Good day everyone.
Please advise on the following. In principle, the question generally applies to tube amps for guitar, but in this particular case I will take the example of the Mesa Boogie Dual Rectifier.
In many cases it is already possible to adjust the bias for the power tubes yourself by means of a trimpot. An example is the Marshall JCM2000 series.
However, at Mesa they believe in fixed bias. By default nothing can be adjusted by the user and Mesa would like you to buy the tubes through them. This means that nothing more needs to be adjusted.
Now it is of course possible to include a trim pot in the current design, so that it is still possible to adjust the bias yourself.
But now I came across the following.
https://www.audioamp.eu/module-ab-q...3vac-filament-supply-is-referenced-to-ground/
This makes it possible to set a bias that is desired. This circuit then regulates the correct bias voltage for each tube.
The advantages according to the manual:
The advantage of using the AB-Qi module is that it will extend output tube life, in a lot of cases it will also improve the sound and last but not least, you will never have to worry about adjusting the bias again.
• Instead of a pair of output tubes being biased together, now all output tubes are individually biased for greater control and stability
• No need to keep an eye on and/or adjust the bias of the output tubes
• Significant extension of the output tubes life
• Bias is not influenced by the input signal
• Undistorted and improved sound for a great listening experience
• Reduction in hum and noise level of the amp
• No maintenance or additional adjustments are required after the initial setup
• Improved overall operation of the amp due to the reduction in the danger of a tube red plating
• The AB-Qi ensures that bias is kept at the pre-set value (see Initial Setup) even as the tubes age
• The AB-Qi comes with a soft start feature, which slowly brings up bias to each output tube
• The AB-Qi comes with its own 6.3VAC to12.6VAC isolation transformer, designed for use in an amp where the filament supply is referenced to ground
Now I have gathered some information about this and have adapted the schematic accordingly.
Below the original and below the modified schematic.
Besides the benefits that the designer claims, I would like to achieve the following with it; I would like to decide for myself which “brand” tube I want to use. So I don't want to be dependent on Mesa. In addition, it should also be possible in this way when a tube is destroyed, I do not have to order a quartet of tubes immediately, but because of the separate bias I can only replace the fallen tube.
Does anyone have experience with this system? What are your ideas about this? Will this work (well) or will it cause a lot of problems? I like to hear it.
Original:
Modified:
Please advise on the following. In principle, the question generally applies to tube amps for guitar, but in this particular case I will take the example of the Mesa Boogie Dual Rectifier.
In many cases it is already possible to adjust the bias for the power tubes yourself by means of a trimpot. An example is the Marshall JCM2000 series.
However, at Mesa they believe in fixed bias. By default nothing can be adjusted by the user and Mesa would like you to buy the tubes through them. This means that nothing more needs to be adjusted.
Now it is of course possible to include a trim pot in the current design, so that it is still possible to adjust the bias yourself.
But now I came across the following.
https://www.audioamp.eu/module-ab-q...3vac-filament-supply-is-referenced-to-ground/
This makes it possible to set a bias that is desired. This circuit then regulates the correct bias voltage for each tube.
The advantages according to the manual:
The advantage of using the AB-Qi module is that it will extend output tube life, in a lot of cases it will also improve the sound and last but not least, you will never have to worry about adjusting the bias again.
• Instead of a pair of output tubes being biased together, now all output tubes are individually biased for greater control and stability
• No need to keep an eye on and/or adjust the bias of the output tubes
• Significant extension of the output tubes life
• Bias is not influenced by the input signal
• Undistorted and improved sound for a great listening experience
• Reduction in hum and noise level of the amp
• No maintenance or additional adjustments are required after the initial setup
• Improved overall operation of the amp due to the reduction in the danger of a tube red plating
• The AB-Qi ensures that bias is kept at the pre-set value (see Initial Setup) even as the tubes age
• The AB-Qi comes with a soft start feature, which slowly brings up bias to each output tube
• The AB-Qi comes with its own 6.3VAC to12.6VAC isolation transformer, designed for use in an amp where the filament supply is referenced to ground
Now I have gathered some information about this and have adapted the schematic accordingly.
Below the original and below the modified schematic.
Besides the benefits that the designer claims, I would like to achieve the following with it; I would like to decide for myself which “brand” tube I want to use. So I don't want to be dependent on Mesa. In addition, it should also be possible in this way when a tube is destroyed, I do not have to order a quartet of tubes immediately, but because of the separate bias I can only replace the fallen tube.
Does anyone have experience with this system? What are your ideas about this? Will this work (well) or will it cause a lot of problems? I like to hear it.
Original:
Modified:
There was a long thread about valve autobias circuits on the Dutch forum zelfbouwaudio.nl/forum a couple of years ago. It turned out that some autobias circuits that are sold commercially are impossible to get stable, others work but much aggravate high-order distortion, others only work well in class A, yet others work perfectly but are quite complicated. In the end the thread turned into a common design session working out an idea that is simple and works perfectly satisfactory for normal music signals.
