$14.95 octal base bias tester kits

[Prairie Dawg]

With this being a kit you could certainly omit the HiV lead (which is cut down to 0.1% of the actual voltage but shit does happen.) And I would only use it myself. Actually I had picked up some of the octal plugs to build my own bias probes years ago but never got around to it. Speaking of B+ I got very unreliable readings from several of my DMMs which were otherwise very dependable. I think that you have an AC component on the plates which might be confusing the "bad" DMMs. And I think that I had one or two that were actually fried- did not give good readings on any of the scales. So I now check B+ only when I really really have to- and of course use my Fluke.


I could never get one of my bosses to order one of those. It would have been great to figure out what exactly was going on with a furnace or AC 24 hours a day.

Thanks!

Steve Ahola

I'm OK around B+ or plate voltage but I'm awful careful when I start sticking probes in a live amp. I have been using Centec multimeters-the twenty dollar ones from Harbor Freight, and they're pretty good for the usual shop chores. I have managed to blow one up here a week ago in four years of amp work and I didn't feel bad about tossing it in the trash.

Digital Multimeter - Save on this AC/DC Digital Multimeter

I don't do this sort of stuff with my FET multimeter or the Valhalla Scientific multimeter-counter. They're too nice to spoil. And I don't like people poking around live innards either.

[defaced]

Those are some really good instructions. I'll keep those around for the next time I build a probe. I didn't add a B+ divider for mine, but I could definitely see that as being useful, so I'll keep it in mind for the next time.

Hoffman Amps also sells a kit that's around the same price (15 bucks): Tools and Batteries
I found I had most of the parts laying around except for the base, so I just bought that with an order a long time ago (last item, 2 bucks): Tube amp Tube Sockets - Bias Checkers

[Steve A.]

Also the spec lists "AC and DC accuracy: +/-(3.5% of reading + 0.5A)" That +0.5A part won't do for making a bias adjustment.

I thought I'd check the Fluke site for exact specifications on that probe and all that was there was a short blurb: "Battery-powered Hall-effect probe measures 1A to 400 A,1mV/Amp output guarantees easy reading on your meter, Take accurate current readings without breaking the circuit" Kinda disappointing that I could find no specs or user manuals for any of the products on the Fluke site. It's like being back in the 1980's!

Steve

[defaced]

Fluke.com support button, manuals. That will get you to the manual search. The instruction sheet for the 410i has the full specs.

[TomCarlos]

Hey Friends....

I found this thread and want to revisit the Tube Depot Bias Probe. It seems simple and straight forward. You build, use a Volt Meter set at 200mv and can measure the Plate Voltage with the Red Banana and Cathode Current with the White Banana. You have a single Common connection.

But... here is the problem....

If you look at the photos, you'll see that you have to cram 3 resistors inside the tube socket. You make the connections, put in the bare wire, stick everything through the pins ... Man-Oh-Man, that socket gets VERY crowded. And once you press the top and bottom piece together and glue, things move, things break, things short.... Not so easy.

I found an old thread on a Peavey forum and they show a similar approach. In the second set of photos, you insert the 1 ohm resistor between pins 8. This is what measures the Cathode Current. The Black wire is used as the Common (Ground, or Common side of your voltmeter). You bring out another wire, tied to the other end of the 1 Ohm resistor and that will measure your Cathode Current. For the Plate voltage, the diagram shows a Red wire coming out of the tube socket. Yup, that is High Voltage. You can build a small project box and create a Voltage divider in the box. You take the Red wire to a 1 Meg, the 1 Meg goes to a 100 ohm, the 100 ohm to the Common, and use the junction of the 1 Meg and 100 ohm and take that to your volt meter. Your doing the exact same thing that is done in the socket- except that you make that connection in your test box. I think this is what is happening with the Amp Head Bias Tester. It probably has the voltage divider inside the box. The Voltage divider is what allows you to use the single Voltmeter, set to 200mv, so you can read the Plate Voltage and Cathode Current.

Anyway.... I am going going to try an experiment using the Tube Depot kit but building a bias probe similar to the Amp Head Tester. And if that works, I might build a Quad Tester. That will require a bigger project box and a 2 pole, 4 position rotary switch. On one part of the switch, you make the connections for the Cathode Current. On the other part of the switch, you make the connections for the Plate Voltage (taken off the voltage dividers). You then take the common of the rotary switch to a SPST and that is how you toggle between reading Voltage or Current on your Voltmeter.

