Hi everyone, I would like to ask for help. I have a Marshall 9200 amplifier with 5881/6L6WGC. I managed to adjust the PI tubes with the oscilloscope, but the thing I really can't do is the bias, unfortunately I noticed that various methods are mentioned on the internet, they talk about shunt method, switch, 1 ohm resistor in series to pin 3, resistor between Pin8 and ground and a thousand other methods, some a 10 ohm resistor, some 100ohm. that it is not known what they are used for and there is no real explanation for each method applied. Up to now I have only accumulated a lot of confusion, I would like to be able to adjust the bias and I know that this amplifier has a trim pot that regulates all 4 finals together, but at the same time be able to move from one final valve to the other with the multimeter. another to see how much difference there is. Since I have other 5881 tubes I might try to match them as much as possible. Up to now I have tried with 4 sockets with a T3 amptata box which intercepted pin 8 of the cathode and measured directly in mA but gave anomalous measurements on both heads such as 52mA at the minimum negative end of the trimmer. Second method tested, soldering 1 ohm 1 w 1% resistors in series to pin 3 of each valve, I connected the multimeter in mV in parallel to the ends of the resistor and I measured 3.4mV but the most absurd thing is that if I keep the leads on the ends of the resistor in parallel and I select the reading in mA in the multimeter in the order of the units I get 0.034mA which could be plausible but the strange thing is that that figure should be in mV not in mA if I'm not mistaken. I would like to understand whoever had this amplifier and what method he used to adjust the bias. Thank you
You have been confused !
First, don't use anything on pin 3, it's dangerous and can kill you. Ever !
The correct method ( imho) is to open up the amp, break the connection to pin 8 ( cathode) and reinstate the connection but this time with a 10ohm 1w resistor. Do this on all 6L6 sockets.
I guess that it is not convenient to measure the voltage on pin 8 with the amp alive, thus install
some connector solder tag or screwstrip connecor with at least 5 connecors. mount it at a place where it's convenient to
connect a voltmeter while the amp is alive. Connect all of the pin 8 to one each if these tag's ( or whatever you installed) plus
the 5 tag connects to ground close to one of the tubes.
This is your future adjustment connector. With the amp alive you can measure the voltage between thr grouns tag
nd each of the cathode's while adjusting the bias. You goal shoud be in the neigborhood of 0.4 to 0.5V.
As it's 4 tubes all adjusted by the sme voltage you need matched tubes ( forget those you have collected )
The advantage with this is that after the initial work you can chack bias in seconds. The tags you installed are safe
and has no dangerous voltages.
The 10 ohm resistors has an additional feature; they will burn out if any tube has a flash-over, and this faster then
any fuse. ( you need to replace that resistor). Don't install larger then 1w and use "flame-proof" ones if you can get them.
First, don't use anything on pin 3, it's dangerous and can kill you. Ever !
The correct method ( imho) is to open up the amp, break the connection to pin 8 ( cathode) and reinstate the connection but this time with a 10ohm 1w resistor. Do this on all 6L6 sockets.
I guess that it is not convenient to measure the voltage on pin 8 with the amp alive, thus install
some connector solder tag or screwstrip connecor with at least 5 connecors. mount it at a place where it's convenient to
connect a voltmeter while the amp is alive. Connect all of the pin 8 to one each if these tag's ( or whatever you installed) plus
the 5 tag connects to ground close to one of the tubes.
This is your future adjustment connector. With the amp alive you can measure the voltage between thr grouns tag
nd each of the cathode's while adjusting the bias. You goal shoud be in the neigborhood of 0.4 to 0.5V.
As it's 4 tubes all adjusted by the sme voltage you need matched tubes ( forget those you have collected )
The advantage with this is that after the initial work you can chack bias in seconds. The tags you installed are safe
and has no dangerous voltages.
The 10 ohm resistors has an additional feature; they will burn out if any tube has a flash-over, and this faster then
any fuse. ( you need to replace that resistor). Don't install larger then 1w and use "flame-proof" ones if you can get them.
If you have a multimeter with a 200mVdc range, you can probably get away with 1-ohm sampling resistors in the cathodes of the output tubes, rather than using 10 ohms, which may lose a bit of power when the amp is playing music. Of course, the loss in the 10-ohm ones could well be insignificant. Big thing is to get the sampling resistors to all have the same value. Buy enough resistors so that you can find a group in them that all measure the same. Sameness of the readings is more important than what the actual value is. Wire-wound is the preferred type of resistor in that application.
in the meantime I had ordered the tad biasmaster 4, it can always be useful to me in the future. I don't quite understand what type of screw connectors you're talking about and these tags, sorry I don't know the acronym. Do you have some images of the objects in question to better understand what it is about. As for multimeters, I have 3, the one with the lowest scale starts at 20mV dc. As far as the formulas are concerned, I'm a beginner, could you kindly explain to me on the basis of choosing a 10 ohm resistor rather than a 1 ohm or 100 ohm resistor in the bias circuit? Thank you
As for the "screw connector" see an barrier strip
10 ohm is one possible value that will make measurment convinient but still safe and will not create losses. A
typical compromise. expecting 0.5V is a range that is easy to measure correct.
100 ohm will reduce available voltage swing in the power tubes with 5V whch starts to be a measurable power loss
1 ohm would also work however the expected voltage would be 50mV which excludes some instruments.
10 ohm is one possible value that will make measurment convinient but still safe and will not create losses. A
typical compromise. expecting 0.5V is a range that is easy to measure correct.
100 ohm will reduce available voltage swing in the power tubes with 5V whch starts to be a measurable power loss
1 ohm would also work however the expected voltage would be 50mV which excludes some instruments.
10 ohms gives you a reasonable voltage to measure accurately and an easy computation. Use Ohm's law to convert the mV reading to current. So a 500mV reading on the meter will correspond to 50mA of current. Measuring the voltage at the 10 ohm resistor for each tube will let you know how well matched they are.
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in the meantime, the tad bias master has arrived to simplify my life without having to continually desolder resistors and cables in a really narrow space. I was partially thinking of solving my problems but the oddities continue. I'm measuring a new quartet of tubes which in theory should be matched. The measured bias is around 39mA at the minimum end of the trim pot. I can't go lower than that, but the BV5 even exchanging with another final it always reads 10mA only the socket in BV5. yet looking at them they all seem to be the same orange color theoretically with 10mA a final shouldn't even be turned on