Different voltage of tubes - best practise

[Goran Lorencin]

Hey there PRS people, a few questions regarding tubes ...

I biased my MT15 today, must say it was really simple using the dedicated ports on the back and wanted to ask for advice regarding your approach to biasing, I mean what do you aim for, do you go by the sound, stick to 30 +/- 5(in regard to my amp, I understand that different amps have different biases), have it somewhere between 25-30 mV?

Today my numbers were 25.5 left and 27.0 right before the biasing(this amp only allows biasing both tubes at the same time), then I put them to 28.0 and 29.5. Is a constant 1.5 mV difference between tubes within the usual boundaries?

Follow up:
Well after playing, I must say that I liked the initial setting more and have dialed it down back to 26mV left and 27.5mV right, for now I think I prefer this sound, will see with time and experimenting a bit more where it takes me. It most likely also depends on what type speaker I use, what kind of pickups I have in my guitar, what kind of space I rehearse in, in what state the electrical installation and so on ... do any of you use a power conditioner or some kind of filter or el.interference blocker?

Cheers!

 

[shimmilou]

I almost always use this formula, works for any amp, any tubes. Tube rated max watts, times 0.7, divided by the plate voltage, equal bias mA, each tube. So for 6L6, 30 watt tube, in an amp with 400 plate volts, would be; (30 x 0.7)/400 = 52 mA per tube. Not sure why some manufacturers bias so cold, other than to extend tube life. Changing the bias level can effect the tone.

 

[Egads]

I tend to start with amp recommendations, then play it by ear. If you aren't using a matched set, get it close and see what it sounds like.

 

[Simon Says]

It all depends on plate voltage of the amp

For what its worth. Just got EuroTubes 'Pro One' Bias meter few weeks ago and I was dying to know whats the stock bias point on my Fillmore 50W head. With idyllic knob settings (all on zero) with STR440 GRN rank I got:
Plate Voltage - 440v
Plate Current -23mA
So it's 55% plate dissipation, super cold.

Can't wait till I go over warranty with it and set it to somehow more to the hot side. As much as I like colder bias in high gain amps I would rather spice up this particular amp. Boogie Fillmore is more classic rock amp than usual Boogie screamers

 

[Goran Lorencin]

I almost always use this formula, works for any amp, any tubes. Tube rated max watts, times 0.7, divided by the plate voltage, equal bias mA, each tube. So for 6L6, 30 watt tube, in an amp with 400 plate volts, would be; (30 x 0.7)/400 = 52 mA per tube. Not sure why some manufacturers bias so cold, other than to extend tube life. Changing the bias level can effect the tone.

Thx for the comment. Maybe it is a bit to specific for me as I do not know the plate voltage of MT15 (and now I know that 6L6(which are in my MT15) are 30W, but I do not want to lose the warranty so I will stick to the factory advised interval between 25 and 30 mV for at least the next 3 years and also I think the sound is good at those voltages.

But what about the last question, the mV difference between the two power tubes which is always 1.5 mV in my case, is there a way to brig that difference closer to 0 (because with the ports on the back of the MT15 I can only increase or decrease the voltage of both at the same time)? I thought that it was possible to regulate that difference, but it seems that I can only bring the voltage up or down for both tubes at the same time. Or is there something I`ve missed?

Thx

 

[Goran Lorencin]

I tend to start with amp recommendations, then play it by ear. If you aren't using a matched set, get it close and see what it sounds like.

I am still using the original factory tubes (do not know if they are matched in the factory) but there seems to be a constant 1.5 mV difference between them. Is there a way to bring them closer? Because biasing seems to lift or drop the voltage on both tubes at the same time.

Thx

 

[Goran Lorencin]

It all depends on plate voltage of the amp

For what its worth. Just got EuroTubes 'Pro One' Bias meter few weeks ago and I was dying to know whats the stock bias point on my Fillmore 50W head. With idyllic knob settings (all on zero) with STR440 GRN rank I got:
Plate Voltage - 440v
Plate Current -23mA
So it's 55% plate dissipation, super cold.

