Confused about precise way to measure bias current with "adaptor probe"

[dvdr]

Hi
A newbie here, searched the forum, but did not find an answer. I have some basic electronic knowledge, but I don't want to open my tube-amp to measure bias current via the voltage drop method. Scared to touch high voltage parts, which I think is reasonable for a newbie.
So I thought using one of these "adaptors", installed between the tube socket and the tube, that basically uses a octal adaptor probe with a 1 Ohm resistor in line with pin 8 and measure the voltage drop. Basic equation lets you calculate the current - you all know that.
Here's what confuses me:

• in my amp -if I properly understand the schematic - there already is a 2.7 Ohm resistor, that connects Pin 8 to ground (that would be my measuring point, if I WOULD open the tube-amp...)
• if I install one of those adaptor probes, doesn't that ADD 1 Ohm to this setup and create a voltage divider?
• so, if I measure the voltage drop across the adaptor's 1 Ohm resistor, doesn't that give me a completely different reading, because I measure the voltage drop at the "upper part" of a voltage divider?
• if the latter is true, what equation should I use to determine the bias current?

To even complicate things more, the manual of the bias measuring device ("Biasmaster" - it offers 4 probes) states: …amplifiers, that have each one cathode resistor per tube or pair of tubes (like McIntosh MC30): using two or more adapters at the same time might lead to errors in measurements. So they suggest to only install ONE probe for certain types of amps - is mine such an amp?

I'll attach the schematic of the amp - it's not the original one, but a reverse engineered one I found on the web, so the 8.2 Ohm resistor connected to Pin 8 is the 2.7 Ohm one in my amp...

Thanks for helping!

[sluckey]

• if I install one of those adaptor probes, doesn't that ADD 1 Ohm to this setup and create a voltage divider? It does but it doesn't matter because you should be measuring the voltage across the 1Ω resistor, not from the 1Ω to chassis ground.
• so, if I measure the voltage drop across the adaptor's 1 Ohm resistor, doesn't that give me a completely different reading, because I measure the voltage drop at the "upper part" of a voltage divider? No, for the reason I stated above. ***Across*** is a key word here.
• if the latter is true, what equation should I use to determine the bias current? The latter is not true.

Measuring the tiny voltage across that 1Ω resistor is only half of the bias procedure. You still must know the plate voltage of the tube. How will you do that? Some fancy bias meters will also read the plate voltage. Looks like the Biasmaster only measures cathode current.

[dvdr]

Thanks for your answer! As far as I understood the instructions for that specific amp and the EL34 (btw.: Svetlana "winged C"), I should aim for 35 to 37 mA bias current. That seems to me having been calculated off the EL34 specs and the 400V (or 393V as seen in the schematic) voltage "running" the tubes. (Please correct me if I'm wrong).
So at a 1 Ohm probe, that would equal 35 to 37 mV, right?

Oh, and could you be so kind and shed some light on the "restriction" they wrote about in the manual about installing just ONE probe vs. allowing to install all FOUR at once?

[sluckey]

Thanks for your answer! As far as I understood the instructions for that specific amp and the EL34 (btw.: Svetlana "winged C"), I should aim for 35 to 37 mA bias current. That seems to me having been calculated off the EL34 specs and the 400V (or 393V as seen in the schematic) voltage "running" the tubes. (Please correct me if I'm wrong).
So at a 1 Ohm probe, that would equal 35 to 37 mV, right?

35mV across a 1Ω resistor means there is 35mA current flowing through that resistor. Simple Ohms Law.

Well, your message subject mentions "precise way to measure bias current with adaptor probe". Doesn't seem very precise if you are gonna just guess at the plate voltage without measuring it. So what will you do if the plate voltage is 500V? That's very common with an EL34 tube in a guitar amp. If you want precision, you will always have to measure.

Oh, and could you be so kind and shed some light on the "restriction" they wrote about in the manual about installing just ONE probe vs. allowing to install all FOUR at once?

That restriction only applies to circuits that have two tubes sharing the same cathode resistor. Does not apply to the schematic you posted because each tube has it's own separate cathode resistor.

I don't want to open my tube-amp to measure bias current via the voltage drop method.

Many times you can only access the bias adjustment pot by removing the chassis.

[PRR]

Draw the whole circuit out. (With actual values and meters.)

There's no "divider".

Measuring "only" current may tell you if it is wrong but can not tell you the amp is biased right.

Get alligator clip leads (not the cheapest!!), pre-connect, and power up with your hands OUT of the amp.

Folks here have repaired MRIs, radars, oil ignition systems, CRT TVs. Big unforgiving electricity. There is always a way to measure without putting your hand in harm's way.

[dvdr]

Just wanted to send a big thank you for your quick and most helpful answers!