Digital Panel Meter For Bias??

[flythair1]

Hello MEF Community

I’ve been a long time follower here and have learned & enjoyed very much. I have some successful builds, a 5E3, 18 watt lite2b, and four from AX84. On a next build I’d like to put a digital panel meter on the rear panel of the amp with the ability to use a momentary toggle switch to display the mv draw(drop)?? across a 1 ohm resistor, of each power tube. I’ve seen cheap meters on eBay and more expensive elsewhere. Some are only 5/8” X 1 ½”.

I’d like your opinion whether or not this would work. I have some high quality switches that I could mount sideways. Momentary on in both directions. If you used a separate display for each tube, you could mount them on the front and let them display continuously.

Would there be a better parameter to have the ability to meter on the power tubes, more directly than what I propose? I am WAY noob. I just thought it might be a nice uncommon feature to have a bias display built right into the amp. Maybe too pricy for industry but not for the home builder.

The pic is the rear panel of my last build, the 18 watt. There’s plenty of room for a small display. This next amp will be a PP 6L6 pair with maybe fixed & cathode but at least variable cathode bias.

Thanks for readin’ and your thoughts are welcome.

Scott

[Jazz P Bass]

I would rather see the total current draw from the mains, if anything.

[loudthud]

Many of the DPMs available require the power supply to float. That means it has no connection to ground. So you have to create the power supply from an isolated transformer winding.

[DIY_Guy]

There is a builder that does this, I just cant rember who it is right now.

[R.G.]

We put an LED indicator in the Workhorse amps. Each power tube had an independent bias voltage set pot, reachable from outside the amp with a #2 phillips screwdriver. The current sense lit a green LED for "just right", red LED for "too hot". There were several switched settings for specific bias currents. The idea was to make it something where you could whip in a new set of tubes between songs if need be. Easy to get it right on the fly, and independent per tube. I even slogged through the error budget/analysis to figure out how much variation there would be when you had two greens and it was telling you that the tubes were biased correctly.

The purists will moan about it not being a perfect setting or not giving fine enough resolution, but it really got the bias set quickly and in terms a guitarist with three beers in him could understand - "green means good".

I considered a meter, but a meter without a reference table for "this much is good, this is too much" isn't much good in a stage setting without a sober road tech to keep your amp in good shape. And metering per tube requires either two meters or a switch - which you can then proceed to get mixed up, biasing the same tube twice instead of each one once.

[flythair1]

Hey Guys, Thanks for the replies.

R.G., I thought I'd use the momentary (on-off-on) toggle mounted between the power tubes sideways so you would switch to one side, the meter would display and you would adjust the bias for that tube and then the same in the opposite direction. The rodie would have to be almost sober. No, really I just thought for a home build it would be fun to try. Can it even be done?

JPBass, would metering the mains current draw allow you to bias the tube separately? It seems by being able to meter each tube you could match or mis-match at will. You might have to be able to bias with a DMM anyway but with the meters built in at a low enough cost it might be interesting.

Scott

[Ronsonic]

I like your idea. RG has to sell amps to normal people. I've seen what normal people do to amps. So has he. He needs to dumb it down for those folks. Since this is for you, it can be as elaborate as you'd like. You would be able to measure each tube separately and adjust each if you design in the adjustment. Sounds like a plan.

[Steve Conner]

As others pointed out, the only hard part is powering the meter. The supplies don't need to be completely floating: it just needs one supply rail above the voltage to be measured, and another below. So if you had a centre tapped heater winding, you could rectify it to get + and - 4v or so. With the centre tap grounded, these voltages would be referred to ground, so you could happily connect the meter across a 1 ohm cathode resistor whose bottom end was also grounded. But meter modules all differ. For instance many of them only need the positive rail, and generate the negative one on board with a tiny charge pump. So read the instructions carefully.

I preferred to use an analog meter for a vintage vibe.

[Enzo]

Then might as well add a switch setting to get plate voltage. Can't calculate dissipation without knowing the plate voltage as well as current.

Monitoring mains current would not facilitate bias of individual tubes, no. SOme just might find it a more useful thing to watch.

I forget now who, was it some Fender model or was it a CArvin? Since the power transformers were a known factor, and all the amps were about the same, they had determined that a properly biased amp drew X amount from the wall. So on the production line, instead of going inside for a bunch of measurements, they set the bias by watching mains draw. When it was set for the target current, then the tubes would be conducting the desired amount. Schematic noted bias adjustment for X mains draw, rather than tube current specifically.

