Active Bias Circuitry

[Dave H]

I have micro-controller simulators and have made my acquaintance with programming them, although I presently lack the means to make PCBs with fine enough traces for them. So I have yet to actually build any uC circuits. I'm mostly just considering transistors and related stuff at the moment.

You need something like this. It's not too hard after a little practice to solder in the uC using the 'swipe' technique.

[exclamationmark]

I doubt the Fender auto bias uses anything as fancy as a FFT. I think it just sets the bias with the audio muted, when you come out of standby.

That's probably true. I can't see them using anything more than an 8 bit controller that costs more than 10 cents (probably those horrible wire bond package things with a big black dot of epoxy over the top of it). A radix 2 FFT is pretty much a glorified sorting algorithm with some addition and multiplication thrown in. Many MCU's can do 256 point FFT's in under a millisecond if they have the hardware to do so (many ARM cores and the higher end PIC32's can do 512 point's in under half a millisecond!) - the main limiting factor is usually amount of RAM (and SRAM is typically the most expensive part of the microcontroller die). That being said, even an 8 bit MCU can do FFT's in a reasonable time frame if it has enough ram (I doubt you would monitor the bias more than once every few seconds anyway).

However, I don't know what you would gain with a system that senses bias (unless you wanted it to do unconventional things), that you couldn't achieve with a potentiometer and LED's that signify a happy amplifier, as per RG's experience. Some unusual things I could think of include a tremolo system (where you could wiggle the bias dependent upon literally anything you can measure) or anything that involves injecting a signal at the cathode like a ghetto signal mixer.

[FredB]

I

Fred I'm pretty sure there's a chip in the UC38xx series that would do what you want. 3842? 3854? I forgot. The LM257x "simple switchers" also can run at 100kHz, and they look pretty stable. I've used them to power some quite demanding analog circuitry.

I remember looking at the UC38xx Series of controllers. They are the grand old workhorse of controllers. There is a newer upgraded series based on them. For some reason I had trouble getting them to work quite right in simulation. It was probably issues with the models.

I also had trouble modeling the Simple Switchers. I know there's the online solution for them, although my operating parameters for a tube amp supply is outside the bounds of the online design tool.

I found I could get the the Linear's LT1725 model to do what I wanted, although only open loop. The controller has current limiting, that effectively slow starts the process. So with that chip is just a matter of regulating the output separately, or running it open loop as a saggy flyback.

There is another LT chip, I'm having trouble finding my sim for it, that I did get the feed back loop running without generating sub-harmonics, although it was too small for me to work with without fabbing a PCB for it.

[FredB]

You need something like this. It's not too hard after a little practice to solder in the uC using the 'swipe' technique.

[ATTACH=CONFIG]22145[/ATTACH]

Yes, I saw the EEV_Blog vids on doing that. I'll have to look into those adapter boards.

[Gingertube]

Did you mean a bias servo??

Bias servos are a little tricky on the current sense side in a Class AB amp where the tubes cut off for part of the cycle. Basically the "traditional" method is to clamp the current sense at 2 x the idle current to "balance" the action of cut off.

A more modern method is to clamp the current sense in a much narrower window around the idle current point. That requires more gain in the servo as a smaller error signal has to be able to develop the full bias voltage.

I'm one of those designers who can't see the point of putting in a microcontroller when a handful of analog will do the same job.

Here is an example of someone elses design:

Cheers,
Ian

P.S. There is also some stuff on bias servos on the John Broskie Tubecad website which is worth looking at if interested.

[FredB]

Well, I suppose I meant a B+ servo, to implement an AB sag dynamic into a SE amp. It senses the current in the output tube via a 1 ohm cathode resistor, like a bias tap resistor. It could well be the bias tap resistor for that matter.

The servo circuit then modulates the output impedance of the power supply, via a pass device, although once I worked that out, I realizes that the same effect could be had though modulating the bias using the same monitoring method.

Although from another thread, I just learned that power supply sag, and bias squish are different things, that could sound well different.

That circuit's interesting, what is the purpose for it?

To constantly maintain a fixed bias, regardless of power supply sag?

I suppose it could be modified to create an output sag though bias modulation.

How would you re-bias the tubes, by changing the voltage reference?

[Gingertube]

FredB,
Yes to keep a fixed idle current in all power tubes. Each tube gets its own copy of that circuit, all of them are fed from a common reference (and common raw bias supply). So one adjustment adjusts all tube idle currents simultaneously AND idle current balance in the output is guranteed regardless of whether you use matched tubes or not.
Example: Set 0.4V on the reference and all tubes will idle at 40mA

I can't think offhand how to do your AB Sag dynamic in a SE Amp simply. The problem being that a SE Amp is Class A by definition and has a constant current draw. It would almost certainly require a MOSFET regulator which was controlled dynamically.
You could set up the MOSFET as a current regulator to control the current into the final flter cap OR as a true series voltage regulator after the final filter cap but in either case you would need to feed the regulator with the dynamic control signal. Coupling the signal to the mosfet is fairly easy using an optocoupler (which aslo gives you isolation between the control circuitry and the high voltage sections).
In the first instance I would probably try the current control into the final filter cap method.

