To rebias or not to rebias????

dirtycooter · Post by **dirtycooter** » Thu Jan 29, 2015 4:31 pm

Ok.
So if I recall correctly Dave F said bias at normal wall voltage then drop with variac and the bias falls where it falls.
Some here are biasing at low voltage that they are running at.

I am a bit confused. If we bias at wall voltage correctly then drop shouldn't everything scale down together operationally?
Wouldn't rebiasing at lower voltage kinda be like being too hot of bias then at 90 with all things scaled down being equally? I mean you couldn 't raise it back up to 120 when biased close to max at 90.... or you'd be way to hot then.

This is a turd I gotta stir here. I find it interesting. Seems like ending up at cold bias at low voltage would give some more lifespan to the tubes. They would be running a bit more cool. Maybe some crossover distortion added in but...
Then the fact Eds bias pots were jacked. Now thats confusing as well here to.

Krinkle · Post by **Krinkle** » Thu Jan 29, 2015 10:51 pm

DF said to bias at 50mA with the variac set to 90VAC in the spill the beans thread. With my amp biased at 120VAC, when I set the variac to 60VAC, my amp was waaaaaaayy cold. My volume was down but I HAD to use the MXR EQ or it sounded like my guitar volume knob was at 7, not clean but not nearly enough dirt. The big problem I had is when I re-biased to 50mA at 90VAC, my volume really jumped back up when I set the variac at 60VAC (compared to how loud it used to be), screwing up the benefit of the variac.

If you bias to 50mA at 90VAC and plug your amp into straight 120VAC it will be hot, not sure if it will be dangerously hot, I'd have to do it, measure the bias and calculate, don't want to put my tubes/amp through that.

The bias voltage does not linearly track with the B+ when using the variac. London Power's Power Scaling has a separate bias tracking circuit to ensure that the bias is correct when changing the B+.

dirtycooter · Post by **dirtycooter** » Fri Jan 30, 2015 2:34 am

Whats weird is I am getting pretty much near twisty's gain in his vid-without a 6 band or the micro amp boost....
So THAT is kinda strange. All I did was dial down the variac. Of course I am usin probably a alot hotter pickup and have the extra paralleled 4th preamp tube. So gain is not an issue or nor do I seem to have the issue Rob was. I am at about 70+V. Volume is totally sane. Probably should check and look at my bias setting.
How much volume came back when you rebiased at 90v? Some or alot? ( I know describing volume is not easy to do))

Krinkle · Post by **Krinkle** » Sat Jan 31, 2015 4:52 pm

I would describe the volume increase as very significant. It makes the amp very loud again. To the point where I certainly don't want to be sitting in front of it and if my wife was not understanding there would be some thumps on the floor telling me to turn it down.

dirtycooter · Post by **dirtycooter** » Sun Feb 01, 2015 5:41 pm

Good to know-was gonna do this rebias today but snowed in at my girlfriends place literally right now

I think what I am gonna do is set this up with my analog MA meter probe plugged in and carefully try different voltages/volumes as I tweak the bias in carefully at the same time while moving the variac to different settings little by little and see if there is a magic combo of the two.

I think modding for test points and external bias adjustment would be a wise mod at this point don't you?? It would make dialing it in sooo much easier. I am sure there are combonation points that sound much better than the probably extremely cold bias I have right now

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To rebias or not to rebias????

To rebias or not to rebias????

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