Bias mod?

[brucesinger]

Anybody do a bias mod to their Express? I have been playing with different tube combinations lately and wonder if I should install a bias pot. Some Lonestar users have done this and claim it really opens the amp up.

 

[gadgetfreak]

another one looking for an answer

Bump

 

[Newysurfer]

Anybody do a bias mod to their Express? I have been playing with different tube combinations lately and wonder if I should install a bias pot. Some Lonestar users have done this and claim it really opens the amp up.

Well 5:50 is already opened up imo but hehe..........why don't you mod yours and report what effect it had :mrgreen:

 

[NomadExpress]

Anybody do a bias mod to their Express? I have been playing with different tube combinations lately and wonder if I should install a bias pot. Some Lonestar users have done this and claim it really opens the amp up.

Well 5:50 is already opened up imo but hehe..........why don't you mod yours and report what effect it had :mrgreen:

+1 for the 5:25 also

 

[a2dconverter]

Since my Express was out of warranty when I bought it, I modded mine with bias test points/ bias control shortly after. I do this with all of my amps. My Marshall JCM800 has an internal bias pot, but I wanted an external adjustment with test points for Cathode current and B+ to allow fast tube replacements and set-up. here's the JCM800 Mod..

I love being able to buy any tube, from any manufacturer, and fine-tune the output tubes for the best dissipation...

I do not recommend this mod due to the dangerous voltages present in a tube amp and the possibility that you will damage your amp, here's an example of what I did...

Installing the Bias Pot and the test-points is the easy part....to set the tube dissipation properly, you will need to monitor the cathode current (which will give you an approximation of the Plate current) and you will need to monitor the B+ (DC Voltage feeding the Plate). Multiplying the current and voltage will give you Watts (power).

This is an involved mod that requires you to remove the small PCB with the EL-84's attached and remove the sockets to allow you to break the connection between the Cathode and ground so that you can install 1 Ohm resistors to monitor the voltage across to help calculate the cathode current as follows...

At rest the Bias is usually set to 70% of the Max. (on an EL-84 the Max dissipation is about 12 Watts, so, at rest you want to set the dissipation to 8 Watts). I find that monitoring the cathode current is a good representation of the actual Plate Current...If I set the TP 2 mV higher than the calculation for 70% dissipation, I am in the ballpark.

Using the Transformer Bypass method to measure the Plate current I get...
[Tube 1] I = 18 mA plate current
[Tube 2] I = 18 mA plate current

Using the Cathode Resistors
[Tube 1] Voltage across the 1 Ohm resistor = 20 mV (which actually equals 18 mA plate current)
[Tube 2] Voltage across the 1 Ohm resistor = 20 mV (which actually equals 18 mA plate current)

The resistors read about 2 mA higher than the actual plate current so I set the TP's 2 mV higher.

With a 400 VDC B+ Voltage the dissipation is 0.018A x 400 VDC = 7.2W

so much for the electronics mumbo-jumbo....have fun reading!... please let me know if I can help further.
seeya

Joe

 

[Krek13]

I love being able to buy any tube, from any manufacturer, and fine-tune the output tubes for the best dissipation...(to date JJ's are my favourite adjusted to 70% dissipation at rest)

you can buy any tube from any manufacturer without the bias mod...you just need to buy tested tubes that comply with the bias of mesa express...most of online stores sell tested and matched tubes...I use JJs in my express and I buy them from eurotubes...they choose the tubes that match the bias of my amp...

 

[a2dconverter]

I love being able to buy any tube, from any manufacturer, and fine-tune the output tubes for the best dissipation...(to date JJ's are my favourite adjusted to 70% dissipation at rest)

you can buy any tube from any manufacturer without the bias mod...you just need to buy tested tubes that comply with the bias of mesa express...most of online stores sell tested and matched tubes...I use JJs in my express and I buy them from eurotubes...they choose the tubes that match the bias of my amp...

yeah, of course I know that... , but, I've been working on tube amps for over 30 years and I love to "tweak"!

here's my highly modified 1982 JCM800...

