1st post - Bias Probe question

[halfmassive]

Hello,

I have a quick question regarding bias probe readings.

When I measure my JTM45 kit amp bias via the internal 1 ohm resistors I get a reading of 38 mV but when I measure via a eurotubes bias probe which requires me to measure at mA I get a reading of 31 mA.

I have an external bias pot installed and would like to read my bias via the bias probe but something doesn't seem right any ideas on the problem?

Thank you for your help

[Structo]

I'd like to hear an answer to this as well.

I suspect it has something to do with the screen voltage?

[halfmassive]

BTW I'm using el34s

[Jana]

either one of two things is going on:

Is the bias probe measuring from the cathode to ground or is it measuring the current from the plate to the OT primary? This could make a difference and it would be very close to the readings you are seeing.

It could also be the tolerance of the 1 ohm resistors. Unless they are exactly 1 ohm, the readings will not be accurate.

What can you do to find some answers?

turn power off of amp, remove the power tubes and measure the resistance of the primary from center tap to pin 3 of each tube. record those readings. They will be a bit different because of the way the OT is wound, this is not a worry. Let's say, for example, one side is 54 ohms and the other is 66 ohms. While you're at it, measure the exact resistance of your screen grid resistors, record those readings.

Now, insert tubes, power up the amp, let it warm up good. Measure the voltages from the center tap of OT primary to each pin 3 of the tubes. Record readings. Measure the voltages across your screen grid resistors, record readings. Now, using ohms law I = E/R, calculate the current from those 4 places.

Let's say you come up with 31 ma on the primary and 5 ma on the screen grid of side A. Add those together, they should equal the reading you get on the cathode resistor.

(All this is at idle, no signal)

[halfmassive]

Which is the center tap? I've learned a lot recently but am still pretty new to some of this. I'm using a weber 6m45 kits if that helps at all.

[Structo]

The center tap of the OT is usually connected to the B+ at the first filter cap in the supply. It is usually a red wire. So you have a red, blue, and brown of the OT primary, the red is connected to the first filter cap.

Do you have a layout of your amp?

[halfmassive]

here is the link for the weber kit I'm using.
https://taweber.powweb.com/store/6m45_layout.jpg

From my OT I have red blue and brown. The red is connected to the standby switch which I believe has a wire running to the filter caps from that. Is this my center tap? And do I measure these pins with the amp on but standby off?

[halfmassive]

I figured out the first part. With no power and no tubes I read

60.8 ohms from center tap to pin 3 of power tube 1
61.9 ohms from center tap to pin 3 of power tube 2

468 ohms across both screen grids

I'm slightly confused about step 2. Do I turn the amp on but not standby and then read from center tap to pin 3 on both tubes and then across screen grids? Don't want to get a shock or mess something up. I haven't measured stuff in a live amp without one probe grounded on the chassis.

[Jana]

you will have to measure these voltages with the amp on and in the operating mode. You will have to use your probes to measure the voltages in the same way you measured the resistance, ie, one probe is not going to ground. So, yes, you will be measuring from the center tap to each pin 3 and across the 470 ohm resistors. While your readings will be only a few volts, it is all at a high potential.

If you are uncomfortable doing this then I highly suggest that you don't. Everything will be live, hot, and ready to bite.

[halfmassive]

So now I measured every thing and got:

resistance
60.8 ohms from center tap to pin 3 of power tube 1
61.9 ohms from center tap to pin 3 of power tube 2

468 ohms across both screen grids

Volts
29.6 mA from center tap to pin 3 of power tube 1
29.4 mA from center tap to pin 3 of power tube 1

5.3mA (Screen grid)

if I add the mA for the primary and the screen grid I have 34.7 mA does this all look right and is this my accurate bias reading?

Thanks again!

[Jana]

I would trust these reading more than I would either the 1 ohm resistors you have in there or the bias probe. The margin for error is much less with the measurements across the larger resistance values.

In any case, you now know that your 1 ohm resistors read a bit high so if you use those to set bias, set it a bit hot. When it says 38ma it is really about 35ma. If you use the bias probe you know that it is reading a bit low. If you want 35 you want to set it so that the probe reads about 3 or 4 ma less.

At any rate, all of these readings are within a safe zone for you, they might be off a bit, but a few milliamps isn't going to matter that much.

[halfmassive]

I'm not sure about the ohms law math. Should I simply be adding the volt readings for the primary and the screen grid or do I need to do some division to get the right reading to get the 34.7 I just added the mA readings on the primary and grid.
for the I=e/r do I need to for example do 29.6mA/60.8 ohms =current or 2.9A/60.8 ohms? this gives me readings in the 40s mA?

the math is confusing me a bit (I can do it but I'm not 100% sure on what my numbers should look like)

[Jana]

Here's an example from an amp I have open right now:

Tube A
35.5 ohms on the primary
measured 1.07 volts

1.07 / 35.5 = 0.030 amps (30 ma)

1K screen resistor (didn't measure it, stated value, didn't feel like pulling the tubes, but for an example this is close enough)
1.4 volts measured across it

1.4 / 1000 = 0.0014 amps (1.4 ma)

.030 + .0014 = .0314 amps (31.4 ma)

31.4 ma is the current flowing up from the cathode through the tube.

repeat for the other tube:
28.5 ohms
measured 0.85 volts

measured 1.67 volts on the 1K

this equals 29.9 ma and 1.7 ma for a total of 31.6 ma.

[halfmassive]

I measured the voltage in mA instead of volts. Now I've got the correct reading.

Thanks for all of your help!

[mlp-mx6]

I measured the voltage in mA instead of volts.

This makes no sense to me whatsoever. You cannot measure voltage in amperes. The ampere is a measure of current, not voltage. The volt is a measure of voltage, not current. They are NOT interchangeable.

If you are using a 1ohm resistor, you are measuring voltage. Period. Due to the simple math of a 1ohm resistor (Ohm's law), you can directly convert millivolts to milliamperes. But you are measuring VOLTS. Your meter must be set to reading voltage.

Please, folks who understand this - be VERY CLEAR when instructing someone on this who does not understand. Incorrect information regarding the inner workings of a tube amp can kill. Don't forget that!