Can't wait till I go over warranty with it and set it to somehow more to the hot side. As much as I like colder bias in high gain amps I would rather spice up this particular amp. Boogie Fillmore is more classic rock amp than usual Boogie screamers

I presume I would have to open the housing to measure those parameters or? And finding out where to measure on an MT15 is the next task

 

[shimmilou]

...But what about the last question, the mV difference between the two power tubes which is always 1.5 mV in my case, is there a way to brig that difference closer to 0 (because with the ports on the back of the MT15 I can only increase or decrease the voltage of both at the same time)?

No worry, that is very close, could be considered matched.

 

[Goran Lorencin]

No worry, that is very close, could be considered matched.

Now that`s good news for me thx Shimmilou, really wanted to know this info, will sleep better now.

 

[Simon Says]

I presume I would have to open the housing to measure those parameters or? And finding out where to measure on an MT15 is the next task

Not with this meter. It measures the plate voltage from the socket

https://www.eurotubes.com/store/pc/Eurotubes-Pro-One-Bias-Probe-for-octal-power-tubes-p144.htm

 

[László]

Even when you buy matched sets, they're often going to measure a few millivolts away from each other.

As to brands, you want the output tubes to be the same brand and spec. Preamp tubes are more flexible.

Not that you necessarily want to get into the whole tube rolling thing, but the best tubes are "NOS" (new old-stock, unused). Since they're built as a vacuum envelope, unused old tubes last for a lot of years - there are tubes made in the 1930s that test new - aficionados buy them to restore vintage radios from that era.

Back then, the best tubes were made in the US and Western Europe. Companies like GE, RCA, Philips, Mullard, Sylvania, Western Electric, Tung-Sol, Brimar, Telefunken, and others made the real-deal high quality stuff. Critical equipment used tubes - jet fighter aircraft used tubes for certain electronics into the '70s because of the danger of fried transistors if there was a nuclear EMP. Medical equipment used tubes, and of course, consumer electronics used them.

When the West stopped making tubes around 1980, there were lots of unused ones in boxes gathering dust. These are now NOS.

A few enterprising folks bought the old names, so now there are new Mullards, Telefunkens, etc., but rest assured, these are just current Czech and Russian production tubes, and they aren't built like the old ones. I've compared them. They don't sound like the old ones, either. Don't be fooled by the names Mullard, Tung-Sol, etc. They're different than NOS Mullard, Tung-Sol, Telefunken, etc.

In 1979 and 1980, the tube manufacturing machinery itself was already old, and was sold off to the Eastern Bloc and Far Eastern countries. As you can imagine, the machine tooling here was at a higher standard when it came to parts, maintenance and the condition of the machines themselves, and now the equipment has another 40 years of wear and tear. Also, the knowledge of how tubes were made back in the day is literally dying off as the people with the know how aren't all around any more.

I have some photos somewhere of NOS vs current production tubes, and you can easily see the reduced quality of workmanship, parts, and subassemblies in new tubes compared to the old ones. It matters sonically. The new ones are more microphonic, they tend to rattle more, they don't last as long. Even the mica spacers that had little pointy bits to keep from rattling against the glass now don't have those. The wire leads from the pins to the cathode and plates aren't as good. The legs holding everything in place aren't as robust. The plates are off at weird angles. Everything is cheapened on the new stuff.

Whether that's deliberate corner-cutting, or just what happens when the old stuff that tubes used to be made with isn't available, I can't say, and of course, it doesn't matter. The point is that it's different than the old stuff.

Unfortunately, NOS are usually expensive. I retube all of my amps with NOS, and I think it's worth it. But I'm in the recording biz, and that probably clouds my judgment, if not my sanity.

I'm really saying this not to convince you that NOS tubes are something you should run out and buy. Just keep it in the back of your mind if you ever decide you have a few extra bucks floating around and feel like driving yourself bananas for a few days of tube rolling experimentation.

 

[Goran Lorencin]

Even when you buy matched sets, they're often going to measure a few millivolts away from each other.

As to brands, you want the output tubes to be the same brand and spec. Preamp tubes are more flexible.

Not that you necessarily want to get into the whole tube rolling thing, but the best tubes are "NOS" (new old-stock, unused). Since they're built as a vacuum envelope, unused old tubes last for a lot of years - there are tubes made in the 1930s that test new - aficionados buy them to restore vintage radios from that era.

