Guys, perhaps a sticky could be generated for how to measure and adjust bias settings on amplifiers.
But for now, to get to my personal question:
I acquired a Meng X1 amp, which uses 8 6P1's in the end stage (rather than teh 4 6p1's on the Meng Yue Mini).
At some point i will want to measure, and potentially adjust the bias settings for the tubes. Unfortunately i do not have any schematics, which makes working on this unit more difficult.
I would imagine that on my amp I have to adjust the bias on 8 tubes rather than 4?
Could someone tell me how I physically measure the bias on a 6P1 tube? Would I have to break the circuit or is there another way to determine the current (I believe you adjust current for bias, rather than voltage?), by e.g. measuring the voltage over the bias resistor and calculate the current using the measured voltage and the resistor value? This last method would be easily possible as i don't have to break circuits on a circuit board (and then fix it again). Which pin on the 6P1 would be biased, and what current should I aim for?
But for now, to get to my personal question:
I acquired a Meng X1 amp, which uses 8 6P1's in the end stage (rather than teh 4 6p1's on the Meng Yue Mini).
At some point i will want to measure, and potentially adjust the bias settings for the tubes. Unfortunately i do not have any schematics, which makes working on this unit more difficult.
I would imagine that on my amp I have to adjust the bias on 8 tubes rather than 4?
Could someone tell me how I physically measure the bias on a 6P1 tube? Would I have to break the circuit or is there another way to determine the current (I believe you adjust current for bias, rather than voltage?), by e.g. measuring the voltage over the bias resistor and calculate the current using the measured voltage and the resistor value? This last method would be easily possible as i don't have to break circuits on a circuit board (and then fix it again). Which pin on the 6P1 would be biased, and what current should I aim for?
The issue with adjusting the bias is that you need a mechanism to adjust the bias with, such as a potetiometer (for each tube).
If there are not 8 Pots to adjust the bias, you would have to modify the circuit to make the adjustments.
One could measure the voltage (DC with no input signal) across each cathode resistor, and divide each voltage by each the cathode resistor value to get the current for each tube. This is a rather tedious process as you also have to measure and record each resistor value with no power applied to the amp, (and being sure there is no charge left on the capacitors!).
You have not indiated what your experience level is with regards to tubes and high voltage. Please be aware that potentially deadly voltages and currents are present in Valve/Tube Amplifiers.
There are sticky threads about tube basics and safety that should be studied before proceeding with any measurements.
If there are not 8 Pots to adjust the bias, you would have to modify the circuit to make the adjustments.
One could measure the voltage (DC with no input signal) across each cathode resistor, and divide each voltage by each the cathode resistor value to get the current for each tube. This is a rather tedious process as you also have to measure and record each resistor value with no power applied to the amp, (and being sure there is no charge left on the capacitors!).
You have not indiated what your experience level is with regards to tubes and high voltage. Please be aware that potentially deadly voltages and currents are present in Valve/Tube Amplifiers.
There are sticky threads about tube basics and safety that should be studied before proceeding with any measurements.
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Bias setting is generally only changed on designs that use a separate bias voltage supply, and have a pot provided for each tube to be biased.
In the case of the Mengs and most small-tubed amps, biasing is automatic by virtue of the cathode and grid resistors and unless you want to redesign the bias circuit, you cannot alter it. Nor should you need to unless something is clearly wrong (red plates in the output tubes etc)
Read up on cathode biasing, as that is the norm for small amps.
Gary
In the case of the Mengs and most small-tubed amps, biasing is automatic by virtue of the cathode and grid resistors and unless you want to redesign the bias circuit, you cannot alter it. Nor should you need to unless something is clearly wrong (red plates in the output tubes etc)
Read up on cathode biasing, as that is the norm for small amps.
Gary
I can't speak for the new Mengs, but the Mini has/needs no adjustment.
If you were reading my thread about the Meng schematic, and saw us playing with bias values, it wasn't implied that they were needed. We were just experimenting to see what improvements could be made to these amps.
Most amps with small tubes like these have no need for variable bias setups. It's generally reserved for larger ones like the KT88, EL34 etc.
I very much doubt that any of the 6P1 or EL84 designs have bias adjustments from the factory.
Gary
If you were reading my thread about the Meng schematic, and saw us playing with bias values, it wasn't implied that they were needed. We were just experimenting to see what improvements could be made to these amps.
Most amps with small tubes like these have no need for variable bias setups. It's generally reserved for larger ones like the KT88, EL34 etc.
I very much doubt that any of the 6P1 or EL84 designs have bias adjustments from the factory.
Gary
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