Formula behind "tube amp bias tester"

[oldman55]

Acquired one to compare results for 7355 and 7591 tubes which came with no documents/schematics. I can see it has a 1 and a 100 ohm resistor on its board,Using a nos tube, I get the following readings in circuit with 425v plate voltage:
By way of this device's test leads; 18.5vdc on the VOM. When just testing across this amps 10 ohm cathode resistor on the same tube, I get .405vdc.
Any idea what the "tester's" reading is calculating?

Oldman

 

[s-petersen]

I am guessing, but I think the socket is measuring between the grid and cathode to display bias voltage. The .405 is tube dissipation of 40.5 ma I believe.

 

[gadget73]

Showing bias voltage is somewhat useful, but showing the current would be more useful. What pins do those test leads connect to according to the ohm meter ?

 

[oldman55]

The amp will show current through its own cathode resistor. Its the value given to me by the tester that is confusing. I cant really trace where the test leads come from. I can only see whats inside from the angle of the photo. I was hoping that the value would lead to the formula.

I can see where maybe it is for cathode biased units?

The jumpers appear fixed and the tester does not open. I have only used it on the vdc of the VOM.

 

[Rockon1]

Man its been years now since I tinkered with my guitar amps. I bought two Amp-head probes- one for small pin (EL84) amps and another for large octal tubes. Nice simple design where you could read plate voltage and current draw at the flip of a switch. No probing with multi meter. I believe they have 1 ohm resistors from cathode to ground in the sockets. Switched to them after using a type that broke the cathode connection and measured directly...until one amp went into oscillation as my leads got to close to the guts of the amp... Heres a good read on this stuff.

http://www.aikenamps.com/the-last-word-on-biasing

Heres my Amp-Heads...

 

[whoaru99]

If you have a meter, apparently so, you could (out of circuit) measure resistance from the adapter sockets to pins and leads to the pins to map out what connects to what and the circuit resistances.

 

[Blast]

This is wrong. So completely wrong. A Donovan "Greatest Hits" album without "Riki Tiki Tavi" on it? Please.

 

[oldman55]

I did find this one online but dont know if it is the same as the one I have.
They are testing Ma in this advertisement. I have no affiliation with the product sale BTW.

https://www.ebay.com/itm/2pc-Tube-a...t88-6550-5881-7024-mV-test-mode-/191102191148

 

[Rockon1]

I did find this one online but dont know if it is the same as the one I have.
They are testing Ma in this advertisement. I have no affiliation with the product sale BTW.

https://www.ebay.com/itm/2pc-Tube-a...t88-6550-5881-7024-mV-test-mode-/191102191148

As I mentioned in my last post its been a while- I was in deep years ago with my guitar amps. That said I'd steer clear of ones that measure in Ma. They break the connection and measure in line . As mentioned my leads were too close to something inside the amp with this style probe and it started to oscillate, tubes redplated and a few resistors got real hot before I had the chance to shut it down. Measuring across a resistor in mv style probe is safer in this regard and why I switched probes to the Amp-Head. I should add it what happened with the ma style may have been a coincidence, but upon reading more about them ,I doubted it was and got rid it it in favor of a mv style. HTH Bob

 

[oldman55]

I check current thru the 10 ohm cathode resistor on a 7591 and I get .42vdc.

The question is that if I put this tester in between the tube and tube socket and I get a reading of 23vdc (not .23v), what is it telling me?

Edit: And if I use on a cathode bias 7355 to check tube, what is the figure telling me?

 

[whoaru99]

I check current thru the 10 ohm cathode resistor on a 7591 and I get .42vdc.

The question is that if I put this tester in between the tube and tube socket and I get a reading of 23vdc (not .23v), what is it telling me?

Edit: And if I use on a cathode bias 7355 to check tube, what is the figure telling me?

It's a bit speculative as eluded to in post #2, since you have not told us what pins are contacted by the leads of the tester.

If you want a definitive answer then you'll need to provide the information requested about the adapter pinout.

 

[Rockon1]

I check current thru the 10 ohm cathode resistor on a 7591 and I get .42vdc.

The question is that if I put this tester in between the tube and tube socket and I get a reading of 23vdc (not .23v), what is it telling me?

Edit: And if I use on a cathode bias 7355 to check tube, what is the figure telling me?