Cheers,
Ian

[FredB]

I see, that would be good for toroidal output transformers.

You are insightful, that's exactly how I worked out the sag generator with a sort of level monitor from dynamic compressor circuits, an opto-coupler from SMPS circuits, and the sag control from the power scaling circuit.

Easy-peazy...

[imaradiostar]

Another vote here for Schmart boards- I have one on my desk at work right now.

I'm not sure about the swipe method though. I guess it's fine for the pre tinned schmart boards but for other small run SMD stuff at work I end up flooding the pins with solder and wicking off the excess.

I have a tube distortion pedal built with a nixie power supply and the whole light load audio modulation noise thing is a mess. I found that by running the SMPS at only 6 volts in (and 200 out) I was able to prevent at from making awful noises. I added a 7806 to the pedal to make it possible.

I wish I could convince someone out there to make a small 200 to 400 volt smps for musical instrument use, with an emphasis on keeping things quiet for audio. I haven't the time to do it myself!

jamie

[exclamationmark]

There's a couple problems with those "one size fits all" adapter boards - mainly the lack of positions for decoupling capacitors and oscillators. The schmartboard's solve this problem to a degree, in that there are some pads for decoupling caps, but the difficulty of fitting an oscillator is still a problem. Personally I think they're sort of a rip off - If you wanted to, you can simply copy the layout and get them fabbed at one of the numerous chinese services. A lot of places offer 5x5cm boards for about $10 for 10 copies. Of course the wait time is a major inconvenience and most people will pay a premium for it.

I've had similar experiences with those nixie power supplies - I have a feeling that they're essentially the bare minimum for a switchmode converter to even function, and since it's a for a nixie tube, noise is probably irrelevant. I found if you stick a suitably sized resistor so it draws a large quiescent current (compared to transients), you can sort of stop the horrendous noise (presumably from the feedback loop spazzing out). A couple of companies make SMPS's for tube audio, but reviews are scarce - I dunno if I'd trust them.

[FredB]

I recall that some one has already done that. They sell them for about $75. If I can find it, I post a link.

Here's something else I just found:



Did you sim your nixie circuit?

Have a schematic?

The 555 Nixie supply was never meant for audio appications. I had to completely redesign it in order to get it to work well for pedals. I have yet to build one though. All my bread boards are presently full of various other things.

I realize what your talking about by using 6 volts input. The 555 Nixie power supply regulates the output voltage by turning the oscillator off and then back one. The horrible noise comes from the number of times a second that the oscillator shuts down and then starts back up. With a SMPS controller chip that I did bread board had a bust mode, which is basically the same idea. The frequency of the bust mode turning on and off under light load conditions turned out to be in the audio spectrum as well.

You see by lowering the input voltage down to 6V your are making the the whole circuit work harder to maintain the output voltage. As a result it avoids ever cycling the oscillator off and then back on, which is what creates that horrible noise.

My fix was to switch the switching element from a MOSFET to a BJT, then regulate the output by biasing down the BJT, while leaving the oscillator running full time. Like magic the noise goes away, although the new feed back loop can take a while to tune.

[Steve Conner]

Re emulating sag in a single-ended amp: The average plate current of a single-ended power stage might not change all that much when you drive it, but the screen current certainly will. So put a RC time constant on the screen supply.

[FredB]

If the amp is being designed you could increase the plate voltage and then use a large value screen grid resistor to create a larger than usual drop due to the screen grid current increase under load. I'll have to sim that too.

[imaradiostar]

If the amp is being designed you could increase the plate voltage and then use a large value screen grid resistor to create a larger than usual drop due to the screen grid current increase under load. I'll have to sim that too.

or, taking a page out of the Bob Pease book, you could just breadboard some stuff.

I used to sim a lot of tube circuits. Now I have a big ugly piece of plywood with a turretboard and tube sockets and a huge assortment of shunt resistors and controls on it. I can cobble together most any output section in minutes and I've learned a lot more that way than I ever did in spice. Of course, I'm fortunate to have access to some nice scopes and true rms voltmeters so I'm able to experiment and measure rather than speculate and simulate.

I guess it depends on the kind of circuit you're building.

jamie

[FredB]

I've gotten the simulation speech before, although at the moment I am financially paralyzed by poverty. So, with only plenty of time, there's little else to do...

It's kind of like when I started working in construction trades, the old fellas would say book learning is one thing, although experience is another.

I've got plenty of equipment, and have bread boarded and built enough things to know there are differences between simulated circuits and as built circuits and that adjustments often need to be made.

During my impoverishment I have worked out preliminary designs for a few amps and half a dozen pedals, that are just waiting until I can afford to order parts for them.