There is a YouTube video where a guy shows you how to build this idea. I believe he is using a Single Voltage divider in his box (for the Plate voltage measurements). So you run the Pin 3 connections to your Rotary switch and the common to your voltage divider.

Anyway.... I wanted to say that the Tube Depot kit is cool but the assembly instructions suck. I like the idea of the 1 ohm resistor being the only one in the socket and doing the voltage divider in a test box. I wish the Amp Head Tester was in a kit form- for either a single tube socket of for a quad.

Anyone see a problem with doing the voltage divider outside the tube socket?

Tom

[g1]

It's mostly a safety issue. However, when you start extending wires like that you can sometimes run into oscillation issues.

[TomCarlos]

Good point Greg.

Perhaps there is a better way to lay out the resistors inside the tube socket. I found this pic from a Weber Kit- looks identical to the Tube Depot (probably the same kit). In this pic, the resistors are laid out on the exterior wall of the socket. But I am thinking the 1 Ohm could be attached to the top part of the socket (just like the other wires connecting the pins then pushed down into pin 8.

For $20, it might be worth an afternoon to try an alternate way of assembling the socket.

[eschertron]

I ordered and built the bias scout last year. Yes, the components get crowded during assembly, and some common sense needs to be applied given the not-so-detailed instructions. I did have a time getting the halves of the socket together, but didn't actually break a sweat - or anything else - while putting it together.

Could there be a better design? Sure. Is this kit a dog? No, it works just as advertised.

[überfuzz]

Some years back I took a tube and a socket and did a make shift probe. I did what it supposed to. Lo and behold, some months after this I bought a bias probe. I think it's very handy to be able to check, or control the voltage/current levels from the out side, so to speak.

[Audiotexan]

Thanks for the additional post/links Tom. Some more cool ideas/reading/watching there!

[B]There is a YouTube video where a guy shows you how to build this idea. I believe he is using a Single Voltage divider in his box (for the Plate voltage measurements). So you run the Pin 3 connections to your Rotary switch and the common to your voltage divider.

Tom

For those that followed the video series to the 3rd video of the series (entitled "part 3 calibration")... The only hitch I can see in his giddyup is: Why on earth would he bias the amp (on his make-shift variac) for 115v use??? Especially when his line voltage was 123v.

Unless he's running that amp powered by "that variac" 100% of the time...his bias setting could/would be totally different than what he's setting it for at 115v.

I'd tend to want to bias it at real world (working) voltages.

Is this just an oversight on his part (perhaps in mentioning)? Or mine (missing an assumed translation, and the shift in plate voltage would equate to the same shift in bias for a 100% match [maintaining that 70% setting, as mentioned])?

[TomCarlos]

I caught that too RS... and in the Part 3 video, I think he says something like "I don't need to calibrate... it's already close enough."

My guess is that he was taking the probes to the rotary switch, then going to the voltage divider, then to the switch to select volts or current.

Anyway, looks like an interesting project. Like he says, the tedious part is constructing the probes.

[g1]

the shift in plate voltage would equate to the same shift in bias for a 100% match [maintaining that 70% setting, as mentioned])?

I think this is the assumption that is often made, which doesn't always turn out in practice. Best to set bias for the line voltage it will be most used with IMO.

[nosaj]

This is the one I use from Hoffman. Hoffman Bias checker

$15.91 entire kit
Mojotone Bias Checker Kit by Hoffman Amps - Mojotone.com

nosaj

[Audiotexan]

Oooooh I like what he did with the pcb there! Nice little additional way to stabilize things. And quite possibly could give you other easy wiring options!
Not a bad price at all, considering the hassle it might save (finding sockets/breaking down junk tubes/alignment). Just seems like it'd be alot easier come assembly time.
Wonder how they are on shipping?

Now... I wonder if anyone's seen one yet for EL84's?

[tedmich]

EL84 version
SRS BiasTool PVT
(not a kit)

I would like to have the voltage divider to be able to read plate voltages in the Mojotone, I mean if you're going to make a PCB...