.42 thru the 10 ohm should be .042vdc or 42ma. minus the screen current which is only a few ma.so say 38 ma. That would give you a dissipation of about 16 watts which seems hot. (unless its cathode biased) Cant really speak for the numbers that probe are coming out with. They dont make sense to me but Im rusty at this biasing stuff now! Bob

 

[oldman55]

It's a bit speculative as eluded to in post #2, since you have not told us what pins are contacted by the leads of the tester.

If you want a definitive answer then you'll need to provide the information requested about the adapter pinout.

Its buttoned up with adhesive and not easy to trace. I was hoping between the principle and the results that there would be a way to crack what it was testing. It seems to me that it would be current as that is ultimately what we are after.

It is for 7355 and 7591. Online instructions for similar device:

For sale are new Tube Amp Bias Tester Adapters for vacuum tube types 7355 7591 7591A 6BG6 6BG6G and 6BG6GA. Price is per adapter. You don't get meter with this listing.

Instructions- Turn your amp off. Turn your meter to ma scale. Plug the test adapter into your meter and plug power tube on top. Turn on amp (minimum volume). Read current after it

stabilizes after about a minute. You may wish to adjust your bias. Turn off amp and let amp sit several seconds before removing test adapter, and returning your power tube as normal.

I guess I can use it for relative strength of different tubes assuming that that higher the voltage, the more plate current.
Old

 

[gadget73]

you can poke at that thing with an ohm meter to get an idea of where the leads go. Testing between the top and bottom pins will indicate where the resistors are in the circuit too.

 

[oldman55]

What's the question?

Why report volts to us?

Original question was what in regards to the results given by this "tester". Apparently any tube with a pin 5 cathode can be measured in mA and I was trying to figure out the circuitry so I knew the meaning of the results.

It was an attempt to easily test tubes with cathode bias.

However, when I put it in line with a 7591 and set the vom to Ma, it blew my 1200 ohm/10 w resistor going to the screen. I should have trashed the tester as soon as I heard a loud hum when starting up (short?).

As far as reporting volts, I do have a cathode resistor on the amp so that I could more or less verify the results given by the current test.

 

[billyz]

I have several of these plug in bias checkers. All the ones I’m familiar with use a 1 ohm resistor in series with the cathode. You set your meter to dc Volts not Milli Amps . The voltage reading is then easily converted to equivalent milli amps. For a 7591 you need the appropriately wired socket adaptor . The socket adaptors are not universal. The socket for 6l6 , 6v6, 6550, el34 etc are the same but will not work for 7591.
The 7355 May or may not be tested with the 7591 adaptor, you will need to verify how the socket is wired.

 

[gadget73]

7355 and 7591 have basically the same pinout. Only difference is the 7591 has the screen on pin 4 and pin 8 where the 7355 has it only on pin 8.

 

[oldman55]

Looks like a nothingburger to me. I finally received the following:

hello,7591 tube pin5 is cathode pin,using your mulitimenter setting at mA mode and plug tester jack to ma hole and comm hole direct test,here is schematic.
Thanks!


So basically it is just bringing pin 5 to the topside to monitor? For some reason I'm not so sure about that schematic as it fried the 10w resistor in my amp. And I am getting 1 ohm from pin 5 of device to the red lead....from the inside of the socket only.

 

[oldman55]

Where is the (-) lead going out of the meter. Need the whole picture.

I agree and believe the schematic to be incorrect. You get what you pay for I guess.

 

[whoaru99]

I agree and believe the schematic to be incorrect. You get what you pay for I guess.

You could measure, as suggested a few times now, and then know for sure.

Based on the description, it appears the red lead should come from #5 socket of the adapter (where the tube plugs in) and the black lead from #5 pin of the adapter (where the adapter plugs into the amp).

When the adapter is not connected I'd expect low resistance from #5 socket to red lead and low resistance from #5 pin to black lead.

You have to measure mA between the red and black leads. If you were to use only the red lead and measure to ground, you'd be grounding the cathode. In a fixed bias amp usually that would not cause a problem because the cathode is usually grounded anyway. Some amps do some funky things with cathode so I used "usually".

But, in a cathode bias amp doing that would not be good because you'd bypass the cathode resistor and the tube would go full on, so to speak. Damage to something quite possible to happen.