Amp bias indicator add-on (moved from the "microprocessor bias" thread

[bob p]

1. Would you agree with a many-turn screwdriver adjustable trimmer for a bias pot, based on human factors? Does it need to be single turn? Or multi?

Either is fine.

2. What's the smallest power rating you'd use for a 10K bias adjuster pot in an amp? I did my usual heavily-derated design, and the real power is probably under 1/4W, but it makes me uneasy for a bias application.

Agree with others that by scaling the appropriate amount of series resistance with the pot, the power dissipation rating of the pot can be decreased without sacrificing too much range of bias.

3. One could argue that fast, accurate biasing is *worth* $5.00 a tube; what do you think?

May not be worth it in an amp with a tube pair, would be more worth it in an amp with a quartet, a definite must for an amp with a sextet that is hard to keep matched. Being a user of sextets II am more concerned with convenience and the cost of replacing tubes, not so concerned about the cost of good support circuitry that saves money elsewhere and makes life easier. I would not be deterred by the price.

4. I was focusing on a small add-on board that could be done by a DIYer and put into their amp, hence the parts are deliberately small and close together, including the bias pots and indicators. Comments?

It doesn't have to be as small as you might want to make it.

On the subject of emitter followers, if you're going to use transistor control and a PIC, then you should get rid of the user adjustments entirely and make the circuit auto-biasing. I mean come on, why go half way?

[NateS]

I know that's not where RG was going with this. He has BTDT already. But..this does sidestep RG's biggest concerns from that thread, in that this is a user intervention in the back of the amp. It's not a once per turnon sort of automatic thing. Just dial in the desired current, the lights tell you if a tube is out of tolerance. Pop in new tubes,you don't need to tweak, just let em warm up, and press the button, and they all come up to 23mA and save the values. Decide it sounds better a little hotter, dial it up to 35mA and push the go button.

[Enzo]

And I have no plan to ever sell these. It's just for the good of humanity.

But then I have to have the 473rd version of the argument with the people who think that if silicon is within ten feet of their amp, it ruins the tone somehow.

Given the first, screw the second. You aren't selling them, so make them like they work, not like some moron in a music store wants them to be.

[R.G.]

Another idea rises from the ashes... again.

I (finally!) did enough of a circuit, layout and physical design to get an early view of how this would work. You can see it here:
http://geofex.com/Article_Folders/Tu...er%20Panel.pdf

Drawing up pretty pictures of operator panels is all well and good, but this one was generated based on an actual PCB layout behind it, so it's fairly real. The page shows the panel real size, which is 1.5" by 4". There are two PCBs behind that, one with the switches, LEDs and controls on it and the other with the controller circuit, power supply and wiring to the rest of the amp.

The circuit is powered by 6.3Vac from the heaters, through an isolation transformer. A suitable transformer can be bought new for $5 to $12. Well, I guess you could spend a lot more, but it can be as low as that. The wiring to the amp is through a terminal block that you stick wires into and screw down to clamp.

I'm still thinking about how to mount this in an amp given all the conflicting requirements. It needs to be in the amp chassis to keep the critical bias wires safe inside the chassis, but the panel needs to be accessible for adjustments. I thought of something like hanging the panel from the lower side of a Fender combo chassis. The mounting is the most ticklish part of this.

There may have to be several versions to make mounting easy. But there will soon be at least one version that works.

[Gingertube]

Guys some start points for a bias servo system. These would then need over current comparators, LEDS etc. but the control system is there.
1) The Norman Koren "TINA" Scheme
The Emperor's New Amplifier
This scheme uses a philosophy of manually setting the bias on one of the output tubes as a "Master" and then has the other tubes track it.
This means the bias servos can be VERY simple but has the problem that if the Master Tube goes short then the others will try to follow its problem.
2) Robs scheme
Most Class AB bias servos work by clipping the current waveform monitor at X2 the bias level to balance the "clipping" as the the tube cuts off.
Rob's circuit works similarly but clips in a narrower window around the bias setpoint, look here:
Motional feedback intro
This is a varaiation on the Menno Vdv/Guido Tent system which can be purchased from Tentlabs.
That is it is a variation of this scheme:

Cheers,
Ian

[R.G.]

I designed and implemented an auto biasing system - then did not ship it in the amplifier line. There were a couple of reasons.

The biggest is that tube amps just don't need to be rebiased all the time. There's not much incremental value in that. And although there are schemes for messing about with the biasing in the face of signal going through the amp, you don't really need it. Most tube amps get rebiased once or twice per set of tubes, if that, unless the owner is a tinkerer.

