Bias Checker with Plate Voltage?

[Fresh_Start]

Had a potentially dangerous idea this morning:  since we need both plate current and plate voltage to accurately measure bias, could one make a bias checker which had TWO bannana plug outputs - one for cathode current and the other for plate voltage?

IOW start with Doug's nifty bias checker but add a second line out with one end attached to the cathode (pin 8) and the other to the plate (pin 3).  Not a bypass like the cathode bypass used to measure cathode current, but just reference voltages.

Having plate voltage exposed on a dual bannana plug seems like a BAD idea.  What if I had the output from the plate/cathode cord going to the "Dean type" connectors that Doug sells with the female end toward the tube socket and put the bannana plug on the other end?  That would allow me to put the bannana plug into the DMM first, then connect the other end with the Dean connector to the bias checker.  At least that way, you'd have to stick something into the female end of the Dean connector to get a serious electric shock (or worse).

I certainly wouldn't recommend this as a consumer item, but do you all think it's just a stoopid safety risk?

We can always open up the chassis to get plate voltage, and that gives us a chance to measure screen grid current too for the most accurate reading possible.  It's just that the bias checker seems geared toward checking bias without opening up the amp.

Thanks for your thoughts,

Chip

[FYL]

Easy : just use IEC1010 safety sockets and connectors, 2 mm versions are rated at 600 V continuous, 4 mm versions at 1 KV.

[Fresh_Start]

Easy : just use IEC1010 safety sockets and connectors, 2 mm versions are rated at 600 V continuous, 4 mm versions at 1 KV.

Sorry, I don't know exactly what "IEC1010 safety sockets and connectors" are.  Searching Mouser, Allied and Pomona turned up a bunch of things but I'm not sure which you were referring to.

I did find bannana plugs with a retractable sheath: 
http://www.pomonaelectronics.com/pdf/d_ai_122505a_6a_2_01.pdf
Not cheap, but possibly a clean solution.

Are those "Dean type" connectors intended for multiple connections/disconnects or are they more of an every once in a while connector?

Thanks,

Chip

[eleventeen]

IMO it's not worth the hazard to expose plate voltage, especially on something like a banana plug.

If you are doing this on an already-built amplifier, then whatever that amp runs at is pretty much what it's gonna run at, so I'd say, just measure B+ and take that as a fixed voltage.

Now it's kind of a different story on my tube matcher, and I'm not saying I have the solution or the good way or anything else. I'm still not entirely happy with how and where this thing is at this juncture. The B+ originates in a voltage doubler running from a variac, so I can go anywhere from near zero to about +425. I elected to use a completely separate transformer for bias, and that circuit is regulated by an 0A3 tube = 75 volt gas regulator. After the regulator I have a voltage divider set up so I can bias anywhere from about -3 volts to about -65 volts. So I can cut off any sort of conventional output tube. I can also blow up almost any output tube, LOL. Realize that I am not trying to set up the output tubes to pass a particular current value and there isn't any push-pull or duty cycle or anything like that. I'm just putting the tubes 100% in parallel (under static conditions, of course) and seeing how much they conduct with identical plate, screen, and bias voltages applied.

Incidentally; I've pulled 3 quads of output tubes out of commercial-unit tube type power supplies: 2 x 4 x 6L6 and 1 x 4 x 6550. These tubes are manufacture-date "matched" and (I assume) were installed in their respective supplies at the same date and have probably passed identical numbers of electrons over their lives. They end up being *phenomenally* closely matched. I believe the operation of the supplies in which they sat for years served to match them, if such a thing is possible. The higher emitters emitted more and the weaker emitters emitted less. Maybe I am fooling myself but as the tubes now exist, they appear to have been "formed" into quasi-matched sets.

But as for a commercially built amp you are trying to bias, or an amp you are building where there's a design-center voltage (eg; B+ = 350-375 volts) or even a specific PT you've selected, my reco would be not to expose that potential to the outside world.

