Biasing Sovtek Mig-100

[Structo]

Yeah, it's probably the only trimmer pot in that amp.

Were these amps really made in Russia?

[selloutrr]

Yes with all Russian military surplus parts. actually they are great amps. It's a ton of volume but with a power brake and a good cabinet these amps sing and take pedals better than any Marshall JCM800 I've heard.

Imported by new sensor and cloned by gibson's Red Bear

[RJSkypala]

Yes its really a great amp. I'm really excited to get some new tubes in it and be able to work on it myself. I bought it for $400 in 9th grade, I will never sell it, it was my first tube amp and from what I hear it is a great amp to learn about amp design with.

Thanks for all your help.

[RJSkypala]

Another question I have:

Do I need to have a load on the output when biasing or doing work on the amp that is not producing sound?

[martin manning]

Yes. It is always producing some audio signal in the form of noise, but there is also the possibility of an oscillation. There's a good project for you- everyone needs a dummy load.

[RJSkypala]

There's a good project for you- everyone needs a dummy load.

Any links you recommend?

[Phil_S]

As far as I'm concerned, any time a tube amp is powered up, there should be a speaker or dummy load connected. I suspect most of us keep a cheap pair of 8 ohm speakers on the bench, easily reconfigured to 4 or 16. If you aren't putting a signal through it, the power rating of the speaker is less of a concern though I guess it could still blow under the right (wrong) conditions.

Why take the risk? It is an ounce of prevention.

[Firestorm]

The best dummy load is a speaker with no cone (just the voice coil). It provides a proper impedance versus a straight resistance. And we all have semi-destroyed speakers laying around by now. But the voice coil has to be intact and functional: you can't use a speaker that you've fried; only one where the cone was damaged.

You don't need to load the output if no output tubes are installed, but obviously you can't bias an amp that way.

[Structo]

I got one of these.

[img:500:500]http://www.parts-express.com/images/pro ... -020_l.jpg[/img]

http://www.parts-express.com/pe/showdet ... er=019-020

[Firestorm]

I got one of these.

[img:500:500]http://www.parts-express.com/images/pro ... -020_l.jpg[/img]

http://www.parts-express.com/pe/showdet ... er=019-020

Whoa! Okay I'm ordering one. Non-inductive, but very cool.

[Structo]

Yeah, I mounted mine on a slab of aluminum to help the heat dissipation.

Be sure to get the right resistance for your amp.

[martin manning]

Here's mine. Updated an old thread since this one is really another subject: https://tubeamparchive.com/viewtopic.ph ... 643#131643

[Structo]

Yeah, if Parts Express would have had a 300w load I would have bought that.

Perhaps putting two 100w 4 ohm in series would be safer?

[Firestorm]

Yeah, I mounted mine on a slab of aluminum to help the heat dissipation.

Be sure to get the right resistance for your amp.

I bought 2 so I can go 4-ohms, 8 or 16.

Thanks for the suggestion.

[RJSkypala]

Alright I got the amp opened up, immediately saw a fried screen resistor and replaced it. Measured resistance from P1:CT and P2:CT - Both 37 Ohms.

Plate voltage is at 545v, for which I calculated 38.5ma for each tube as per Weber Bias calculator (6L6GC) running at 70%, which for 2 tubes is 77ma. Calculated (.077*37) that I need a -2.85v drop from the CT to plate.

I am getting about a .3v difference between each set of tubes for this reading, so I set one reading lower to -2.8v and the hotter pair are at -3.1v

Am I missing anything?