Author Topic: BIAS Probe (Read 8779 times)

PhilKing · « **Reply #15 on:** May 22, 2008, 05:28:08 PM »

Not thinking there, but Nik did originally tell me that he was going to make it cathode bias (like the 18W is). I guess he changed his mind and went with the standard JTM 45 power design.

martinw · « **Reply #16 on:** May 22, 2008, 05:38:12 PM »

No reason why he couldn't have done that, it's a very easy mod to do.

The only trouble with those external bias points is that you still have to access the inside of the amp to measure Va, unless you assume it stays the same (which it doesn't) when you change the bias voltage.
It kind of works doing that, but it's a bit inaccurate.
That's assuming you knew what Va was in the first place.

Nice amp anyway!

hamfist · « **Reply #17 on:** May 22, 2008, 07:26:27 PM »

Quote from: martinw

The only trouble with those external bias points is that you still have to access the inside of the amp to measure Va, unless you assume it stays the same (which it doesn't) when you change the bias voltage.

Of course you're absolutely right. I forgot about that. So perhaps not as useful as I originally thought.

martinw · « **Reply #18 on:** May 22, 2008, 07:32:07 PM »

OK on something like the Marshall DSL50, where all the variables are known and Marshall publish figures to set the bias to.

PhilKing · « **Reply #19 on:** May 23, 2008, 11:44:38 AM »

Reply from Nik at Ceriatone when I asked if he had decided to go with fixed bias in the JTM45/18 and also if all I needed to do was check the bias on standby using the probe sockets:

Quote

As for the JTM45/18, yes indeed. The cathode bias sounds a bit too woolly/farty driven so I made it fixed.

As for the bias, correct. We already install the 1ohm resistor onto the tube socket (pin 8/1 to ground). You just measure across this resistor, with meter set to DC Volts. Do one at a time.

Your reading will be milivolts. Shoot for 35-40mV.