I don't know in which category your module belongs. It could be great, it could be crap.
I don't know in which category your module belongs. It could be great, it could be crap.
Fixed bias can be a great thing, particularly if the extra punch is required of amps.
And controlling the grid bias to prevent saturation in relation to power supply "sag" is also easy.
David Gillespie's novel and quite simple regulation method works like a charm for me on one of my tube amps.
And controlling the grid bias to prevent saturation in relation to power supply "sag" is also easy.
David Gillespie's novel and quite simple regulation method works like a charm for me on one of my tube amps.
Do not see why a module can not do a self calibration at startup and then leave the bias there for the remander of the day. If you have a hifi amp it may need to re-calibrate after a period of time if you leave the amp on continuously.
Yes, that was one of the methods that should work. I haven't a clue if it is the method used on Ricotjuh's module, though.
I don't recall Mesa selling me an amp with a note "always use #7 power tubes in this amp." You were never dependent upon Mesa. Mesa made amps with non-adjustable bias for years. It is one thing to fidget with hifi amps, but this is only a guitar amp, not precision lab gear. This amazing circuit says it will be better? What is better? More like hifi? This is guitar amp land, where recordings have been made with amps having holes poked in the speakers with a pencil. Who is going to tell Eddie Van Halen that when he runs amps with only three power tubes instead of four, it will be better if he uses all four and does a bunch of bias fiddling? And so on.
There is no "it must be THIS way" standard for bias. That 70% rule is just internet lore. Surely Fender never shipped amps set that hot.
There is no "it must be THIS way" standard for bias. That 70% rule is just internet lore. Surely Fender never shipped amps set that hot.
I certainly don't tell the dead anything.
They'd put me in a padded cell if I tried to discuss something with Van halen.
Well, a servo output tube bias meant for a HiFi audio amp may behave differently in a guitar amp, depending on how you play through it. Thinking the % of time that 100W output stage is going to be at the high end of it's power, when pushed with an electric guitar vs "music". The circuit take an average of the bias current and compares that to a reference set point. It may have to average out more wild swings with the guitar, is all I'm thinking.
You could setup 4 bias controls, one for each tube and a small value resistor between the cathode and ground, with a rotary switch and a panel meter. That way you can check and adjust yourself as you warm up that beast at each gig.
The only reason they dont provide that is it would cost more, than just running all 4 off the same (-) voltage. Perhaps plus the fact most customers wouldnt know what to do with it -
You could setup 4 bias controls, one for each tube and a small value resistor between the cathode and ground, with a rotary switch and a panel meter. That way you can check and adjust yourself as you warm up that beast at each gig.
The only reason they dont provide that is it would cost more, than just running all 4 off the same (-) voltage. Perhaps plus the fact most customers wouldnt know what to do with it -
Another reason is that if an amp is properly designed for the task, it's not a needed item.
Have you asked Pavel, the designer?
My concern would be the large signal swings of guitar, especially with boost and other pedals if used, would be causing the autobias board to constantly adjust. In that case, you might find the tone suffers tremendously.
I'd add adjustability to the existing bias circuit instead. The bias supply feeds all 4 output tubes, so you still want a pretty well matched set.
My concern would be the large signal swings of guitar, especially with boost and other pedals if used, would be causing the autobias board to constantly adjust. In that case, you might find the tone suffers tremendously.
I'd add adjustability to the existing bias circuit instead. The bias supply feeds all 4 output tubes, so you still want a pretty well matched set.
I own 2 of Pavel's autobias boards.
There is a long thread in the audiokarma tube hifi forum on this autobias unit. Pavel was very responsive to requests about his device. I second GKTAUDIO's suggestion...contact Pavel.
Steve
There is a long thread in the audiokarma tube hifi forum on this autobias unit. Pavel was very responsive to requests about his device. I second GKTAUDIO's suggestion...contact Pavel.
Steve
You have to consider that some musicians tend to LIKE distortion caused by overloads, and disregard any "hifi" applications.
Absolutely, Wiseoldtech. I'm one of them. All the amps I build are intended to generate pleasing distortion. One reason voltage and current regulation isn't really used on guitar amps (other than chokes).
Thanks everyone for the contribution.
Yes I have been in contact with Pavel.
I had put the question to him and he then came up with an outline of the situation. I then drew it in an overview and it can be seen in the opening post.
And I completely agree that a guitar amplifier should/may have completely different specifications than a hi-fi amplifier. And certainly in the case of a mesa, in many cases the intention is to let the power tubes overdrive a bit. Especially in the Hi-Gain sound.
And my preference is to keep it as simple as possible.
However, I thought that the bias voltage was independent of the input signal and should therefore remain the same at all times.