The bias mod on the Express 5:25 or 5:50 is not for the faint of heart, but, there is an advantage to being able to run the tubes hotter or colder for experimentation... especially if you have an old set of tubes that you can "burn" to get a specific sound in the studio.

Even running colder bias has a specific sound that some people might prefer.

Anyway, brucesinger asked a question and I thought I'd share my experience...

seeya

Joe

 

[Krek13]

that JCM looks sexy 8)

 

[gadgetfreak]

Joe you are a very intelligent man. I now know that I will not be getting this mod done, Reason being I don't trust any amp tech around me to do the work you have done and do it properly. I am satisfied with the amp as is but like you love to tweak and get different sounds. Thanks so much for answering this question and providing as much information as you did, I found your post extraordinary. Thanks and nice amps!!!

 

[edward]

Hey there Joe,

What a clean and well-thought-out "panel" you've created ...kudos! I was thinking, "wow, that looks great and I'd love to do that" ...then I read on. Um, yeah. I think doing the "pretty part" of the mod is exatly what you said, the easy part. I think I'll pass on the rest
Besides, I very happy with my tube recipe I have arrived at ...so long as no one lets me hear their Express with properly biased tubes, I think I'll stay happy

Edward

 

[a2dconverter]

Thanks for all of the kind comments!

seeya

Joe

 

[h_ruukki]

Hey there Joe,

Besides, I very happy with my tube recipe I have arrived at ...so long as no one lets me hear their Express with properly biased tubes, I think I'll stay happy

Edward

The way I read the 5:25 manual, I'll never think of tweaking things that have been tweaked well enough by some of the best professionals in the amp business. Why would the amp sounds so great otherwise? I understand that since the model is not the most expensive it must have some compromise... but I do trust them when they claim the amp is biased and schmiased all through without me or anyone else trying to improve it. Just my opinion, though.

 

[a2dconverter]

Hey there Joe,

Besides, I very happy with my tube recipe I have arrived at ...so long as no one lets me hear their Express with properly biased tubes, I think I'll stay happy

Edward

The way I read the 5:25 manual, I'll never think of tweaking things that have been tweaked well enough by some of the best professionals in the amp business. Why would the amp sounds so great otherwise? I understand that since the model is not the most expensive it must have some compromise... but I do trust them when they claim the amp is biased and schmiased all through without me or anyone else trying to improve it. Just my opinion, though.

As far as tubes go, the biasing only affects the Power Tubes... so you can still swap out preamp tubes without worry.

You are correct that Mesa amps leave the factory, fully tested, with Mesa tubes installed...when you have to change the power tubes, sometimes it is hard to trust whether a non-Mesa tube, that is supposed to be compatible with your specific Mesa amp, is actually the right tube and is operating properly...

With Bias Test Points you can verify that whatever tubes you buy, even if they are Mesa Factory tubes, are working properly and are closely matched. This is the only way to guarantee that you are getting the most out of the expensive tubes that you are buying that claim to be Mesa-compatible and properly Matched Pairs or Quads.

If you do have the ability to bias your amp, which many other amps (Marshall, Fender, Hiwatt, etc.) do have, it's nice to have power tube options without worrying about buying specific "color, number or recipe" of Mesa compatible tubes from different retailers.

You can just pick one off the shelf, or pick one from any manufacturer in your NOS tube collection, and know that you are not damaging the power tube or the amplifier because you can measure the power dissipation and adjust the Plate Current as required.

