Checking Bias on a 5F6A

[dpm309]

Was checking the bias on my 5F6A using the bias current checker that I built and am getting very high readings (>20A).  Checking the bias across the 1ohm resistor, I am getting normal readings - 42 +/- mv (PV is 425VDC).  I have used this checker on a couple of other amps with no problem and I checked to make sure the probe is wired up correctly (continuity, shorts, etc).  What could be causing this?  Would it have anything to do with the 1 Ohm resistor to ground?

Posted this here first as it might also be a topic for the Amp Tools forum.
Thanks,
Dan

[sluckey]

Got a schematic of your bias checker? What meter do you have that can read >20A?

The 1Ω resistors shouldn't be a problem, but need to see your bias checker to know for certain.

[dpm309]

Sluckey,

I built the one on Doug's info page - the oldest version using an 8 -pin base and socket.  I connected pins 1 to 7 to the base and connected pin 8 base to a shielded wire and pin 8 socket to the ground on the shielded wire.  Connected the shielded wire to the banana plugs and plugged into my multi-meter. I meant to say exceeded 10A (highest on my meter).  The schematic is shown on Doug's info page.

Thanks,

[sluckey]

I think there's something wrong in your procedure or the current function on your meter has gone south. The 1Ω resistor is definitely not a factor.

I connected pins 1 to 7 to the base and connected pin 8 base to a shielded wire and pin 8 socket to the ground on the shielded wire.

This could be a factor for some meters ---IF--- you are connecting the center lead of your shielded cable to the red meter jack and connecting the ground/shield lead to the black/common meter jack. Auto polarity sensing DMMs don't care, but some other meters can be damaged when connected reverse polarity.

Look at the attached pic and notice the arrows. They point in the direction of current flow. Pin 8 of the socket should attach to the red/positive meter jack. Pin 8 of the base should connect to the black/negative/common meter jack.

[dpm309]

For some reason your picture did not come through.  I tried reversing the banana jack in the meter with no change in readings.  There must be something wrong with my probe since I am getting good readings measuring the bias across the 1 Ohm cathode resistor. Unfortunately I epoxied the thing together so I will have to build another one.

Thanks,
Dan

[sluckey]

For some reason your picture did not come through.

Sorry, my fault. Here's the pic...

[dpm309]

I got it now.  I have attached a picture of my probe.  The base is connected to the ground (black) and the socket is connected to the socket just like your diagram.  I have double and triple checked all connections for continuity and shorts so I am still at a loss.  Popped the probe in my 5E3 (6V6) and am getting the same results.  When I measure across the 1 Ohm resistor, everything checks out fine.  Need to bias a Soldano Decatone for a friend so I want to make sure this probe is working properly.  Any other suggestions short of building a new one?

Thanks,

Dan

[eleventeen]

I am highly confused as to how

1: You are getting reading ">20 A" which is beyond the capacity of your meter (and would fry it)
2: How this can not work in one amp yet works in another.

All that's going on is to break the pin 8-to-ground connection from the tube and inserting the 1 ohm resistor. I myself frown on measuring current with meters. Call me Neanderthal, it's left over from the days of analog meters, but that is (or was) the classic way to destroy them (other than measuring volts on the ohms scale)

You ARE or SHOULD BE measuring VOLTS across that 1 ohm resistor, ("everything checks out fine") measuring the voltage differential using the VOLTS setting. Not amps! If the tubes is pulling 23 mils, your meter should read, per Ohm's law: E = IR....E = .023 * 1 so E should read .023 volts. It will be a teeny bit higher because of the resistance of your test leads. Myself, I prefer using ten ohm resistors because the numbers are a little closer to the decimal point.

[sluckey]

Any other suggestions short of building a new one?

I don't suspect anything wrong with your probe. I suspect the current function on your Extech meter has pooped.

You can prove your probe is good with a simple experiment. Connect your bias probe as usual. Simply put a 1Ω resistor across the banana plugs at your meter. Set the meter to measure volts. If you get a reasonable milliVolt reading, then your probe is fine. If so, you need another meter.

