to do that anyway though, since feeding the cathode current out of the amp can cause instability which will at best give a spurious reading.

If you can open up the socket unit you could easily bridge the connections with a 1-ohm resistor.

yeah I may try and open up the probe and fit a resistor. I was thinking though.....on the actual valve base in the amp the earth wire actually connects to pin 1 and then there is a small wire bridging across from pin 1 to pin 8. Usually I break the wire between pin 1 and pin 8 and then connect one end of the resistor to pin 8 and then bolt the other end of the resistor to ground on the chassis, the original earth wire is still connected to pin 1. Couldn't I just solder the 1 ohm resistor across pins 1 and 8 on the base after removing the original wire link?

ICBM

rawk100 said:

yeah I may try and open up the probe and fit a resistor. I was thinking though.....on the actual valve base in the amp the earth wire actually connects to pin 1 and then there is a small wire bridging across from pin 1 to pin 8. Usually I break the wire between pin 1 and pin 8 and then connect one end of the resistor to pin 8 and then bolt the other end of the resistor to ground on the chassis, the original earth wire is still connected to pin 1. Couldn't I just solder the 1 ohm resistor across pins 1 and 8 on the base after removing the original wire link?

Yes, if that's easier than opening up the socket adaptor.