Switchable external bias points/probes?

[alfi27]

Hi guys,

I'm considering to drill my amps up for external bias points, one of them already has an external bias pot which is really convenient; when I have my TAD Biasmaster available.

The easy way of doing it is of course to just wire up three probes, one for each tube and ground. However, I had a thought yesterday that it would have been even a step further towards ultimate convenience with just two probes and a switch. Then I can put the test leads into the probes, and just flick that switch back and forth while adjusting the bias instead of moving the test lead back and forth.

Are there any potential issues with this, and would the switch see any significant current? Could I use a mini switch or would it have to be a big one similar to a power switch?

Cheers!

[xtian]

Your plan sounds fine. The current across the 1R bias sense resistors is small, like 40mA. Any switch will do.

[martin manning]

If you are measuring voltage at the cathodes with a 1Ω resistor to ground, then the voltage is just what you see, 40 mV or so, and you could indeed use a (SPDT) mini toggle to switch the jack connection from one power tube to the other. If you're really lazy, you could use an On-On-On as shown below and let the meter save you doing the math to get the difference between the two.

[tubeswell]

Or with a SPDT and 2 resistors

[R.G.]

I like two resistors and just switching the node you "look at" with your meter.

For extra points, you could recognize that your meter has at least 1M and probably 10M input impedance, so you could stick 10K resistors between the cathode resistors and that switch, or between the switch and the bias sensing node and not affect things, and have 10K worth of isolation from you and your meter and those cathodes.

And you ... could ... I mean, as long as you're messing about with bias, ditch meters entirely and use a circuit that lights a RGB LED blue for not current enough, red for "too hot" and green for the tiny just right current setting. I did this in the Workhorse line of amps, and it worked really well. People completely unfamiliar with biasing their amps could set bias on a Workhorse in about 15 seconds - including the time spent hearing what to do.

[gingertube]

I like separate monitor points for each tube plus the "ground".
I set idle current for each tube by measuring from its monitor point to ground
BUT
I do the final balance tweak by measuring between the 2 tube monitoring points and adjusting for a reading of zero.

Cheers,
Ian

[martin manning]

...use a circuit that lights a RGB LED blue for not current enough, red for "too hot" and green for the tiny just right current setting. I did this in the Workhorse line of amps, and it worked really well.

And what would this circuit look like? can you elaborate?

[R.G.]

Sure. The production circuit I did back in '05 was (per tube) a window comparator with a tiny window and a selectable reference voltage. The reference voltage was set to be the nominal voltage for the cathode resistors when biased properly. The window was narrow, a few millivolts. The outputs of the comparators ran through a bit of simple logic to select an LED color. The circuits were mounted on the board with the LEDs, and a tube led from the back of the amp down to nearly the LEDs.

If I were doing it again today, I'd use a $0.50 microcontroller with an A-D input and drive the LEDs directly from the pins not used for reading A-D. I did a test case for this and it worked well. The controller would allow you to make the display much fancier at no extra cost.

This amp was intended to be a working man's amp, as named. The design was deliberately conservative in many ways, and focused on the application of being hauled to a couple of different gigs every weekend, forever. The LED indicator biasing allowed the owner to put in a new set of spare tubes and bias them up literally between sets, if not between songs. Tubes will drift, especially when new - OK, re-bias after the next six songs. Want a warmer bias? Fine, flip the bias-targeting switch from "normal 6L6" to "hotter 6L6". Want to play with EL34s? Fine, flip the bias switch to "EL34". The ease of resetting the bias made it entirely practical to play with correctly biased tube at every gig or every practice session. Or to get them right once and then forget about it until you changed tubes next year like happens with most amps.

The "green" window was that, a window. The tubes could not be guaranteed to be perfectly the same current with this scheme. However, the window was so narrow that it was danged touchy to get the green light to stay on, so the adjustment was pretty close to identical for adjustment with a single turn bias pot. You also didn't get the ability to adjust your tubes for 37ma instead of 35ma, only normal and hotter. However, these could be adjusted by a resistor change in the reference if it was really needed. Nobody ever asked how to change that, although we expected they would.