I see a lot of people doing a bias mod to adjust the cathode current. And then the question remains whether it offers added value to continuously monitor the bias and adjust it where necessary.
But if I hear you like that, this is not desirable with a guitar amplifier.
Yes I have been in contact with Pavel.
I had put the question to him and he then came up with an outline of the situation. I then drew it in an overview and it can be seen in the opening post.
And I completely agree that a guitar amplifier should/may have completely different specifications than a hi-fi amplifier. And certainly in the case of a mesa, in many cases the intention is to let the power tubes overdrive a bit. Especially in the Hi-Gain sound.
And my preference is to keep it as simple as possible.
However, I thought that the bias voltage was independent of the input signal and should therefore remain the same at all times.
I see a lot of people doing a bias mod to adjust the cathode current. And then the question remains whether it offers added value to continuously monitor the bias and adjust it where necessary.
But if I hear you like that, this is not desirable with a guitar amplifier.
Most gain occurs in the preamp stage. Preamp tube distortion has a different character than power tube distortion, and both have their adherents.
Then cathode bias is the answer, but not readily accomplished on your Dual Rec, nor advisable IMO.
Bias is set at quiescent level. It will always vary with signal level because of changing voltages and currents in the tube in response to signal in.
Example, please. Are you referring to cathode biased amps, or a mix of grid and cathode biasing?
It sounds like your primary goal is to be able to adjust bias so you can use different tubes. I think the autobias board is getting you onto a side-track. Why not just add the appropriate resistor/trimmer network to achieve this? Done successfully, you should only have to bias your tubes occasionally, assuming the amp functions properly.
When a grid is driven into clipping/saturation (positive voltage in relation to cathode), the tube is prone to distorting until the coupling cap can lower its charge from the previous overdriven stages.
The amount of time that it takes to "settle back down" is pretty much dependant on the size of the coupling cap, and the following grid-to-ground resistance.
People go nuts (internet-driven blabber) with using high value coupling caps (0.22uF - 0.47uF?) thinking that their "bottom end" will improve and rediculously allow the amp to play into subsonic regions.
The result is lousy performance and higher chance of overload distortion.
In reality.... and I like to stay in that sane area, smaller-value coupling caps are actually better (0.047uF being common maximum) and don't seem to suffer from low-end or overdriven issues.
Tubes are extremely high impendence devices compared to BJT transistors, and don't need those silly high amounts of grid coupling caps.
And as I previously mentioned, David Gillespie's neat and effective power supply biasing modification works amazingly well to help control the effects of overdrive distortion.
The amount of time that it takes to "settle back down" is pretty much dependant on the size of the coupling cap, and the following grid-to-ground resistance.
People go nuts (internet-driven blabber) with using high value coupling caps (0.22uF - 0.47uF?) thinking that their "bottom end" will improve and rediculously allow the amp to play into subsonic regions.
The result is lousy performance and higher chance of overload distortion.
In reality.... and I like to stay in that sane area, smaller-value coupling caps are actually better (0.047uF being common maximum) and don't seem to suffer from low-end or overdriven issues.
Tubes are extremely high impendence devices compared to BJT transistors, and don't need those silly high amounts of grid coupling caps.
And as I previously mentioned, David Gillespie's neat and effective power supply biasing modification works amazingly well to help control the effects of overdrive distortion.
And here's one last response from Pawel:
For bias voltages -51V 6L6 and -39V EL34 at anode voltage 400V, the
cathode current at both tubes will be 20 mA.
This is how the tubes are set up for very cold operation and maximum
durability.
Because the operating points are at the beginning of the anode
characteristics, the tubes should be paired.
The amplifier set in this way works with a larger distortion (about 8 to
12%), which does not matter with a guitar amplifier (sometimes it is
even required).
If you use an autobias module, you set the cathode current to 20 mA and
you don't have to pair the tubes. It will also not matter if you use
tube 6L6 or EL34. Atobias always sets the cathode current to 20 mA in
the same way for all tubes.
Pavel
For bias voltages -51V 6L6 and -39V EL34 at anode voltage 400V, the
cathode current at both tubes will be 20 mA.
This is how the tubes are set up for very cold operation and maximum
durability.
Because the operating points are at the beginning of the anode
characteristics, the tubes should be paired.
The amplifier set in this way works with a larger distortion (about 8 to
12%), which does not matter with a guitar amplifier (sometimes it is
even required).
If you use an autobias module, you set the cathode current to 20 mA and
you don't have to pair the tubes. It will also not matter if you use
tube 6L6 or EL34. Atobias always sets the cathode current to 20 mA in
the same way for all tubes.
Pavel
As I mentioned in post #2, some poorly designed autobias circuits aggravate high-order distortion. They do so particularly at low levels. This applies to circuits that compare the momentary current through each valve with the set value and then try to control the duty cycle of the comparator output by controlling the grid bias voltage. Maybe such a design would result in an interesting sound in a guitar amplifier.