[Fresh_Start]

FWIW Doug told me that the "Dean type connectors" are NOT suitable for applications requiring multiple connects/disconnects.

I do NOT want to expose plate voltage on a normal bannana plug.  However, my thought was that there may be times when you want to bias an amp without ever opening it up to measure plate voltage.  So far, the bannana plugs with the retractractable sheath are the best solution I've found, but I'm still not sure what FYL had in mind.

As an aside, I'd love to figure out how to make a bias checker for 9-pin EL-84s.

Cheers,

Chip

[eleventeen]

The standard way is to get one or more of the type of things shown on this page.

http://www.radiolaguy.com/miscTools.htm

But to interrupt the cathode requires opening them up, and not all of them permit this on a non-destructive basis. Still, it becomes a lifetime tool that you'll use many times.

Not too difficult to gather some dopey octal TV tubes, break them, gouge out the internals, (wear gloves!) unsolder the leads & clean out the pins, then epoxy an octal socket on top of the open end with wires running pin to pin inside. Most octal tube bases will be missing pin 6, but that isn't a problem for any octal power tube type I know of.

9-pin 6BQ5 style, much more difficult to find/improvise something that will work.

[FYL]

Sorry, I don't know exactly what "IEC1010 safety sockets and connectors" are.

Have a look at what's used on modern test gear... Check for instance http://www.alliedelec.com/search/productdetail.aspx?SKU=8850384

[Fresh_Start]

FYL - Thank you.  I'd found that part based on your first post but didn't quite get how the panel jack fit into the picture.  I think that my DMM already has that type of banana jack in it.  (My other meters don't.)  Those banana plugs with the retractable sheath I linked to above seem like they're intended to be used with that type of jack.  I've emailed Pomona to get more info.

Thanks again & have a good weekend,

Chip

[PRR]

Three 1/2W 330K plus a 1K. Plate to cathode.

[Fresh_Start]

Three 1/2W 330K plus a 1K. Plate to cathode.

A voltage divider to make it safe.  Gotcha.

I just have to figure out a reasonable way to mount those resistors.

Thanks,

Chip

[PRR]

I agree with eleventeen. The voltage varies with current, but unless the power supply is VERY saggy or you are being TOO fussy, it won't vary that much over the range of reasonable idle currents. 550 no-load, 450V full-roar, it will stay 500V-525V while biasing, and we just don't sweat the 5% change.

[Fresh_Start]

I agree with eleventeen. The voltage varies with current, but unless the power supply is VERY saggy or you are being TOO fussy, it won't vary that much over the range of reasonable idle currents. 550 no-load, 450V full-roar, it will stay 500V-525V while biasing, and we just don't sweat the 5% change.

I completely agree if you have opened up the amp and know what the plate voltage is. 

I want a tool which will allow me to bias an amp without going inside the chassis.

Thanks again for your thoughts,

Chip

[PRR]

without going inside the chassis.

With a bias-tool, you could pull one tube, meter the plate-pin hole of the empty socket, compute current for 100%-120% rated Pd, bring the other tube up to that point, read the plate-hole again, and shut-down. That voltage reading is "close enough", and errs on the safe side (two tubes at 60%-80% Pd will sag lower than one tube at 100%).

Me, I never NOT go inside the chassis. But I've only worked on salary or for play. Piece-work is quite different.

[sluckey]

Put a 1 ohm resistor between pin 3 and pin 3 and connect the banana plug across the resistor. That way you have the plate voltage available and can also measure current (using the millivolt scale). DON'T USE DOUG'S DUAL BANANA PLUG! Use individual insulated banana plugs like the ones used with a Fluke DMM. It would be handy if you could get one more banana plug lead into the socket adapter and connect it to cathode pin 8. Then you could measure plate voltage between plate/cathode and get plate current across the 1 ohm resistor.

Be careful!

[Fresh_Start]

Thanks for the suggestions.

I WILL be VERY careful.

Chip