What is really useful is to make manual biasing fast and easy, with low error. Humans are very good at turning a knob until a light turns a different color.

The first system I did had comparators and such, but today, it's simpler to let a $2.00 uC with many A-D converter channels just read the current, then decide what to do.

In this particular design, the controller watches the cathode currents all the time. It does not light the indicator LEDs unless the "bias me now" button is pushed. But it does look for the biggest currents. It uses a preset "current too high level", which can be adjusted manually, as well as a "overcurrent for too long" setting. In operation, the thing ignores overcurrents that last less than the timeout for overcurrent. So it can be set up to not interfere with normal playing any more than the existing manual bias pots, which is what these pots are. But when a tube shorts, or the bias wire to the tube breaks, it can open the B+ line and save a tube - or more importantly, a power or output transformer.

[Enzo]

I imagine it falls in the category: for every extra sale it gets from someone really digging that feature, it loses ten sales from the increased price. At least in a commercial amp. MAybe as an add-on kit...

[R.G.]

I'm pretty sure you're right, Enzo.

[bob p]

Resurrecting this old thread ...

RG, did you end up going anywhere with this since we last spoke about it? I see that Behringer/Bugera is doing the auto-bias thing with their Infineum line of amps ... and making some outlandish claims about auto-bias improving tube life by 20x. (!) For that to happen, they have to be biasing them cold.

[R.G.]

I wrote and tested the uC code for the RGB indicator and dis a trial layout for a PCB for the 4-tube version. Then real life interfered and I didn't go ahead and order prototype boards. Having tested the code, this is one of those things where the technical difficulty involved is so low that it's nearly certain to work.

I did not pursue the fully automatic biasing any further. For the reasons listed in this thread, I consider it to be unnecessary and perhaps detrimental to an amp; the only condition I can see where it would be useful is as advertising where the amp already has a processor on board and the incremental cost is nearly zero.

[nickb]

Resurrecting this old thread ...

RG, did you end up going anywhere with this since we last spoke about it? I see that Behringer/Bugera is doing the auto-bias thing with their Infineum line of amps ... and making some outlandish claims about auto-bias improving tube life by 20x. (!) For that to happen, they have to be biasing them cold.

I think they reduce the current when there is no signal and that is where the (likely exaggerated) life extension claim comes from.

[R.G.]

I'd love to see the raw statistically-significant data that they used to back up that claim

If any.

[Chuck H]

Well there IS a charge and dump time constant for a real tube amp power supply that is partly dependent on where it starts. I've considered what it might take to make an amp just hold in cutoff unless a signal is applied, at which time the bias initiates. Considering breaks between songs (or practice stalls) and breaks in songs, passages and any musical situation it does seem like a good idea. I never took it beyond the consideration though for the reasons mentioned. It might work a treat for amps that virtually NEVER overdrive the power amp. As is the case with most modern, uber gain channel switching type designs. I had the idea long before the "Infineum"design but it wasn't applicable to the designs I was doing at that time. Not to mention that I looked it over and figured it would probably require a "logic" circuit. And that's beyond my depth. Still a good innovation for the right application, I think.?.

[R.G.]

Once again, the devil is in the details. Power stages are >> designed << to make noise when current changes in the output tubes. When I proto-ed the uC bias, changes to one tube at a time were overwhelmingly noisy. Changes to both tubes were less so, but still somewhat noisy, as one of the screaming advantages of auto-biasing is not having to use matched tubes. The bias servo takes care of matching the DC bias point for the OT's advantage, and most power tubes of the same type number are close enough in AC gain for it not to be a problem. But having the bias circuit tinkering with the power tubes' currents whenever it felt like it was ugly. I think that it could be even worse to let the thing decide when to bias based on whether it had had signal in the last X seconds. That's almost always going to be a non-musical decision I think.

[Chuck H]

"non-musical" I couldn't have said it better. At least as it applies to overdriven power tubes. And only maybe could it work for a clean power amp IF the power supply were designed accordingly. This as it might apply to a triggered bias. Not even considering auto bias for current matching dissimilar tubes.

What I had considered was something like a mosfet gated by the signal to provide the necessary bias point only when signal is applied. That would seem simple enough. In one mode there is a negative voltage that holds the tubes in cutoff and then signal triggers a mosfet to counter with a fixed positive voltage that results in the operational bias current. Easy peezy. But then there's those pesky time constants that affect the envelope of the power supply. I can only say that from my observations of what an overdriven tube amps power supply is doing, it doesn't seem like it would work without consequences. But then I'm not cool enough to design something like 'Infineum'.