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previous: liam I'd be pretty happy if I could get ... -- 3/13/1998 6:01 AM

Re: Biasing Marshalls

3/13/1998 10:04 AM
Jim S.	Re: Biasing Marshalls The first thing you should do is make sure there really is a problem with the bias adjustment pot or circuit. If there is, then you should fix it. Find out what range of negative bias voltages you can achieve. It should span at least 10 or 15 volts (for example -30 to -42 volts). Measure this right at pin 5 of the output tube sockets. Once you get the bias adjust issue resolved, you can go back to the task of setting the bias for the set of EL34's you have installed. I STRONGLY recommend that you do it by measuring the amount of current drawn through the cathode, either by installing 1 ohm resistors between cathode and ground or by using a "bias probe" type device which consists of an adapter (or set of adapters) that plugs in between the tube socket and the tube. Even if you ultimately wish to use a scope to help find your ideal bias point, you really need to know how much current is going through those tubes so that you have a reality check. The trouble with using the scope method alone is that you're not measuring anything! Being a tech, I like to measure things - it gives me a warm feeling that I know what's going on with the circuit and the tubes. Good luck!

3/13/1998 10:04 AM

Jim S.

Re: Biasing Marshalls

The first thing you should do is make sure there really is a problem with the bias adjustment pot or circuit. If there is, then you should fix it. Find out what range of negative bias voltages you can achieve. It should span at least 10 or 15 volts (for example -30 to -42 volts). Measure this right at pin 5 of the output tube sockets.

Once you get the bias adjust issue resolved, you can go back to the task of setting the bias for the set of EL34's you have installed. I STRONGLY recommend that you do it by measuring the amount of current drawn through the cathode, either by installing 1 ohm resistors between cathode and ground or by using a "bias probe" type device which consists of an adapter (or set of adapters) that plugs in between the tube socket and the tube. Even if you ultimately wish to use a scope to help find your ideal bias point, you really need to know how much current is going through those tubes so that you have a reality check. The trouble with using the scope method alone is that you're not measuring anything! Being a tech, I like to measure things - it gives me a warm feeling that I know what's going on with the circuit and the tubes.

Good luck!

Replies:

Derrick I strongly agree!! I use the outpu... -- 3/13/1998 10:33 AM
Bruce I think Jim also ment to imply that... -- 3/13/1998 9:03 PM
liam What bias current -- 3/16/1998 4:44 AM