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How close is close enough for setting bias?
How close is close enough for setting bias?
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Old 10th December 2015, 05:35 AM   #1
katana1100 is offline katana1100  United States
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Default How close is close enough for setting bias?

I have a jolida JD 1000, has 8 l34 tubes.
Using ez bias, it's biased. Spec is 400mv
+\ _ 40mv
I checked and a couple are at 400, most are
In the 380 range. Yes, in spec , but should I
Try to tweak and get them all at 400mv?
Any real power to be gained?
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Old 10th December 2015, 05:45 AM   #2
petertub is offline petertub  Sweden
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You won't gain any power by adjusting bias, but you will find that distortion may
be reduced when the load is shared equally between the tubes.
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Old 10th December 2015, 08:49 AM   #3
DF96 is offline DF96  England
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10% is fine.
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Old 11th December 2015, 04:23 PM   #4
kevinkr is offline kevinkr  United States
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How close is close enough for setting bias?
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.
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Old 11th December 2015, 04:54 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

The bias can only be as accurate as the resistor tolerances.
Likely anything closer than 5%, is a moot point, but there
is no harm in trying to get them all close to 400mV.

rgds, sreten.
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Old 11th December 2015, 05:18 PM   #6
cerrem is offline cerrem  United States
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Keep in mind that is CATHODE current.... is PLATE + SCREEN current....
A El34 will have roughly 4 to 6 mA of SCREEN current at idle at the typical operating voltages..
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Old 12th December 2015, 12:00 AM   #7
katana1100 is offline katana1100  United States
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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
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Old 12th December 2015, 04:50 AM   #8
trobbins is offline trobbins  Australia
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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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Old 12th December 2015, 02:17 PM   #9
hpeter is offline hpeter  Slovakia
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Quote:
Originally Posted by cerrem View Post
Keep in mind that is CATHODE current.... is PLATE + SCREEN current....
A El34 will have roughly 4 to 6 mA of SCREEN current at idle at the typical operating voltages..
so a current resistors should be mounted on plate wiring?
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Old 12th December 2015, 02:34 PM   #10
jane is offline jane  Norway
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Quote:
Originally Posted by hpeter View Post
so a current resistors should be mounted on plate wiring?
If you want to know the plate current, you should measure the plate current.
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