Biasing Sovtek Mig-100

[RJSkypala]

Hello All,

This is my first of hopefully many posts in the amp building community. I have spent the last few weeks reading up as much as I can on tube amp theory and studying and analyzing designs. I am not 100% confident with some details (particularly math) but I can surely look at a schematic and map things out. After the new year I plan to build my first amp, likely a 5E3 circuit and I am very excited. I have experience with low voltage stuff like effects pedals and synthesizers and have been taught proper soldering technique over the years from the many electrical engineers in my family.

With that said, before I delve into amp building I need to install a new set of tubes and bias my old (got in 9th grade, 2002) Sovtek Mig-100. I am very much looking forward to taking the thing apart and seeing the inside, as I did alot of part replacing back when I was about 14 (switches, jacks, lamp, etc.) and I can't wait to see the slopping wiring I did and correct it. I am looking at the schematic for the amp and I think I have located the bias circuit (red), bias resistors @ grid (blue) and point to measure bias voltage (green). I have attached an image with these sections blocked out. There also appears to be a trim-pot in the bias circuit, however I don't remember this being in the amp last time I looked at it (im thinking this may be a difference between the 100 and 100h, schematic is for 100h).

I will not be able to get to the amp for about a week, but I want to prepare to know what I am looking at. I have a new set of SED 6L6's I am excited to install and get this thing going.

I appreciate all of your help!

[jelle]

Looks like you will have to do the transformer shunt method on this one. unless you want to install 1 ohms (5% 2W metal oxide is what I use) resistors at the powertube cathodes to ground.

With the transformer shunt method, make sure you know what you are doing! You will measure bias current for two tubes at once, so keep that in mind. Also the B+ is high on these so make sure you set the bias current acording to that high B+ voltage.

Good luck! Monster amps....

[xtian]

Some resources for you. Bias calculator: http://www.webervst.com/tubes1/calcbias.htm

You'll measure your plate voltage (pin 3 on power tubes) and enter into the calc. Say you've got 450v on the plates, your 70% target current is ~40mA.

So, as Jelle suggests, you've not got to measure the current, and there are a handful of ways to do this.

http://www.aikenamps.com/Biasing.html

Once you can make a measurement, you'll adjust the bias trim pot until you get the target current. Let us know if you need more help!

[martin manning]

You've got the bias circuit identified, and it has a trimmer to adjust the bias voltage, but what you want to measure is bias current. I would measure the DC resistance of each half of the OT primary (call it Rcta)with the amp off, then measure the voltage across each half (CT to plate lead, call it Vcta) with the amp powered-up and use that to calculate the current flowing on on each side.

Since plate dissipation Pa = plate current times plate voltage = Ia x Va, and you can get Ia from Vcta/Rcta, Pa = (Vcta x Va)/Rcta.

A good place to start is to set Pa at around 70% of the rated Pa Max, so you want Pa = 0.7 x Pa Max = (Vcta x Va)/Rcta.

Rearranging, you can get Vcta = (0.7 x Pa Max x Rcta)/Va

Pa Max for 6L6GC is 30W, but use 60W since you are measuring current for two tubes. Therefore you want Vcta = (42 x Rcta)/Va

So measure Va, calculate the taget Vcta for one side and dial that in with the bias trimmer. Measure Va again (it will move), and repeat. Check the other side to see if it's current is close to the same as the first.

[jelle]

Also, look at this:

http://www.duncanamps.com/technical/lvbias.html

And yes, please do verify the current, the static current, that is.

[RJSkypala]

Thanks for the help. I will do some of the suggested reading and report back when I am actually with the amp. It does not seem that difficult. I assume that both sets of tubes will not be operating at exactly the same current since there is only one adjuster, am I looking for the best compromise between the two and my desired current?

[jelle]

Different schools of thought on that one:

1) balance and match everything: both static current of the powertubes and the transconductance of each half of the push pull section. This includes the phase inverter.

2) just adjust for safe operation conditions and play rock and roll.

The second method has more powerstage distortion and that may be a very desirable thing in that amp. This method is less reproducible, though...

[RJSkypala]

Would a bias probe help in any of this? As I said before I am hoping to embark on a lifetime of amp building and I rarely mind spending money on tools to improve the process.

[cbass]

I copied this from Aikens site

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side

[cbass]

*This is really plate current Ia plus screen current, so the result here will be a little conservative.

Martin Doesn't thiis method measure current before the screens.Or amInot understanding something

[martin manning]

Oops! you are correct CB! Thanks, I'll edit my post above.

[RJSkypala]

Thanks for all your help, I am very excited to get working on this when I get back home to Philly.

Preparing to work on amps, I am thinking about some new tools that may come in handy. I already have a nice hakko soldering station, DMM and some pretty decent hand tools that I have picked up here and there, but I was looking at these and they looked like they would be pretty nice:

http://www.amazon.com/Xcelite-General-S ... B002FCHEC8

and

http://www.amazon.com/Xcelite-Profile-E ... _indust_24

Anybody ever use these?

[selloutrr]

gut shot

[RJSkypala]

Is this the bias pot?

[roberto]

You can find the bias pot easily on all amps.

Just search (around the PI circuit) in order:
- a reverse polarized diode
- a cap with positive to ground
- a resistor
- a cap with positive to ground
- a trimmer

So yes, that trimmer is the bias trimmer.