[R.G.]

I thought I'd use the momentary (on-off-on) toggle mounted between the power tubes sideways so you would switch to one side, the meter would display and you would adjust the bias for that tube and then the same in the opposite direction. The rodie would have to be almost sober. No, really I just thought for a home build it would be fun to try. Can it even be done?

Sure, it can be done. And for an amp where you're the only user, it's fine.

On the issue of metering mains current, Enzo is correct as usual. In a properly functioning amp, the power used by the entire amp is mostly determined by the bias on the output stage. It's one of those end results of a string of conclusions and approximations, and it's a great one for manufacturing lots of amps. It's kind of the opposite end of the spectrum from the it's-just-my-homebuilt-amp though.

In my view, output tube biasing with micrometer-like accuracy is an interesting personal toy or a studio-amp tool, not something that is critical for normal day to day amplifier use. Amplifier sound does vary with bias, especially on the cold end. But when you get the crossover just taken care of, there is a broad range of sounds-a-little-better-as-I-get-hotter as you turn it up towards full class A. The only issue there is is can the tubes and power supply take the power dissipation.

But for an amp you haul to the club for your every-Tuesday-night gig, you probably need it biased just enough and not too hot.

It is pretty easy to get off the deep end on biasing.

The real objective is to get the output tubes just on the good side of making crossover distortion and canceling hum. Crossing that threshold gives you the biggest bump in sound quality, and more current gives you slight improvements along the way, at the cost of more heating and less output power (which goes down as you get further towards Class A). Ideally, you set the plate currents to equal on a pair of tubes with matched AC gains.

It's a real advantage to measure near ground instead of several hundred volts up at B+, so measuring plate current plus screen current at the cathodes is a simplification, and probably justified because the screen current tends to be a semi-fixed percentage of plate current. This is another of those 80-20 things. You get most of the value with the simplest efforts. Getting the other 20 percent requires more and more effort.

More esoteric biasing gets involved. Measuring plate dissipation and setting a fixed percent of dissipation is one of those, as it adds not only a current measurement and a voltage measurement, but also a calculator.

I've done a paper design of a $2 microcontroller that measures cathode current and plate voltage, multiplies the two together and displays that on a $10 LCD. It's entirely feasible and not all that tricky, but probably a vast overcomplication for most amps. Might be interesting for a biasing box that plugs into a DIN plug in the back of an amp and reads you the stuff auto-magically as you twiddle the bias pots every now and then, though.

The dodge that measures AC power line current is the end result of a chain of simplifications that goes tube dissipation -> plate current -> cathode current -> B+ current -> AC power in. And I probably skipped over several intermediate things that could be used.

On the subject of matched tubes, there are at least three things that go into matching. These are DC current versus bias voltage, AC gain, and AC nonlinearities. Independent biasing sets the DC currents independently, and again is the easy chunk of this 80-20 task. You can match AC gain by selecting tubes, or you could twiddle the AC signal from the phase inverter, a variable version of the fixed version built into amps with different plate loads on a differential phase inverter. You could do this with a pot with its wiper attached to the B+ and its lugs feeding the plate resistors of a PI. I've messed with this, and it works. It gets you another chunk of the remaining 20% (except I found that my ears like them slightly unbalanced).

Matching nonlinearities is an interesting possibility. You could preselect tubes for matching distortion spectra, which used to be impossible but with sound card applications offering Fourier analysis these days, you could make up a fixture to let you match spectra on output tubes. That's a whole lot more esoteric than I ever need to get.

After all, it's only rock'n'roll.

[Jazz P Bass]

Monitoring mains current would not facilitate bias of individual tubes, no. SOme just might find it a more useful thing to watch.

I forget now who, was it some Fender model or was it a CArvin? Since the power transformers were a known factor, and all the amps were about the same, they had determined that a properly biased amp drew X amount from the wall. So on the production line, instead of going inside for a bunch of measurements, they set the bias by watching mains draw. When it was set for the target current, then the tubes would be conducting the desired amount. Schematic noted bias adjustment for X mains draw, rather than tube current specifically.

Crate Blue Voodoo 120 used the mains current.
Interesting alternative is suggested on the scheme to adjust for slight crossover.