Rebiasing a Marshall JTM 60

[emento]

Can some one help me out? I'm a bit confused.

This is what I know.
Trying to bias a 1997 Marshall JTM 60 using a Bias King which measures cathode current.
Amp uses EL34 tubes...I'm using Svetlana EL34s.
I got them from www.thetubestore.com. They rate the tubes, "Our Perfect Pair system is the most accurate because we use the actual measurement of the tube’s plate current for the numbering system."
My tubes have a plate current rating of 43.

Maximum DC plate voltage is 800V
Maximum Screen Voltage is 500V
Maximum Control Voltage is -100V
Max DC cathode current is 150mA
Max Plate dissipation is 25W
Max Screen dissipation 8W

All these figures and terms have left me confused. Where does plate voltage come into this? How do I figure out plate voltage?
And then what's my cathode current range?

Please help me!

[John Phillips]

You don't sound like you really know what you're doing (yet), and some of these measurements involve very dangerous voltages! Be VERY careful! You will have to work with the chassis out and the amp powered up.


You can ignore those maximum ratings, since they will not be reached in this amp - except for the maximum plate dissipation, which is where biasing comes in.

You need to measure the amp's plate voltage. This is easiest done by measuring the voltage from pin 3 to ground on one of the power tube sockets. You will need a voltmeter with at least 500V DC rating. Make sure that the test leads are rated for this too, and well insulated. It's best if the ground lead has a clip on it, so you can work 'one hand' which is inherently safer than using both.

Clip the ground lead to some point on the metal chassis and touch the positive probe onto pin 3 - the amp must be fully on BTW, not on standby. Be very careful not to let the probe slip - you could seriously damage the amp or yourself. Try to work only holding the probe - keep your other hand well away from the amp. Don't worry if you don't know which is pin 3 - simply go round all eight until you find the one with the highest voltage (it will certainly be in the range of 400 to 500V on this amp). Write it down somewhere, since you don't really ever need to do this more than once, not in an amp with a solid-state rectifier anyway, like this one.

Let's assume for now that you read the plate voltage at 450V.

Now... current.

In a Class AB amp, the maximum you should set the plate dissipation at is 70% of the tube rating - so in this case 17.5W. This equals the plate voltage multiplied by the tube current, so the current should be no greater than 17.5/450 = 0.039 amps, or 39mA. This is the upper limit, the amp will work well (and the tubes will last longer) set somewhat below this too. 30 to 35mA is probably the best range - there isn't really a precise 'right' value.

(You can effectively ignore the screen current too, since it's quite small and not accounting for it will err on the safe side anyway).

Put the Bias King into the socket and the tube into it (put the amp on standby to do this). Set your meter to read mV (millivolts) - I'm assuming this model is one with a resistor and is designed to read volts, not one which is designed to read current directly which are a potential hazard. Turn the amp back on and you will get a current reading.

You can experiment with different bias settings to see whether you like the sounds, as long as you stay below the maximum (use the measured voltage value from your amp to calculate it of course). If you set it too low, you'll probably get crossover distortion, which sounds bad but won't do any harm. But there's normally a fair range in the middle.

Personally, I like to bias amps a little cooler (ie lower current) than has become popular recently - I find it gives a clearer, more defined sound, and certainly improves tube life, but it's up to you.

[emento]

Thanks John!
I really appreciate it!