I like to tweak... so, I always mod all of my amps with Bias and B+ Test-points, an external bias adjustment pot and Hi-frequency Snubber circuit. That way I can keep and eye on things and possibly forsee a failure before it happens. :shock:

 

[h_ruukki]

I like to tweak... so, I always mod all of my amps with Bias and B+ Test-points, an external bias adjustment pot and Hi-frequency Snubber circuit. That way I can keep and eye on things and possibly forsee a failure before it happens. :shock:

Gotcha. On the other hand, for the average user, Mesa tubes won't cost too much and getting them is nowadays no problem, either. I do understand the need to tweak everything we can get our hands on, of course. To make it quite clear, I don't exactly know why I stuck my nose into this discussion... I'm probably trying to save someone (as inexperienced as I am myself) from a lot of pain by trying to suggest that Mesas don't need tweaking. I don't even know anything about the business, except for what I'm reading here and elsewhere right now. In any case, quoting from the manual (for those who have second-hand Expresses without a manual and no better information): "So the end of the short answer is: Since a bias supply needs to put out the right voltage and never vary, I wanted to build amplifiers that were individually hard wired to the correct values and NEVER needed adjustment. And for 25 years, that's how MESA/Boogies have been built. Time to change tubes? Just plug our tubes into one of our amps and you're DONE."

Mind you, just in case, that I'm not trying to be snide or to belittle anyone's knowledge. Just bringing up what I just learned myself. Plus, I get the point about being able to foresee a failure.

Also, the warning label is the best so far!

Henri

 

[a2dconverter]

Thanks Henri

I like to make analogies to hot rods... before fuel injection, in the old days of the carburetor, the mechanic had to adjust the air to fuel ratio to get the car to perform at it's peak... but all of this is moot if you don't have gas gauge to tell you how much fuel you have left.

 

[a2dconverter]

Thanks Henri

I like to make analogies to hot rods... before fuel injection, in the old days of the carburetor, the mechanic had to adjust the air to fuel ratio to get the car to perform at it's peak... but all of this is moot if you don't have gas gauge to tell you how much fuel you have left.

I think Mesa makes their amps non-adjustable so that they can sell more tubes....just like HP makes you buy their printer cartridges....consumables make up a great portion of their businesses.

 

[h_ruukki]

I think Mesa makes their amps non-adjustable so that they can sell more tubes....just like HP makes you buy their printer cartridges....consumables make up a great portion of their businesses.

That is, of course, possible... but at least the first set of tubes has a half-year warranty. Also, when I had my Mark II, I never swapped any tubes and it sounded just great for maybe 3 years. It was already second-hand then. I'm all for healthy scepticism, but I tend to trust Mesa based on my experience. I hope they won't prove me wrong! Here's to quality products!

Henri

 

[Newysurfer]

Thanks Henri

I like to make analogies to hot rods... before fuel injection, in the old days of the carburetor, the mechanic had to adjust the air to fuel ratio to get the car to perform at it's peak... but all of this is moot if you don't have gas gauge to tell you how much fuel you have left.

I think Mesa makes their amps non-adjustable so that they can sell more tubes....just like HP makes you buy their printer cartridges....consumables make up a great portion of their businesses.

ahh please explain your logic here - don't understand

I swapped out the Mesa tubes within 6 months of buying mine.
Made a big improvement and will never go back to Mesa tubes.
The great thing about fixed bias is that I could do it all myself in 30 mins and didn't need an amp tech.
Long live fixed bias :mrgreen:

 

[a2dconverter]

Thanks Henri

I like to make analogies to hot rods... before fuel injection, in the old days of the carburetor, the mechanic had to adjust the air to fuel ratio to get the car to perform at it's peak... but all of this is moot if you don't have gas gauge to tell you how much fuel you have left.

I think Mesa makes their amps non-adjustable so that they can sell more tubes....just like HP makes you buy their printer cartridges....consumables make up a great portion of their businesses.

ahh please explain your logic here - don't understand

I swapped out the Mesa tubes within 6 months of buying mine.
Made a big improvement and will never go back to Mesa tubes.
The great thing about fixed bias is that I could do it all myself in 30 mins and didn't need an amp tech.
Long live fixed bias :mrgreen:

Firstly, I'm a tech so my comments are biased (pun intended)..I will agree that if you live outside of a big city where amp techs are not easily available then being able to plop in a 3rd party Mesa graded tube is great!

