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How often do you bias your tubes?


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My previous power amp I always just got it biased by the people who sold me the tubes and have to admit to have never adjusted them myself afterwards being scared to touch it while it was running.

 

My new power amp came with instructions for biasing the valves and a tool to do it with. I checked the bias shortly after installing it and everything was nigh on perfect. After reading a reviewer say he checks tubes at 0, 100 and 200 hours, I then checked it again after 200 hours clocked up on the tube counter and most had drifted down slightly (.5-1mV) and I adjusted it. That's certainly not enough for a noticeable sonic difference and while I've read lots about how often the bias should (or should not be checked), I'd like to hear from others with tube amps regarding how often they check their biases.

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General rule for me (if you don't have drift problems after reaching operating temp), when I was into tubes was, once for the start of Winter (on a cold day) and again at the start of Summer (on a hot day).

 

Cheers George 

Edited by georgehifi
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Guest Muon N'
2 hours ago, Ittaku said:

How much do you find they drift in that time?

It variable and I never use the amp during that time the same, so any figure would be meaningless.

 

 

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Keep in mind that when tubes/valves were mainstream (before solid state) they were in everything and never needed rebiasing.    Even when replacing a valve, most consumer equipment had no adjustment, but it all still worked.  I guess it will depend a bit on the design decisions made by the manufacturer, but I am betting most will not be that finicky.

 

I am not saying to never check it, but as you noted, small amounts of drift will not be noticeable.  A small amount of movement up and down the valve curves is not a big problem.   I'd do it when new valves are installed, then maybe a week or three,   a month or two, after that depending on amount of use.  After that, only if I was bored :)

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1 hour ago, aussievintage said:

Keep in mind that when tubes/valves were mainstream (before solid state) they were in everything and never needed rebiasing.    Even when replacing a valve, most consumer equipment had no adjustment, but it all still worked.

That's because many were Cathode Biased (auto biasing) Sure you loose a couple of watts doing this, but it's very convenient. You can if your good enough convert your fixed bias to a cathode biased system. Guitarist's say it "sustains longer and sings"compared to fixed bias.

 

Cheers George

Edited by georgehifi
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38 minutes ago, georgehifi said:

That's because many were Cathode Biased (auto biasing) Sure you loose a couple of watts doing this, but it's very convenient. You can if your good enough convert your fixed bias to a cathode biased system. Guitarist's say it "sustains longer and sings"compared to fixed bias.

 

Cheers George

The odd thing is that calling it auto-bias makes it sound more fancy, when it fact it is the simpler method of doing it.  Not as accurate either, but that was part of my point, it isn't that critical.

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2 minutes ago, MC240 said:

Why so ?

Well as quoted above.

9 hours ago, aussievintage said:

The odd thing is that calling it auto-bias makes it sound more fancy, when it fact it is the simpler method of doing it.  Not as accurate either, but that was part of my point, it isn't that critical.

Autobiasing isn't as accurate as manually biased and...

7 hours ago, Noum said:

Bias out of wack  a little and the sound stage can go to the crapper, i always adjust if It's needed.

The reason for bias in the first place is for optimal performance. ARC still use manual bias for example; it's not like they're unaware of the (ancient) technology behind autobias.

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8 hours ago, Ittaku said:

That's not in itself necessarily a good thing.

There's nothing wrong with cathode biasing, it holds tube in perfect bias unless they go faulty. The only penalty is, you lose a couple of watts off you max output wattage, no biggie.

My Rogue M150 monos were auto/cathode biased.

 

Cheers George 

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Just now, Ittaku said:

Well as quoted above.

Autobiasing isn't as accurate as manually biased and...

The reason for bias in the first place is for optimal performance. ARC still use manual bias for example; it's not like they're unaware of the (ancient) technology behind autobias.

