ID this tube?

[angelodp]

Hi, if anyone can kindly help ID his tube? 12AX7, maybe you recognize the graphics?

[bepone]

ECC81 most probably , need to put tube on the tester to see the currents

[angelodp]

I have a very basic tube tester, please expand on possible testing. So that 81 on the bottom is
a clue yes?
Thanks A

[maxkracht]

Sometimes if you pop the tube in the freezer for a bit, the condensation will show an impression of the worn off screen print when you take it out.

[angelodp]

Found this method -

Measure dc voltage either side of the 100K plate resistor (pins 1 & 6) divide voltage drop by resistor value, measure dc voltage at the tube side of the 1.5K cathode resistor (pin 3 & again divide voltage by resistor value.

Connect a 4ohm dummy load to the speaker jack, apply a low AC voltage to the high gain input until you see say 5VAC at the dummy load (volume wide open) and see what AC it takes at the input to generate that (for differing speaker loads you will need to fine tune the VAC you look for - we're not really concerned with the amp's clean W RMS rating, we just want to see what impact the preamp tube has). Then either stick to that AC input voltage and mark the difference in tubes & output, or always hit the same dummy load voltage and mark what input it took to do that. Do one method, or the other, Don't mix. Record AC voltages at 12AX7 pin1 plate, 12AX7 pin 6 plate, 6V6/power tube pin 3 plate.

Remember that any tube specs are nominal, there will be small variations from brand to brand, even from tubes of the same brand.

I can already tell you that typically a 12AX7 tube will show ~68% of the dc voltage supplied to the 100K plate resistor from the B+ rail, at its plate (pin 1 or 6) in this type of circuit, ~50% for a 12AT (that was a JJ, they show more voltage that typical NOS), ~48/49% for a 12AY, ~30% for a 12AU7. Even tubes that show the same current draw/voltage still sound different...then you get bored measuring & just use the above figures as a rule of thumb, diagnostic tool!

[Stevem]

Due to the large area of white lettering that tube has and the size and shape of the plates and the round getter I think this is what your tube is, or atleast the brand that it is.

[angelodp]

Thanks Steven, may I ask how you made that ID? Did you use a google search? other? Compared to what you have?

Very kind.

[Stevem]

It rang a bell to me since I have had a few great Marconi tubes over the decades, and instead of diving into one of my amps that has one in it to get a photo for you I went to the Tube World Express site for a photo.