Different voltage of tubes - best practise

[László]

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.

Halogen bulbs will crack when the oils from fingers heat up. But audio tubes are probably different, and there doesn't seem to be much evidence that it'll damage the tube to handle it with bare hands - - obviously, don't do it when the tube is hot!

I often wear a clean pair of leather gloves to pull tubes, simply because leather's slightly tacky on the glass, so I don't have to squeeze hard, and the tubes don't slip out of my hand. After installing a tube, I'll gently wipe finger oils off with a dry microfiber cloth. It's probably an unnecessary precaution, but it only takes a couple of seconds and doesn't hurt anything.

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

I'd say that what works with tube bias depends a lot on the tube itself, and also depends on how the amp was designed.

In the case of the MT-15, you have two 6L6 GC tubes; usually, you find 2x6L6s in a 50 Watt amp. The MT-15 runs them at only 15 Watts. To do that, the circuit has to be designed differently than your typical 6L6 amp.

It would make sense that the manufacturer's recommendation on biasing the tubes should be followed.

Generally, biasing a 6L6 hotter than 26 mV doesn't add extra noise (again, the tubes matter), and most are set at the manufacturer at a higher bias. But the MT-15 is a different kind of amp.

 

[Simon Says]

This might be useful:

https://robrobinette.com/Tube_Bias_Calculator.htm

Yeah, you can get bias calcs as an app on your phone too. The one I'm using is FTBC Lite

 

[Goran Lorencin]

An update regarding "cold" factory biasing of tubes, they are 25-30mV AT IDLE, but when you start playing it goes up quite a bit, up to more then 50mV, today I've measured from 45-53 when playing at 25% volume and a bit less than 50% gain. And I was surprised that the 6L6 were Ruby tubes and not JJ as stated on Thomann.de shop.

 

[László]

An update regarding "cold" factory biasing of tubes, they are 25-30mV AT IDLE, but when you start playing it goes up quite a bit, up to more then 50mV, today I've measured from 45-53 when playing at 25% volume and a bit less than 50% gain. And I was surprised that the 6L6 were Ruby tubes and not JJ as stated on Thomann.de shop.

I believe you're supposed to disconnect the guitar or pedalboard, and run the amp at idle to check the bias.

 

[shimmilou]

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.

Class A simply means that the tube is always conducting and never turns off as an AB would, and typically is 100% idle wattage, and typically Cathode biased. My example is specifically for AB circuits, at 70% idle wattage, which is typically the high end for AB amps, the lower end being 55%, which is too cold/sterile IMO.

 

[Goran Lorencin]

I believe you're supposed to disconnect the guitar or pedalboard, and run the amp at idle to check the bias.

Yes, for biasing I did exactly that (nothing connected except for the speakers), but I left the probes inside to see how the voltage behaves when playing as it does not affect anything. It was interesting to see how high the mV go when playing different chords.

 

[Goran Lorencin]

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.

Amazing film! What an operation that was. This was so incredibly sophisticated for that era, they almost had robotics, well they actually had mechanical robotics in a way ... I wonder how long it took them to build all that machinery and to train all those people?! But those were different times and discipline was at its peak imo.

 

[elvis]

Class A simply means that the tube is always conducting and never turns off as an AB would, and typically is 100% idle wattage, and typically Cathode biased. My example is specifically for AB circuits, at 70% idle wattage, which is typically the high end for AB amps, the lower end being 55%, which is too cold/sterile IMO.

Fair statement, though Class AB doesn't actually have the opposite drivers turn off, they just go to low drive, depending on the idle bias. If you bias Class AB hot enough, it is Class A. Class AB is a continuum from Class A to Class B (opposite driver turns totally off). So by setting as hot a bias point as you recommend, you are nearly Class A. If that is your preference, fair enough, but it's dissipating more power as heat, and the amp may not be properly designed to dissipate this heat. Most Class AB amps are set in the 30-35mA idle current range, not to run the tubes "cold", but to set the tone as more AB than A (punchier, less creamy) and to limit heat dissipation.

 

[László]

Amazing film! What an operation that was. This was so incredibly sophisticated for that era, they almost had robotics, well they actually had mechanical robotics in a way ... I wonder how long it took them to build all that machinery and to train all those people?! But those were different times and discipline was at its peak imo.

It really is interesting!

No doubt so much of it was about the training, and the machinery was pretty darn sophisticated. But that was the height of the Mechanical Age. I remember being a little kid and going to see the presses of the Detroit News. They were as big as a locomotive, and appeared to be very complex devices, manned by lots of workers.

Today, the workers are far fewer, and software runs things. But sophisticated manufacturing machines were pretty well-known back then.

I've seen tube videos of manufacturing processes in the Far East, and there's still a lot of hand assembly. Are the workers as well-trained? Who knows. So much tube manufacturing know-how has already passed into the Great Beyond!