OK, but I only question the need for pinpoint accuracy, and this gets back to the OPs question.  I have built many valve amps (I know weird, but I love the technology)  and experimented with bias points.      I find it makes more difference to the sound when you change the bias on preamp valves, but they aren't the ones that you typically adjust in a commercial amp, it's the outputs.  If you're not changing the bias so much that you alter the class of the amp, i.e. class A, class AB, class B  then  small bias adjustments don't change the sound much in my experience.  The adjustments help you balance slightly unmatched valves, and let you set the bias for valves to account for the manufacturing differences between different makes, and even different designs and batches.  The very small amount the bias drifts, after initial settling in, should not effect the sound to any great degree.

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Guest Muon N'
12 hours ago, Ittaku said:

The reason for bias in the first place is for optimal performance. ARC still use manual bias for example; it's not like they're unaware of the (ancient) technology behind autobias.

Not sure what you are attempting to tell me here.

 

I know what Bias is and what it is for. I use a cathode bias in my amp.

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12 hours ago, Ittaku said:

Well as quoted above.

Autobiasing isn't as accurate as manually biased and...

The reason for bias in the first place is for optimal performance. ARC still use manual bias for example; it's not like they're unaware of the (ancient) technology behind autobias.

Is thata fact ?

I forget to also mention in my previous post there is need to match the valves with the mc240 let alone bias it. Btw thank you for your reply.

Edited by MC240
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22 minutes ago, MC240 said:

I forget to also mention in my previous post there is need to match the valves with the mc240 let alone bias it. Btw thank you for your reply.

Do you mean NO need to match?

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2 hours ago, Ittaku said:

Do you mean NO need to match?

Yep

When I first got it it had a couple of different branded 6l6 in it changed them out for a matched set of svetlana 6l6gc winged valves buggered if I could hear and great difference, how ever I did replace those with a  tung sol quad of kt66 unmatched they sounded different but I wouldn't say better.

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1 hour ago, Noum said:

Not sure what you are attempting to tell me here.

 

I know what Bias is and what it is for. I use a cathode bias in my amp.

I was just pointing out that some high end amps choose one or the other approach. ARC (which I mention as example, not saying it's better than everyone else) choose manual, and they run their valves way below their power capacity, so If the pinpoint accuracy makes no difference and they can afford to sacrifice a few watts, then there must be other reasons... though sometimes the reasons for choices in manufacturing have more to do with legacy choices than any continued advantage. I'm not trying to say autobias is necessarily worse either.

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29 minutes ago, Ittaku said:

I was just pointing out that some high end amps choose one or the other approach. ARC (which I mention as example, not saying it's better than everyone else) choose manual, and they run their valves way below their power capacity, 

Just curious about this.  Not familiar with the ARC design before now. 

 

A quick Google found a REF 75 amp - pair of KT120s?    Choice of power supply, and bias point can result in less power, but maybe also less distortion.  It's a bit of an art.  I used to use a spreadsheet with valve models and graphs to play with operating points.  I suspect this is what they have done - chosen a different operating point to the more normal one which would be optimised for power output,  and sacrificed power for less distortion. Maybe even improved valve life if they move further away from the max. dissipation curve. 

 

However, having chosen the operating point, and designed a power supply to allow it,  they could bias the valves to sit at that operating point using any method they want.  When you adjust your bias, you are maintaining the amp at that operating point, but I am just saying a little drift away from it shouldn't be making a huge difference.  But hey, can't do any harm making sure it's spot on. :)

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Yeah, the current (SE) amps use KT150s. The 75 uses a pair per channel in push pull enriched A/B for only 75W. They also run their tubes at lower voltage and cool them actively with fans to prolong their life (Yes I have an ARC amp which is why I know about them.)

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1 hour ago, Ittaku said:

Yeah, the current (SE) amps use KT150s. The 75 uses a pair per channel in push pull enriched A/B for only 75W. They also run their tubes at lower voltage and cool them actively with fans to prolong their life (Yes I have an ARC amp which is why I know about them.)

Aha, "enriched AB"  yeah, that's just sales talk for running the tubes hot.  They are running the tubes biased higher -towards class A.  Small wonder they need run lower voltage and use fans to cool them so they don't burn out.  Not a recipe for long life if they didn't cool them.   The result is lower output power because of those operating points.   Still, if it sounds good, maybe it's worth it.   

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