Back then, the best tubes were made in the US and Western Europe. Companies like GE, RCA, Philips, Mullard, Sylvania, Western Electric, Tung-Sol, Brimar, Telefunken, and others made the real-deal high quality stuff. Critical equipment used tubes - jet fighter aircraft used tubes for certain electronics into the '70s because of the danger of fried transistors if there was a nuclear EMP. Medical equipment used tubes, and of course, consumer electronics used them.

When the West stopped making tubes around 1980, there were lots of unused ones in boxes gathering dust. These are now NOS.

A few enterprising folks bought the old names, so now there are new Mullards, Telefunkens, etc., but rest assured, these are just current Czech and Russian production tubes, and they aren't built like the old ones. I've compared them. They don't sound like the old ones, either. Don't be fooled by the names Mullard, Tung-Sol, etc. They're different than NOS Mullard, Tung-Sol, Telefunken, etc.

In 1979 and 1980, the tube manufacturing machinery itself was already old, and was sold off to the Eastern Bloc and Far Eastern countries. As you can imagine, the machine tooling here was at a higher standard when it came to parts, maintenance and the condition of the machines themselves, and now the equipment has another 40 years of wear and tear. Also, the knowledge of how tubes were made back in the day is literally dying off as the people with the know how aren't all around any more.

I have some photos somewhere of NOS vs current production tubes, and you can easily see the reduced quality of workmanship, parts, and subassemblies in new tubes compared to the old ones. It matters sonically. The new ones are more microphonic, they tend to rattle more, they don't last as long. Even the mica spacers that had little pointy bits to keep from rattling against the glass now don't have those. The wire leads from the pins to the cathode and plates aren't as good. The legs holding everything in place aren't as robust. The plates are off at weird angles. Everything is cheapened on the new stuff.

Whether that's deliberate corner-cutting, or just what happens when the old stuff that tubes used to be made with isn't available, I can't say, and of course, it doesn't matter. The point is that it's different than the old stuff.

Unfortunately, NOS are usually expensive. I retube all of my amps with NOS, and I think it's worth it. But I'm in the recording biz, and that probably clouds my judgment, if not my sanity.

I'm really saying this not to convince you that NOS tubes are something you should run out and buy. Just keep it in the back of your mind if you ever decide you have a few extra bucks floating around and feel like driving yourself bananas for a few days of tube rolling experimentation.

Thanks for the explanation LSchefman, I always wondered why NOS can be so expensive. You would think we would start making them even better today, with all the modern measuring devices, modern materials and robotic electronics. Looks like mechanical analog remains the thing of the past ... isn`t it ironic, don`t you think a human touch makes such a difference. Well, the thing is that to really appreciate the subtle differences in sound you would have to buy 5-10 different power pairs and a couple of pre-amp tubes to really experiment with the combinations. I`ve learned something new, thx guys and gals

 

[László]

Thanks for the explanation LSchefman, I always wondered why NOS can be so expensive. You would think we would start making them even better today, with all the modern measuring devices, modern materials and robotic electronics. Looks like mechanical analog remains the thing of the past ... isn`t it ironic, don`t you think a human touch makes such a difference. Well, the thing is that to really appreciate the subtle differences in sound you would have to buy 5-10 different power pairs and a couple of pre-amp tubes to really experiment with the combinations. I`ve learned something new, thx guys and gals

You're so right about the human touch. I'm fascinated by how the tubes were in fact manufactured. There was a lot of hand assembly. Whether that could be reproduced today or not is an open question.

 

[Deleted member 5962]

I've often wondered why "someone" doesn't create a NEW tube factory, and make all the classic tube styles with old school quality, but new technology and machinery that could make them even better. But the truth is, there isn't enough market for even a small scale operation. Our society no longer even WANTS to make things that may last for a lifetime! But, doing some math... if a super high quality 12ax7 sold new for say $50.00 or even $75.00 and you put 3 in your tube amp and never again replaced them, they'd make less than $15 tubes that you replace every year. So, you have 2-3 or even 5-6 amps. You buy $300 worth of pre and power tubes from them, then never buy again.

25 years ago, they told us tubes would be gone, and out of use by now. We all know that's wrong. Not only is old stuff still around that uses them, but new amps and hifi gear still being made uses them. But even with both guitar and higher end audio equipment still using them today, there just isn't enough of a market to support a truly "new" tube manufacturer here in the US.