If you look closely at a new tube and hold an old one next to it, you can easily see things that look different, such as thinner internal wiring, fewer (if any) stand-offs on the mica spacers, thinner legs on the assemblies, less neat assembly on the plates, etc. Since a tube is a device whose efficiency is determined by the flow of electrons from the cathode to the plate, the physical assembly seems to matter, as does even the materials coating the plate, and so on.

There's no doubt in my mind that Chinese and Russian workers are entirely capable of making very high quality products. All you have to do is look at a Soyuz microphone or an Apple Computer to see very high quality manufacture and assembly.

My guess is that any problems are a combination of know-how, the age of the machinery, and materials availability. Price pressure probably also plays a role, though back in the day, tubes weren't expensive items and there was probably pricing pressure back then, too.

I hail from the days when drugstores used to have large tube testing machines and shelves with new tubes to buy if your tube tested bad. I remember one store in particular in my neighborhood that had a whole aisle of RCA, Sylvania, and other U.S. tubes available for negligible amounts of money.

 

[shimmilou]

....I hail from the days when drugstores used to have large tube testing machines and shelves with new tubes to buy if your tube tested bad. I remember one store in particular in my neighborhood that had a whole aisle of RCA, Sylvania, and other U.S. tubes available for negligible amounts of money.

I certainly remember that too, Hooks was the drugstore. I think this, coupled with the thriving repair businesses involving tube equipment, is a pretty good indicator that the good ole days of tubes weren’t quite the romantic magical time that we would like to remember, tubes still failed just as much then as now. Many of the modern Russian and Chinese manufactured tubes are just as awesome as yesteryear tubes, and some are even repros of the famed tubes like the RCA Blackplates and GE Greyplates and seem to be just as good as the old tubes. In comparing them, I found the GT Blackplate and Greyplate tubes to be outstanding, and many times are even better than old. Sound is another subject, certainly subjective, but I find it very rare that any NOS tubes are worth the ridiculous premium price over modern production IMO. Buy from a reputable dealer and all is usually well.

 

[László]

I certainly remember that too, Hooks was the drugstore. I think this, coupled with the thriving repair businesses involving tube equipment, is a pretty good indicator that the good ole days of tubes weren’t quite the romantic magical time that we would like to remember, tubes still failed just as much then as now. Many of the modern Russian and Chinese manufactured tubes are just as awesome as yesteryear tubes, and some are even repros of the famed tubes like the RCA Blackplates and GE Greyplates and seem to be just as good as the old tubes. In comparing them, I found the GT Blackplate and Greyplate tubes to be outstanding, and many times are even better than old. Sound is another subject, certainly subjective, but I find it very rare that any NOS tubes are worth the ridiculous premium price over modern production IMO. Buy from a reputable dealer and all is usually well.

I've found quite the opposite. I certainly respect your opinion even though I disagree with it.

The reproduction tubes are not as well made, and that's easy to see if you look carefully at new v old tubes. I have some comparison closeup photos I will try to dig up that demonstrate the difference in manufacture and assembly.

From 1966-1980, I didn't have one tube go bad. Never had to replace one on any of my amps. With new tubes, replacement was frequently necessary. Tube replacement was more of a thing back in the day because everything was tube powered. Radios, TVs, car radios, you name it. While the chances of failure were smaller, there simply was a lot more stuff that had tubes in it. And the fact is that tubes wear. A lot of tube replacement was simple wear and tear on the tubes.

In terms of sonics, I find that the NOS tubes have transformed some of my amps from 85% great amps into 100% great sounding amps. This has been especially true of my Mesa amps - in fact, I didn't love the Lone Star at all until I swapped the tubes for NOS. I was ready to sell it before the tube swap. But it was also a thing with my Two-Rock amps.

On amps that started out great, like my HXDA, replacement with NOS has simply made the amp more responsive, satisfying to play, and more tonally balanced. My DG30 came equipped with NOS, so no comparison was necessary.

Admittedly, tone improvement is a subjective thing. But on a $3,000-4000 amp, I find that a few hundred spent on NOS tubes is worth the expense if it turns an amp into a lifetime keeper.

And it's entirely possible that as with all human beings, we simply listen for different things when we work on music. I spend my life in the studio. For me, small differences are easy to hear, but I'm that guy who listens for them.

 

[László]

Here’s a real NOS Mullard 12AX7. Note the tooth-shaped standoffs on the mica spacers, the wiring from the pins to the plates, etc. The purpose of the standoffs is to keep the mica from being loose and they prevent rattling inside the glass envelope.

Here’s one of my BRIMAR box plate 12AX7s. Again, note the mica spacer standoffs and the quality of the wiring from pins to plates and cathode:

This one’s a JAN (military but identical to their consumer version) GE 12AX7. Again you’ll see the mica spacer standoffs, the quality of the wiring and assembly.

This next one was one of the new stock tubes that came in my Lone Star, branded Mesa, that I replaced with NOS JAN GEas shown in the previous picture. While it has standoffs, the mica spacers are a bit warped, and the wiring is a comparative rat’s nest. This is a Chinese tube, I believe.

