How to measure and adjust bias

ksporry · 18th February 2010, 12:11 PM

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?

TheGimp · 12:49 PM

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.

rotaspec · 18th February 2010, 06:58 PM

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

ksporry · 18th February 2010, 08:53 PM

Gary, so what you are saying that the Mengs are auto biased and therefore don't need adjusting?

rotaspec · 18th February 2010, 09:30 PM

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

18th February 2010, 12:11 PM	#1
ksporry diyAudio Member Join Date: Jan 2010 Location: Shanghai	How to measure and adjust bias 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?

18th February 2010, 12:43 PM	#2
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	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. Last edited by TheGimp; 18th February 2010 at 12:49 PM. Reason: Safety Warning

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Way to adjust Acoustat Bias Voltage?	croweproductions	Planars & Exotics	1	9th November 2009 11:12 PM
How To Adjust Bias Current?	Bushulo	Pass Labs	1	14th July 2008 02:11 PM
Independent Bias Adjust for Each Tube?	TubeHead Johnny	Tubes / Valves	4	24th February 2008 12:57 PM
How To adjust Adcom 565 Bias	HiFiMonk	Solid State	1	25th November 2005 12:09 AM
How do I adjust amp bias?	fotzepolitic	Solid State	29	22nd January 2004 05:09 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

18th February 2010, 06:58 PM	#3
rotaspec diyAudio Member Join Date: Nov 2007	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

18th February 2010, 08:53 PM	#4
ksporry diyAudio Member Join Date: Jan 2010 Location: Shanghai	Gary, so what you are saying that the Mengs are auto biased and therefore don't need adjusting?

18th February 2010, 09:30 PM	#5
rotaspec diyAudio Member Join Date: Nov 2007	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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)