I waited until my amps were out of warranty to modify them...now I can plop in any tube and when I am long-gone...the new owner of my amp, or the amp tech that works on that amp, will be able to plop in any tube and adjust the idle dissipation with just a voltmeter! If they can figure out what I've done!

You did say that you swapped the tubes from a Factory Set with a 3rd Party Mesa Graded set and it sounds better to you, this means that you may have a set of tubes that draw more or less current than the Mesa Factory set. Which means that technically you have adjusted the bias...But by how much?...

Fixed Bias is only fixed if you replace the tubes with tubes that draw the same current. Otherwise, all parameters in the output stage change...most importantly the idle current / power dissipation.

Here's a few questions...
Is it possible that another set of tubes might sound even better to your ears? What current are those tubes drawing?

If you find that Holy Grail power tube, will you be able to get another? or, maybe the Holy Grail power tube is a NOS tube that is not available in 3rd party Mesa Grading? So you'll never know (but life is not over!)

Can the tube manufaucturer guarantee that when you buy specific tubes that they will draw the same current? Therefore, giving you the same performance? (or maybe the manufacturer grades them within a certain current range, maybe within 5-10 mA)(or maybe you won't be able to tell the difference!)

Third Party Tube Vendors don't tell you what the current draw is for a specific tube because that depends on the B+ Voltage that they used to test/ grade the tube...what if your amp's B+ Voltage is a lot higher? (They might assume that an Express 5:25 B+ is 400 VDC, but yours might be 420 VDC which means that the tube will run colder in your amp) They couldn't possibly have such a range of graded tubes to cover all of MESA's amp lines and varying B+ Voltages. :shock:

Wouldn't it be nice if you could order some esoteric NOS power tubes and just set them to the proper idle current and know that if you bought another set that you could adjust them to the same idle current and get the same sound?

Many other manufacturers make biasing easier, even for the amp owner, by adding external test points and bias pots for just this reason.

There are also manufacturers, including Marshall and their new Slash amp, and Bugera's Infinitum amps, which have an auto bias feature which sets the idle current correctly, using a microprocessor, regardless of which tube you plug-in.

All of this is futile if you can't tell the difference in an amp's sound when you swap different tubes in and out and this is also futile unless you have an amp tech install test-points, cathode resistors and a bias pot. Buying tubes from Mesa is ok too!

Anyway Fixed Bias has always been an interesting point of discussion as we have proved here!

seeya

 

[Newysurfer]

Thanks Henri

I like to make analogies to hot rods... before fuel injection, in the old days of the carburetor, the mechanic had to adjust the air to fuel ratio to get the car to perform at it's peak... but all of this is moot if you don't have gas gauge to tell you how much fuel you have left.

I think Mesa makes their amps non-adjustable so that they can sell more tubes....just like HP makes you buy their printer cartridges....consumables make up a great portion of their businesses.

ahh please explain your logic here - don't understand

I swapped out the Mesa tubes within 6 months of buying mine.
Made a big improvement and will never go back to Mesa tubes.
The great thing about fixed bias is that I could do it all myself in 30 mins and didn't need an amp tech.
Long live fixed bias :mrgreen:

Firstly, I'm a tech so my comments are biased (pun intended)..I will agree that if you live outside of a big city where amp techs are not easily available then being able to plop in a 3rd party Mesa graded tube is great!

I waited until my amps were out of warranty to modify them...now I can plop in any tube and when I am long-gone...the new owner of my amp, or the amp tech that works on that amp, will be able to plop in any tube and adjust the idle dissipation with just a voltmeter! If they can figure out what I've done!

You did say that you swapped the tubes from a Factory Set with a 3rd Party Mesa Graded set and it sounds better to you, this means that you may have a set of tubes that draw more or less current than the Mesa Factory set. Which means that technically you have adjusted the bias...But by how much?...

