1 ohm cathode resistors

[bob p]

I think the Ampeg paradigm that uses current sense resistors and comparators is very elegant. More companies should be using circuits like these -- they allow us to leverage cheap, solid-state devices to protect valuable, hollow-state devices. You couldn't ask for anything more. The only problems are that is that it takes some time to study the circuit, and that some people just have an aversion to SS devices.

We had a debate about the value of this protection paradigm quite some time ago, when I butted heads with SGM about the modern comparator technology going into the modern "Crate-Peg" SVT. He liked the traditional Ampeg circuits, and I thought the new ones from Loud Technology were better.

I think the circuits are definitely worth deploying, though they're probably out of reach for the hobbyist builders whose level of expertise amounts to copying a Fender or Marshall layout diagram.

[g1]

I think the Ampeg paradigm that uses current sense resistors and comparators is very elegant.

Another example would be the newish Traynor YBA300, with 12 6L6 power tubes and solid-state type protection for each pair.

[Enzo]

Easy to butt heads with SGM, since his arguments were not generally based on real experience nor actual analysis of circuits.

[Mick Bailey]

That's twice he's been mentioned. Three times in the same thread and legend has it that he will appear.

[R.G.]

I think the circuits are definitely worth deploying, though they're probably out of reach for the hobbyist builders whose level of expertise amounts to copying a Fender or Marshall layout diagram.

Maybe. The necessary circuits are about the complexity of a tube screamer clone, more or less. Lots of people build those who have only this week figured out which end of a soldering iron to hold onto. I think it could be cased up into a PCB buildit project.

I've come very close to doing that several times. The first was the "Tube Saver" article on geofex, from about 2000. I did another similar design incorporating RGB stoplight bias setting in about 2015.

The real problem here is not how to do it, or even how to build it. It's how a user perceives it (that is, the "user interface") and whether people could be educated to quit thinking "it's impossible to get tone from a stone" after their internet indoctrination. I could dust off the biaser from 2015 and re-up the design in a week, have boards back in two weeks, and write a building manual while waiting for the boards. But I suspect that even with this level of circuits by the numbers available. adoption would be practically nil.

[bob p]

Want to do something truly useful? Put out a board that provides individual tube bias adjustment for a pair/quad/sextet of tubes, with a built-in comparator protection circuit. Better yet, make it auto-biasing, where the user sets the desired target bias (or tube type) and the board takes care of the rest, automatically adjusting bias as the tubes age. I'd buy it.

[R.G.]

Actually, that's what the board did - offer individual bias pots for up to six tubes, with RGB annunciation of tube bias condition, and a "master" setting that lets you use a meter to set the target bias for what constitutes "green" target bias. Had shutdown, etc..

I'm convinced that automatic bias is not worth the trouble, based on what I saw with the first design. For one thing, the turn-on of the amp makes the tubes do a power-up drift dance, where the "best" bias drifts as the tubes get hotter. Any change in bias for one tube that's not exactly matched by a bias change in the other tube(s) is an audio "click" or thump, unless it's so small and/or slow that it's below audio frequencies. Managing the bias during playing is problematical, as you have to separate out what is the DC bias for the tube from the (large!) swings of audio, and adjust it on the fly in a way that's not audible. All of this can be done, with clever enough programming and circuitry, but what I've seen so far makes me think that having a green target light whenever you think about bias every few months gives you 99% of the good of auto bias with 0.001% of the complications and trouble.

Edit: here's a link to the concept page. What's behind the concept page is a circuit design and layout that would make that work.
Tube Bias Adapter Panel.pdf

The idea is to let you set one master target current with your meter probes stuck into a set of probe sockets in the panel. Once this is set, the circuits watch the current per tube and give you a red/green/blue for hot/right/cold, as well as setting overcurrent trip level and how long to allow an overcurrent to exist before shutting down.

[pdf64]

Want to do something truly useful? Put out a board that provides individual tube bias adjustment for a pair/quad/sextet of tubes, with a built-in comparator protection circuit. Better yet, make it auto-biasing, where the user sets the desired target bias (or tube type) and the board takes care of the rest, automatically adjusting bias as the tubes age. I'd buy it.

Carol Ann amps have done a clever system https://www.youtube.com/watch?v=LPg6SAKNriU

[Gregg]

Managing the bias during playing is problematical, as you have to separate out what is the DC bias for the tube from the (large!) swings of audio, and adjust it on the fly in a way that's not audible.

I don't think that is necessary. You can introduce an user triggered procedure for rebiasing or zero signal rebias every once in a while but not during playing. Let's keep it close to how an amp is operated in real life.

[Enzo]

But you still would need to disable that function during play, otherwise it chases.

[R.G.]

I don't think that is necessary. You can introduce an user triggered procedure for rebiasing or zero signal rebias every once in a while but not during playing. Let's keep it close to how an amp is operated in real life.

Yeah. That's the conclusion I came to. Let the user decide. And prevent her/him from doing it before X time from power on, for stability. I chose that rather than sensing the tube temperature.

On the other hand, if the user has to think about it at all, we found that tweaking a small pot til you get a green light was remarkably easy. We had one of the Workhorse amps set up in the main concourse at NAMM and had anyone that was interested rebias the amp. Took about 10 seconds per person.

Granted, pressing one button that's marked "Do what I want" is faster, but there's not all that much more work in getting green lights.

[bob p]

Separating an AC from a DC component isn't that hard.

I'm not so eager to dismiss the idea as being something not useful. In another thread we've got people complaining that there are no good EL34 available, that they're subject to bias creep and they all fail prematurely.

IMO getting a handle on the bias problem is exactly what needs to be done -- especially when the tubes we have available today are all substandard, as the problem can only get worse.

[Gregg]

But you still would need to disable that function during play, otherwise it chases.

What I meant is there will be user induced rebiasing which will do it's job say for couple of seconds and after it's done it will switch itself off as if it's not there at all. Other functions that don't affect the bias like tracking for overcurrent, shorts etc. will be still active.

[cjenrick]

you can combine self bias with fixed bias to keep tubes from running away, and it sounds nice,

plus you can switch tubes from 34's to 6L6 without rebias, Jerome Brown

Kustom 72 Coupe

[Justin Thomas]

I always call that "Fixode Bias..." loved my Dual Showman that had it.

Justin