Could someone explain how biasing tubes works

[McLaughysSN]

Im in a very tight spot, my sovtek mig-60 has shot power tubes and so its time i replace power tubes and pre-amp tubes. my situation is that the local repair guy is backed up a full 6 months! i need the amp as soon as possible. now im not saying that i will do this myself but i would like someone to explain exactly what it is and possibly how to do it so i have a further understanding.

thanks a lot

[STRATDELUXER97]

Im in a very tight spot, my sovtek mig-60 has shot power tubes and so its time i replace power tubes and pre-amp tubes. my situation is that the local repair guy is backed up a full 6 months! i need the amp as soon as possible. now im not saying that i will do this myself but i would like someone to explain exactly what it is and possibly how to do it so i have a further understanding.

thanks a lot

I don't have one,but maybe a fellow forum brother has one of those tube bias probes...You plug the probe into where the power tube goes and then the tube plugs into the bias probe....Providing your amp has a bias pot,you adjust your bias pot to meet certain requirements that are based on plate voltage and power tube type...

If you are handy with a soldering iron,you can solder a 1% 2 WATT resistor between pin 8(The cathode) of your power tube sockets and ground...You need to know what you're doing though or you can get hurt...Once the resistor is in place,you measure in MV across the resistor and again set to a certain value....I do have some photos on how to install the cathode resistors,but I don't want to see you get hurt or ruin your amp in the process..You also need a meter that reads in millivolts between say about 30 to 40 millivolts...

John

[Thames]

Aah.. those great old russian beasts!!

Yeah. Stratdlxer is right about the resistors, but you must connect both PIN1 and PIN8 together, then to the resistor, then to ground. However, the MIG 60 already has two 3.9 ohms resistors placed there. You can already read the mV/mA thru these resistors. You must set the bias until you read ~80mA each.

[Chaos]

OK, this won’t help you bias your amplifier, but if you want to know what biasing does…
A bit of simplified tube theory:

The filament of a tube is heated by applying a voltage to the filament (like a light bulb).
The added energy liberates electrons from the filament forming an ‘electron cloud’ around the filament.
Electrons (which are negatively charged) are repelled by the tube’s cathode (which is also negatively charged) and attracted by the tube’s anode (positively charged) Opposites attract - Likes repel.

In order to control the amount of electrons flowing to the anode, a grid is added to the tube. The grid acts like a valve that opens and closes to control the flow (wonder why the Brits call tubes valves?). This is accomplished by placing the grid between the filament and the anode and applying a negative voltage to the grid.

The negative charged grid is much closer to the filament than the anode so a small negative voltage can overcome the attraction of the positively charged anode.

If you apply an audio signal (guitar output) to the grid, the voltage will vary up and down a little causing the ‘valve’ to open and close a little. The current flow across the tube will then flow in direct relation to the audio signal. This enables you to control a large amount of current with a very small voltage.

In order for this to all work out, the grid voltage needs to be set at a point where it has optimum control over current flow without exceeding the tube's dissipation limit (so that the slightest change in voltage on the grid will have the biggest affect). Setting this voltage is what you do when you bias a tube…

[McLaughysSN]

thanks a lot guys

i did find a shop about 50 miles away that has a week turn around and is very helpful (always was my faveorite shop) i just have to save the money for the job and the gas

[Neeradj]

I don't have one,but maybe a fellow forum brother has one of those tube bias probes...You plug the probe into where the power tube goes and then the tube plugs into the bias probe....Providing your amp has a bias pot,you adjust your bias pot to meet certain requirements that are based on plate voltage and power tube type...

Wow that sounds so incredibly simple, why are people always making a fuss about biasing by themselfs and why do amptechs ask so much money for such a simple job?

[Thames]

Hey look what I just found !!



Shows exactly what I said... 3.9 ohms resistors, and they even wrote 80 mA suggested setting!

[STRATDELUXER97]

Aah.. those great old russian beasts!!

Yeah. Stratdlxer is right about the resistors, but you must connect both PIN1 and PIN8 together, then to the resistor, then to ground. However, the MIG 60 already has two 3.9 ohms resistors placed there. You can already read the mV/mA thru these resistors. You must set the bias until you read ~80mA each.

1 and 8 is for EL34s...fender 6L6 style amps don't use PIn 1 for that...Depends on the amp? Either way,it's dangerous in there...

John

[STRATDELUXER97]

Wow that sounds so incredibly simple, why are people always making a fuss about biasing by themselfs and why do amptechs ask so much money for such a simple job?

Because the guys that know,leave it as some unexplained "Voodoo" so guys like us have to pay for the 5 minute adjustments...

John

[Chaos]

Here's a good site for a more detailed explaination:
http://www.duncanamps.com/technical/lvbias.html

Or a simple equation:
70% of the max tube dissipation (in watts) divided by the plate (anode) voltage=tube current (if measured at the tube) or tube current times the number for tubes measured at the standby switch (doesn't take in to account the grid current)

here is a explaination of why 70% max tube dissipation is usually used:
http://www.aikenamps.com/Why70percent.html

[Thames]

1 and 8 is for EL34s...fender 6L6 style amps don't use PIn 1 for that...Depends on the amp? Either way,it's dangerous in there...

John

The BFDR I built has PIN1 and PIN8 tied together. It doesnt change anything, but that way you can use EL34 in Fender 6L6 amp (however, there are other parts to swap so just dont do it!!)

And yeah... LETHAL VOLTAGES IN THERE! Please be careful with that stuff, I dont want to see an "Obituary" section on our board....

[The New Guy]

Man, there are a lot of really smart people in here...
as for myself...i like puppies.

[STRATDELUXER97]

The BFDR I built has PIN1 and PIN8 tied together. It doesnt change anything, but that way you can use EL34 in Fender 6L6 amp (however, there are other parts to swap so just dont do it!!)

And yeah... LETHAL VOLTAGES IN THERE! Please be careful with that stuff, I dont want to see an "Obituary" section on our board....

That's fine,but with the 6L6 you don't actually have to use Pin 1...If you tie 1 and 8 together you can use either the EL34 or the 6L6....

John

[STRATDELUXER97]

Man, there are a lot of really smart people in here...
as for myself...i like puppies.

What kind of puppies are we talkin bout?