I've never liked Doug's bias checker for the simple reason that it requires you to use your meter in current mode. Like eleventeen, I also don't like breaking a circuit and measuring actual current. It's much more foolproof to use a 1Ω resistor and check the tiny voltage across the resistor. If I needed a dedicated bias checker, I'd build Doug's but permanently install a 1Ω resistor across the banana jacks. The dual banana plug works very well for installing the resistor.

[sluckey]

2: How this can not work in one amp yet works in another.

Because he is using only his meter to measure mV across a 1Ω resistor in one case (the working case) but he is using the bias probe and his meter to measure mA in the other case (not working). The difference is the function switch on his meter. I feel pretty sure the meter is bad.

[dpm309]

I misspoke about the 20A capacity. What I meant to say is that my meter was beeping indicating that I was exceeding the 200ma scale.  Also, I used the voltage settings when measuring across the 1 ohm resistor in the amp.

I installed a 1 ohm resistor across the banana plugs as per Sluckey's suggestion and am now getting good readings using the DC volts setting.  It seems my meter is good, at least for measuring volts. I was getting the same results with my GB multi-meter as I was with the Extech. 

Anyway, looks like I am good to go with my bias checker.  Thanks to Sluckey and Eleventeen for you help and advice.

Dan

[eleventeen]

"Because he is using only his meter to measure mV across a 1Ω resistor in one case (the working case) but he is using the bias probe and his meter to measure mA in the other case (not working). The difference is the function switch on his meter. I feel pretty sure the meter is bad."

Aha, aha. Thanks for clarifying.

Yeah. 1: use known-to-work procedure, and 2: try to avoid measuring current with your DVM by running circuit current through the meter. Yes, I know the buttons are there. It's far better to gather some 1% resistors of various values (1 ohm or ten ohm for output tube current) and measure volts across same.

[HotBluePlates]

I misspoke about the 20A capacity. What I meant to say is that my meter was beeping indicating that I was exceeding the 200ma scale.  

You have to stop and think for a moment about even 200mA+... Would that not either pop the amp's fuse or cause the tubes to visibly redplate? From experience I can also tell you that once the current runs away and gets over 100mA per tube, you'll start hearing an odd hum show up in the speaker.

It seems you figured out what was going on, but this looks like a good lesson in "don't always believe the meter," especially if you have extra evidence that contradicts what the meter says.

That said, I understand it's hard to be confident in your reasoning, or keep from wondering if you'e doing something wrong in one measurement but not the other.

Last bit: I've killed some meter functions before trying to measure voltage with the meter leads plugged into the current sockets (creates a dead-short from high voltage to ground). Usually, I've found that inside the meter, one of the several fuses popped and the meter was easily repaired by replacing the fuse. The mid-range and nicer DMMs have 2-3 fuses inside for just the situation (my Fluke and a previous Wavetek meter had spare fuses in a holder inside as well).

[eleventeen]

Good point HBP. Fluke meters often have fusible resistors is series with the input leads and while that part is not that easy to source, you can use a (gasp!) non-fusible 1000 ohm resistor in its place and the meter will resume working fine!

My 25 y/o trusty and adored model 77 Fluke started going nuts on me one day. If you google "repair Fluke model ---" you will find some forums where this is discussed. That's how I found out about the fusible resistor thingy. You send the thing in to Fluke, it's gonna be $125, I believe, enough to buy a new meter and then some unless you're talking about the higher end ones.

Once again: I strongly reco against trying to measure current with your DVM by placing the meter in series with the ckt under test.

[sluckey]

Once again: I strongly reco against trying to measure current with your DVM by placing the meter in series with the ckt under test.

Me too!

The real danger is not so much with measuring actual current (aside from overloads) but more with making a voltage measurement after making a current check with the switch still in the current position or the leads still connected to the amp jack. By the time you realize the voltage reading ain't right, it's too late. Lot's of people, including myself have made that mistake. Thankfully, it was the company meter!