When To Check Bias

So I have read from many people that they only check their bias once a week, bi-weekly and as long as once a month.
I use to constantly check my bias when I first entered into the amber light.
So concerned was I.
As time went on I only checked my bias after 20minutes of warm up time and then once again in another hour...
I was listening to my music after 5minutes of turning on the m125's.
What I have discovered is that if I do not check my bias until 30-40 minutes into play it is just as I left it the day before. This new discovery had me scratching my head.
Why is it that way?
I do have a Variac and and a kil-O -watt making sure before I turn on the m125's the voltage reading in my house/system is no more than 119.6. This discovery is new to me and I was wondering why I never read about this conduldrum.

So the longer I have my tubes warm up from first turning on my m125's the more they are in tune (so to speak)
Happy to discover this working fact.

Effectively, tubes take about 20 - 30 minutes to heat up and stabilize fully. I am running vintage Siemans tubes, I will check bias about every 100 hours or so. Rock steady these last 10 years or so.

With new tubes : check at install time , then after one hour.
First days after tube change, check each power on and if stable increase to a week.
If still stable, check after a month, then if still stable, half a year or so.
No need to adjust each time if changes are small, just note value and date.

Good tubes will keep steady bias for many hours.
Note that mains fluctuations will affect bias. Thus if all tubes fluctuates "together" this is a sign
of fluctuation mains. Only if tubes starts to disperse should more frequent check/adjustment
be needed.

Doesn't hurt to clean the bias pots once in awhile with tuner cleaner or some such other and give the wiper a stop-to-stop swing to make sure there's no crud in there to skew your bias.

What is a reasonable mV range to fluctuate within?  

Would it be possible to put the front power sockets to an alternative use by installing (simplest case) an LED on a low voltage circuit + bias and ensuring that all 4 LEDs are illuminated at the same approximate level?  Alternatively, install a split LCD screen that is a digital readout of the current bias?  Something like one of these: https://www.adafruit.com/category/63.

bennytehcat wrote:What is a reasonable mV range to fluctuate within?  

Would it be possible to put the front power sockets to an alternative use by installing (simplest case) an LED on a low voltage circuit + bias and ensuring that all 4 LEDs are illuminated at the same approximate level?  Alternatively, install a split LCD screen that is a digital readout of the current bias?  Something like one of these: https://www.adafruit.com/category/63.

Nominal value is 0.5V, and +- 10% would be benign as long as the tubes follows together.
It's when the tubes spreads that you should take a closer look, usually it's an indication
of tube replacement times.
Tubes are consumables and time for replacement is randomly spread between 1h and 6000h
of use. To keep spread low replace them with matched pairs of same brand.

Peter W. wrote:Effectively, tubes take about 20 - 30 minutes to heat up and stabilize fully. I am running vintage Siemans tubes, I will check bias about every 100 hours or so. Rock steady these last 10 years or so.  

Peter..What amplifier are you using with the Seigman Output tubes...St 70, 120 or the m125's?

OEM 70.

After I wipe any dust off the amp and the VOTTs,  I check and re-adjust if necessary every Saturday morning after a 1-hour warm-up. My KT-88s have settled down very nicely and rarely do I have to do any adjustment now.