• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

How close is close enough for setting bias?

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It is not quite as cut and dried as the last post would seem to suggest, it depends quite heavily on how much DC unbalance an output transformer can tolerate before it will start to saturate at some usually unspecified moderate to high power level, that figure is not universally 10%, but it is what I would consider a reasonable maximum dc imbalance between the two phases in a PP primary.

I aim for 5% or better match over time and try to get something better than than 1% for initial current match.

Initial adjustments may be performed just after power up to make sure output tubes will not self immolate, and then finer adjustment after roughly 30 minutes of running time, followed by occasional checks and tweaks over the first 50 hours or so with new tubes. Check monthly after that unless you believe there is an issue that would compel you to check it more frequently.
 
Thanks!
Totally forgot about the resister tolerance .
I let the amp sit for an hour on idle , then adjusted, got them all
At 400. Next day,rechecked after listening to it for an hour, some were
At 403 some 394, readjusted and that is that.
Maybe pyschological, but it sounds a little better. Before, there were
In the 380 range, now at 400, sounds better. Plus, they are very similar .
I think jolida ez bias is good when you put new tubes in to get them close,
But after that, dig out the meter.
Or, just use the meter
 
katana, do you know if a single common meter or ADC is used to measure the cathode current sense resistor voltages?

I guess the ingenuity of the Jolida doesn't extend to measuring rms or peak current when a constant audio signal is applied? That may allow swapping around the valves to achieve effective matching of dynamic current through the OT. Although as cerrem notes, there is still tolerance on the screen current matching.

Primary winding inductance drop off at small signal level is one consequence of idle imbalance. An OT for a Williamson amp I measured recently showed about 20% drop-off in inductance (at 12VAC excitation) for about of 10% net imbalance of plate idle current (5mADC in that case). Whether that translates to a noticeable change in bass roll off may be dependant on many other factors.

Wrt OT core saturation, I would have thought that a well designed OT would operate flux density sufficiently below a level where noticeable saturation kicks in, so as to manage core related distortion - or is that not a valid comment for modern core materials.
 
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