[M Fowler]

Just a point of view from me.

If the amps are for yourself go with external bias; however, if your building for family, friends, and all other misfits go with internal bias.

My reason is people fiddle with that bias adjust when they know nothing about how to use it. So then the amp comes back to me to set the bias again.

Mark

[tubeswell]

...use a circuit that lights a RGB LED blue for not current enough, red for "too hot" and green for the tiny just right current setting. I did this in the Workhorse line of amps, and it worked really well.

And what would this circuit look like? can you elaborate?

Here's one version (bias window comparator circuit on 2nd page)

[gingertube]

Not sure I like the SVT CL Circuit.

From values on schematic then each output tube typically idles at 24mA.

The Green LED is normally ON
The Green LED gets pulled OFF if any tube is below 15mA

The RED LED is normally OFF
The Red LED gets pulled ON if any tube is above 30mA

These values set by R67, R64, R62 divider chain from the +15V supply.
Seems a bit "COARSE" to me.

That is GREEN LEDS means 24 +/- 7 ish mA each tube.

Cheers,
Ian

[R.G.]

And that's not even taking into account component tolerances, which can wreak havoc on an otherwise nice design. Technology can do a lot of things, but you have to do your homework.

I dug up some of the stuff from the uC bias monitor straw design. It looked at up to six tubes - well, cathode resistors, it didn't actually watch tubes at all. One of the tubes was designated to be the reference tube. It's actual cathode resistor voltage appeared on a couple of probe sockets at the back of the amp for sticking meters into. The procedure to set up the biasing was to first stick in all your tubes and turn the amp on, watching for any signs of runaway. Then, you would meter the reference tube points and twiddle the pot for the reference tube til you got the current you liked on it, ignoring the LEDs for the moment. When the reference tube current gets right, you push the momentary switch that tells the uC to memorize that current as where you want every tube to sit. The LED for the reference tube then goes green, indicating that it's now right.

The uC remembers the -value- of current from that switch press, so that the reference tube is no longer special. It's LED now shows blue-green-red just like the others. But the uC remembers what the current WAS.

Now it's one pot per tube for bias and one LED per tube. Blue is current below the reference, green is within acceptable tolerance on the current, red is over the reference. Adjusting takes one put twiddle per tube. The reference value is remembered until the next setup, which can be any time at all.

The cheap uCs have 10 bit A-Ds, 0-1023 counts. A good starting point for setting this thing up is to scale the readings so the typical (ish) tube reading would be in the middle of the readable range, about 512 counts. So the resolution is roughly 1/512th or about 0.2%. That's about the same as a 1000-count digital meter, and for the same reasons. It is possible to set the programming so that all of the tubes are "green" on only one specific count value. That would mean every tube had to be biased so its cathode current was exactly the same single count. As a practical matter, this makes biasing be incredibly ticklish,

There is a tradeoff between resolution of reading current and speed for biasing. You can tell the uC to give you a green over any range of currents you like. If you demand "green" be exactly the same count on the internal A-D, it's going to be tricky to adjust every tube to be that one, single count. IF you tell the uC to put out a green for a band of, say five counts around the "reference" value, that gets the tolerance band for setting the pots down to 1%. Getting all your tubes within 1% of each other isn't all that bad a target, and is easier to hit with a hand-tweaked pot. The narrower the band of "green" readings around the reference, the easier and quicker it is to bias tubes on the fly, but the less accurate the cross matching will be.

It's cost convenient to use a uC for just biasing, but the poor uC will spend all its time waiting to do the next biasing. Given that, you ...could... have it spend its waiting time between biasing watching the tube currents for overcurrents. It's easy enough to watch the tube currents to catch a loss of bias or tube short, and internally count off a time period of how long you let overcurrents exist before stepping in to save things like power and output transformers. You can have the uC remember which tube went off the reservation and, say, blink its bias LED; and/or turn off the B+ like a B+ fuse; and/or monitor some temp sensors stuck to your power and output transformers; the list of what you can do just goes on. The only problem is picking what you WANT it to do.

Not bad for a $2 part.