Fixed Bias is only fixed if you replace the tubes with tubes that draw the same current. Otherwise, all parameters in the output stage change...most importantly the idle current / power dissipation.

Here's a few questions...
Is it possible that another set of tubes might sound even better to your ears? What current are those tubes drawing?

If you find that Holy Grail power tube, will you be able to get another? or, maybe the Holy Grail power tube is a NOS tube that is not available in 3rd party Mesa Grading? So you'll never know (but life is not over!)

Can the tube manufaucturer guarantee that when you buy specific tubes that they will draw the same current? Therefore, giving you the same performance? (or maybe the manufacturer grades them within a certain current range, maybe within 5-10 mA)(or maybe you won't be able to tell the difference!)

Third Party Tube Vendors don't tell you what the current draw is for a specific tube because that depends on the B+ Voltage that they used to test/ grade the tube...what if your amp's B+ Voltage is a lot higher? (They might assume that an Express 5:25 B+ is 400 VDC, but yours might be 420 VDC which means that the tube will run colder in your amp) They couldn't possibly have such a range of graded tubes to cover all of MESA's amp lines and varying B+ Voltages. :shock:

Wouldn't it be nice if you could order some esoteric NOS power tubes and just set them to the proper idle current and know that if you bought another set that you could adjust them to the same idle current and get the same sound?

Many other manufacturers make biasing easier, even for the amp owner, by adding external test points and bias pots for just this reason.

There are also manufacturers, including Marshall and their new Slash amp, and Bugera's Infinitum amps, which have an auto bias feature which sets the idle current correctly, using a microprocessor, regardless of which tube you plug-in.

All of this is futile if you can't tell the difference in an amp's sound when you swap different tubes in and out and this is also futile unless you have an amp tech install test-points, cathode resistors and a bias pot. Buying tubes from Mesa is ok too!

Anyway Fixed Bias has always been an interesting point of discussion as we have proved here!

seeya

ok I hear you and understand you're an amp tech.
But for me as a player / user you're over complicating things. It's pretty simple for me

THe stock Mesa tubes are just very ordinary Chinese one's - not crap - not great - and in Australia Mesa charge an absurd $70 per power tube and about $40 per preamp tube (no BS).
$340 for a crap tube set. It's a Mesa disgrace.

After 6 months of buying a 5:50 I requested a recommendation from Doug at http://www.dougstubes.com for a complete new tube set that would
a) greatly reduce amp hum / noise
b) increase gain at top end
c) keep same cleans

Doug recommended a mixed set of preamp & power tubes specifically for the 5:50.
He took all the current draw / idle dissipation factors into consideration.
This cost me $110 for high quality tubes.

I've got same advice from Doug on 4 other amps over the years - Fender Twin, Vox AC30, JCM800 and Egnater Rebel 30.
Doug is great imo and his advice is always spot on.
Power tubes he recommended for the 5:50 were either SED 6L6s or the TADs.
I got the SED.

I bought em, inserted them - big improvement - did exactly what Doug said they would and much more.
They also improved tone thru the whole range - smoother , clearer, warmer. Cut amp hum by 60-70%.
I then reported all this to this Forum.
Many other people tried them and reported same improvement.

Very happy and will never return to Mesa stock tubes.
3 years later I've never used the warranty but if I do I'll simply replace the stock Mesa tubes back in before I send it off.
How long is the warranty period anyway ??? - can't remember now

As for the rest of your points - yes I understand but just don't care - not important to me.
What I do is simple and works.
And even though Mesa try their best to rip Aussies off with their absurdly priced Chinese tubes they make up for it partly by the fixed bias.
Then I don't have to pay $50 and drag my 5:50 to an amp tech, wait for it for a week, then go pick it up.

Look I know this loses business for amp techs like you - tube work is your bread n butter but.... its obviously much better for the consumer :mrgreen: