Enzo is right. As usual.
Build or buy a conventional bias probe, plug it into your DVM, plug the tube into it and be done.
PS: Good to see you in these quarters, Enzo.
Build or buy a conventional bias probe, plug it into your DVM, plug the tube into it and be done.
PS: Good to see you in these quarters, Enzo.
I like the bias tool, didn't know it existed. And at ~$35 on ebay that's worth all the hassle. Thanks Enzo....
You should open up your own, Enzo's Electronics Forum.
You should open up your own, Enzo's Electronics Forum.
My experience is fairly specific, involving maintenance and repair and troubleshooting. I look up to the engineering types and their far more advanced knowledge of theory.
This web site is more oriented towards hifi than guitar amps.
You are welcome to join us over at Music Electronics Forum which is oriented towards instrument amps mostly.
This web site is more oriented towards hifi than guitar amps.
You are welcome to join us over at Music Electronics Forum which is oriented towards instrument amps mostly.
Hi Daniel,
Ok, you want to check the quiescent current flowing in the output valves (tubes) of your amp to see if it's correctly biased, and your amp uses fixed bias, not cathode bias.
Either break into the anode circuit and put your meter across the break - the meter is in series with the anode; or, read the voltage across a known resistor that is part of that circuit.
I'll talk here about only the voltage route, use "I=V/R" to calculate current flowing.
You can install 1 ohm resistors in series with the cathodes of the valves. This point of the circuit is nearly always at low voltage wrt ground, so it's safer, especially if you're not very experienced. 1 ohm is perfectly practical, it has virtually zero impact on the amp, it's easy to figure the current flowing - a reading of, for instance 60mV means you've got 60mA flowing. The disadvantage of this method is that the current you read includes the screen grid current, so your reading is a bit inflated.
The other place to read a voltage doesn't need a resistor installing, you use the resistance of the primary winding of the output transformer. With the amp off, measure the resistance of the two legs of the primary. Note which leg is connected to which valve(s). They will differ slightly. Using well insulated leads, connect your meter to the centre tap of the primary, and one of the anodes. If you have two meters, connect one across the "push" winding, the other across the "pull" winding.
Using two meters does give you a check that the two halves of the output stage are well balanced at least as far as bias is concerned.
(On an amp with four output valves, you see two valves' anode currents flowing in each winding).
You can now monitor current flowing in each of the two windings by reading the voltage across that winding's resistance, then "I=V/R".
BE WARNED the meters are floating way above ground at the anode voltages, so if you do it this way, be careful.
Ok, you want to check the quiescent current flowing in the output valves (tubes) of your amp to see if it's correctly biased, and your amp uses fixed bias, not cathode bias.
Either break into the anode circuit and put your meter across the break - the meter is in series with the anode; or, read the voltage across a known resistor that is part of that circuit.
I'll talk here about only the voltage route, use "I=V/R" to calculate current flowing.
You can install 1 ohm resistors in series with the cathodes of the valves. This point of the circuit is nearly always at low voltage wrt ground, so it's safer, especially if you're not very experienced. 1 ohm is perfectly practical, it has virtually zero impact on the amp, it's easy to figure the current flowing - a reading of, for instance 60mV means you've got 60mA flowing. The disadvantage of this method is that the current you read includes the screen grid current, so your reading is a bit inflated.
The other place to read a voltage doesn't need a resistor installing, you use the resistance of the primary winding of the output transformer. With the amp off, measure the resistance of the two legs of the primary. Note which leg is connected to which valve(s). They will differ slightly. Using well insulated leads, connect your meter to the centre tap of the primary, and one of the anodes. If you have two meters, connect one across the "push" winding, the other across the "pull" winding.
Using two meters does give you a check that the two halves of the output stage are well balanced at least as far as bias is concerned.
(On an amp with four output valves, you see two valves' anode currents flowing in each winding).
You can now monitor current flowing in each of the two windings by reading the voltage across that winding's resistance, then "I=V/R".
BE WARNED the meters are floating way above ground at the anode voltages, so if you do it this way, be careful.
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I would always use the series resistor method.
If you inadvertently break the connection with your meter connected in series anything could happen.
If you inadvertently break the connection with your meter connected in series anything could happen.
I'm not sure that anyone here is recommending series connection of a meter - I mention it as a possibility only.
The method that I describe does not entail a series connection. It's a parallel connection across the impedance of the halves of the primary winding.
It's an unfortunate fact that whilst monitoring current in the cathode circuit might be safer, it always includes screen grid current and is thus not as accurate.
What's really needed is a change to amplifier front panels and power switching, to incorporate a signal-mute-&-enable-anode-current-meters. Along with anode temperature gauges of course.
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