So what I've always wondered is this: They say you can get any level of quality you want from MIC goods. The better quality you want, the more you pay, but you can get very well built "stuff" from China. So, is the retooling cost too high for someone to "spec" better tubes even with the existing factories? Or, is the scale of the market not even big enough to support that?

 

[elvis]

I almost always use this formula, works for any amp, any tubes. Tube rated max watts, times 0.7, divided by the plate voltage, equal bias mA, each tube. So for 6L6, 30 watt tube, in an amp with 400 plate volts, would be; (30 x 0.7)/400 = 52 mA per tube. Not sure why some manufacturers bias so cold, other than to extend tube life. Changing the bias level can effect the tone.

This would be more appropriate for Class A (full power dissipation at idle), but most amps are Class AB. The idle current should be quite a bit less than Class A idle current.

Note that the bias current is the current dissipated with zero output power. (This is converted to mV by running through a 1-Ohm resistor, so mV=mA, but bias is current, not voltage). Class A amps swing from zero output current to 2X idle current.

Class AB amps have swing current that is more complicated and not directly related to idle current. Running higher idle current makes them run hotter at zero output power, but they were designed to not run as hot as Class A at idle while still being able to deliver full power with an output.

 

[László]

I've often wondered why "someone" doesn't create a NEW tube factory, and make all the classic tube styles with old school quality, but new technology and machinery that could make them even better.

Well, actually, there's a company in Prague, Czech Republic that still does make the classics by hand. The people were trained by the old Tesla folks to do it the right way.

The tubes are also extremely expensive, and probably have to be, given the economy of scale for this type of production. I wish they made guitar amp tubes - seems to me that they could increase production/reduce prices, as there are undoubtedly more tube guitar amp owners than tube hi-fi owners in the world.

Here's the link:

http://www.kraudio.com

I thought hard about looking into tube manufacture in Detroit. It seems like there's an opportunity to do some good things here. At this point, however, I have my hands full just dealing with the ad scoring business.

 

[ViperDoc]

This might be useful:

https://robrobinette.com/Tube_Bias_Calculator.htm

 

[Goran Lorencin]

Tkanks sexy naked chick with a PRS in her hands that goes by the name ViperDoc (losing focus when looking at your avatar) ... this looks like a very useful calculator for when I get my chops to intermediate level.

 

[Bill SAS 513]

Les...Very cool video...Thanks.
Over the weekend, I had one of my guitar heads not power up. I did the most basic look-over and couldn't find the culprit. Per my repairman, it was due to a weak tube...(I have a great repair guy, and I go to his place so he can work his magic.)
I've really never given a ton of thought to tubes, but it really is another important link to tone. Now I wish I'd listened when he was working, and using all those "big" tube words.
Also, my Grandfather used to repair radios, and TVs...and had a shed attached to the back of his house, with boxes of old tubes. CRAP. Things I wish I'd known then.
Suggestion...(Not sure how true this is,) But I've heard you shouldn't get your hand/finger oils on tubes...I have a couple old pairs of Marching band (just plain old white) gloves to handle the tubes when I mess with them.
You can pick 'em up at any music shop that rents band instruments.

BTW, the repaired head's volume pot also had a lot of travel before actually effecting the volume, (dead spot on the wiper???) so I also had him swap that out. Damn thing slays, now!!!!

 

[Goran Lorencin]

I almost always use this formula, works for any amp, any tubes. Tube rated max watts, times 0.7, divided by the plate voltage, equal bias mA, each tube. So for 6L6, 30 watt tube, in an amp with 400 plate volts, would be; (30 x 0.7)/400 = 52 mA per tube. Not sure why some manufacturers bias so cold, other than to extend tube life. Changing the bias level can effect the tone.

Regarding the formula and my measurements ... I must say that putting tubes to 30mV bias started to sound quite harsh and there is quite some noise present, I am even considering to putting them back to around 26mV, will se today in rehearsing how the sound holds up. I like the amp to be as quiet as possible when not playing, do not like lots of hum or noise. So I imagine you could put the bias to over 30 or even 40mV, but I think the sound would be unusable for what I play and want to hear ... and I would probably have to constantly put the volume knob to mute between playing parts.

So I would say that manufacturers bias so "cold" because the sound is not as usable when biased hotter, you lose detail, definition and tightness. And man do I love my MT15