Next up is the new production Russian Mullard copy. The mica spacers lack standoffs. The mica appears warped. The wiring assembly isn’t the same quality as the NOS I posted earlier in the first pic.

Clearly the US and UK tubes are different from the newer tubes, and are of higher caliber construction. What isn’t shown in a pic is the coating materials that help speed the electrons on their way. I don’t pretend to know what’s different about that.

But if I can see differences in how the tubes are made, and hear differences in how they sound in use, that’s all the convincing I need.

New tubes are ringier and often more harsh in the upper midrange and high frequencies - my biggest gripe. They’re more likely to rattle and are more prone to crapping out. NOS are simply less fatiguing, they aren’t harsh, and in subtle ways, they improve my amps. So I spend the dough on them on the theory that my clients want the best tone I can offer.

 

[Simon Says]

Fair statement, though Class AB doesn't actually have the opposite drivers turn off, they just go to low drive, depending on the idle bias. If you bias Class AB hot enough, it is Class A. Class AB is a continuum from Class A to Class B (opposite driver turns totally off). So by setting as hot a bias point as you recommend, you are nearly Class A. If that is your preference, fair enough, but it's dissipating more power as heat, and the amp may not be properly designed to dissipate this heat. Most Class AB amps are set in the 30-35mA idle current range, not to run the tubes "cold", but to set the tone as more AB than A (punchier, less creamy) and to limit heat dissipation.

To say if the amp is biased cold or not - other than the current you need to know the plate voltage. Not trying to teach you how to suck eggs but this need to be stressed out

 

[Simon Says]

Here’s a real NOS Mullard 12AX7. Note the tooth-shaped standoffs on the mica spacers, the wiring from the pins to the plates, etc. The purpose of the standoffs is to keep the mica from being loose and they prevent rattling inside the glass envelope.

Here’s one of my BRIMAR box plate 12AX7s. Again, note the mica spacer standoffs and the quality of the wiring from pins to plates and cathode:

This one’s a JAN (military but identical to their consumer version) GE 12AX7. Again you’ll see the mica spacer standoffs, the quality of the wiring and assembly.

This next one was one of the new stock tubes that came in my Lone Star, branded Mesa, that I replaced with NOS JAN GEas shown in the previous picture. While it has standoffs, the mica spacers are a bit warped, and the wiring is a comparative rat’s nest. This is a Chinese tube, I believe.

Next up is the new production Russian Mullard copy. The mica spacers lack standoffs. The mica appears warped. The wiring assembly isn’t the same quality as the NOS I posted earlier in the first pic.

Clearly the US and UK tubes are different from the newer tubes, and are of higher caliber construction. What isn’t shown in a pic is the coating materials that help speed the electrons on their way. I don’t pretend to know what’s different about that.

But if I can see differences in how the tubes are made, and hear differences in how they sound in use, that’s all the convincing I need.

New tubes are ringier and often more harsh in the upper midrange and high frequencies - my biggest gripe. They’re more likely to rattle and are more prone to crapping out. NOS are simply less fatiguing, they aren’t harsh, and in subtle ways, they improve my amps. So I spend the dough on them on the theory that my clients want the best tone I can offer.

While we talking tubes - if you or anyone has a Polamp 12AX7 I will offer any money for any volume. I only have three (one in my Boogie, one spare, one to microphonic to be of any use) and I would like to have some spares. There is no tube like this one quite alike. Once I burn trough the ones I have my tone will be gone

 

[elvis]

While we talking tubes - if you or anyone has a Polamp 12AX7 I will offer any money for any volume. I only have three (one in my Boogie, one spare, one to microphonic to be of any use) and I would like to have some spares. There is no tube like this one quite alike. Once I burn trough the ones I have my tone will be gone

There is one on reverb

 

[Simon Says]

There is one on reverb

Can you link me? I only seen 12AT7 on reverb

 

[László]

It looks like Telam is the same as Polamp, so maybe search that as well? They appear to be hard to find!

 

[László]

While we talking tubes - if you or anyone has a Polamp 12AX7 I will offer any money for any volume. I only have three (one in my Boogie, one spare, one to microphonic to be of any use) and I would like to have some spares. There is no tube like this one quite alike. Once I burn trough the ones I have my tone will be gone

Here’s a link to an NOS Telam/Polamp 12AX7a made for RCA:

https://www.ebay.com/itm/RFT-TELAM-...3&pageci=e1c0ea94-b9b0-4cc0-853b-e4237954af6b

 

[Maxime Bousquet]

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

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!

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!

I advise you to have a multimeter that reads the plate current and the wattage at the same time, because when you set the wattage, the plate current also moves, look at the eurotube store it offers the multimeter I am talking about and lots of good advice too! at the bottom 60% is too cold and can create cross distortion, 70% to 75% is a correct setting and gives the lamp longer life.

 

[Maxime Bousquet]

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

the right tools for the right job!