recommend a multimeter for amp biasing

[WADEAL]

I have a Marshall 20/20 Stereo valve power amp, a 1966 JTM45 original head (no it isn't for sale) and a Laney 100w Pro Tube head. Having looked at afew websites, biasing a fresh set of valves looks straight forward enough, but can anyone recommend a decent multimeter, that is cheap and easy to use for this purpose. I generally don't mess with electrics too much as I quite like not being fried, so it only needs to be something fairly basic, just for this task.

[ef37a]

I have a Marshall 20/20 Stereo valve power amp, a 1966 JTM45 original head (no it isn't for sale) and a Laney 100w Pro Tube head. Having looked at afew websites, biasing a fresh set of valves looks straight forward enough, but can anyone recommend a decent multimeter, that is cheap and easy to use for this purpose. I generally don't mess with electrics too much as I quite like not being fried, so it only needs to be something fairly basic, just for this task.

As you say, you do not need anything very grand for bias checking but do make sure that it has 200mV DC range. Then spend a couple more pounds on some insulated croc to croc leads (or buy the bits and make them. You mentioned your aversion to sudden death so here is a reminder of the rules.
Connect the meter to the amp whilst it is UNPLUGGED from the mains supply.
Double check the connection and have the meter switched to the right range.
Power up the amp and adjust the bias with an insulated screwdriver (a Lekkies one with a 90% covered blade is best)keeping ONE HAND IN POCKET! Let the amp settle for 10mins and re-tweak. Don't try for EXACTLY 25mV whatever, it will drift a bit, leave it a mV under rather than over.

NEVER work on an amp alone, that means a responsible adult within eye and earshot.

Send the kids to the pictures(!)

Lock up any pets.

Do not be intoxicted ("Lesh goan' bias tha' fuuging amp shall we?!)

Dave.

[Nico 3313]

I think ef37a assumes that cathode resistors for measuring bias current are already fitted. For an original 1966 JTM 45 they are certainly not. A cathode resistor of 1 ohm converts every mA of current in the power tubes to 1 mV of voltage over the resistor. If there are no resistors you have to break the cathode to ground connection and connect your multimeter in the mA setting between cathode ground. After checking and adjusting the bias you have to reconnect the cathode to ground again. Fitting the resistors makes bias checking and adjusting so mouch easier and have no effect on normal operation.

[ef37a]

My apologies Nico. I had forgotten that not every amplifier manufacturer does it properly!

It is still possible (and safe but you need to be VERY careful!) to measure anode current on any amp using the output transformer method but I will not go into that here. PM me if any of you want details.

But yes, it is certainly much more convienient to fit a one Ohm resistor in the cathode circuit. In fact you could bring wires out from across the sense R and have them above chassis, no need then to expose high voltage giblets. Even neater to fit a pair of 4mm sockets but not everyone wants to drill holes in their amps!

Note, if you fit a 1R make it a biggy! A 10W component is not at all an overkill, an EL34 can dump over an amp if it goes bang easy!

Dave.