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Plate current question EL34

Chrisxo55441

Chrisxo55441

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I have a Maxi Matcher 2 and was testing some Mullard EL34 xf2 tubes and got a plate current reading of 17.6 mA on one at 400v. The chart on the tester says the expected readings for EL34 is between 21-50 mA. im just wondering if there is a minimum mA that EL34 tubes should be before they are considered bad?

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If there are no other issues the tube should be fine. The idle current will change with bias level and voltage. It may be more difficult to find matching tubes if the value is unusually low.
 
JohnT123 said:
If there are no other issues the tube should be fine. The idle current will change with bias level and voltage. It may be more difficult to find matching tubes if the value is unusually low.
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the other 3 test between 19.3 and 19.8 so matched pretty close. the other 3 test around 100% on my TV-7D/U at 48-50. the first one is 38 with 30 being minimum so probably not a long life expected. i bought these as a used matched quad but the numbers quoted were a bit higher using the same tester 21.5 for the one in question and about the same for the others so im trying to decide if i want to return them, they are the dual getters so excellent tubes but that one tube doesnt seem to fit in that quad to me.
 
any data on what sort of grid and screen voltages are applied while testing? If it doesn't reasonably jive with expected current in the datasheets, I'd say that means its too pooped to pop, or at least its just electrically different enough to not be a match.

a power output test would confirm this, but unless you've got a single ended amp or a test fixture for the purpose thats kind of hard to do.
 
Well, you could return them, or consider that you got three good strong Mullards for the money and look for another strong single. Easy for me to say since I don't shop for them and am not up on prices and availability. Either way you don't seem to have gotten a matched quad. If the getter on No. 2 is any indication, that looks pretty worn.
 
zackthedog said:
Well, you could return them, or consider that you got three good strong Mullards for the money and look for another strong single. Easy for me to say since I don't shop for them and am not up on prices and availability. Either way you don't seem to have gotten a matched quad. If the getter on No. 2 is any indication, that looks pretty worn.
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No 2 is the tube in question. i thought about just adding another tube so went through all my 00 xf2's ( i have about 40 mostly NOS) the lowest mA reading i have is 27.5 so the low readings on the this quad make me wonder too. i really didnt need any more tubes but a matching quad of 00 at a reasonable price i thought what the hell it could be a good investment down the road but not so sure now.
 
Is it just me, or is the getter on #2 quite a bit more 'used up' than the others?

I have a feeling it was "used hard" in an amp at some time. Maybe with a leaky coupling cap causing it to run at a higher bias than the others- hence, the wear/lower specs now.

Regards,
Gordon.
 
If you look closely at #4 you'll see it also has some clear glass up top. Definitely have some hours on them.

I had a guitar amp customer that would buy Mullard 34's online and have me install them. Usually took 6-8 tubes to get a matched quad. Occasionally one would just die when hit with a working voltage around 500 vdc. Testers just don't tell the whole story.

Hope the tubes work out for the OP.
 
hm, pic didn't load previously, the info I was wondering about is right on the label. 400 volts, probably plate and screen, and -36v of bias.

anyway, datasheet conditions for 400/400 and -36v should be 34ma per this

https://www.vtadiy.com/loadline-calculators/loadline-calculator/#calculator

Not sure what the official +/- tolerance is but 50% of normal I'd say is whipped. I wouldn't really think the other 3 are all that healthy either based on that data.
 

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Yeah, even the 21ma minimum stated on the Maximatch tester, is less than 70% of 34ma. I would really take that to mean that these tubes are all pretty weak.

Regards,
Gordon.
 
Try testing each tube one at a time. You will find the current value will increase significantly. This is because all four tubes plugged in will put a heavier load on the Maximatcher 2. This is a known issue with these testers, as the heater current draw of all four tubes will load up the power supply.

Also, the line voltage will affect the results as the power supply on the tester is unregulated.

I also own a Maximatcher 2, and they are pretty good testers, but those are well known weaknesses of the design...
 
Kidmoe said:
What is the transconductance reading of the tube? Is it also below indicated values?
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crooner said:
Try testing each tube one at a time. You will find the current value will increase significantly. This is because all four tubes plugged in will put a heavier load on the Maximatcher 2. This is a known issue with these testers, as the heater current draw of all four tubes will load up the power supply.

Also, the line voltage will affect the results as the power supply on the tester is unregulated.

I also own a Maximatcher 2, and they are pretty good testers, but those are well known weaknesses of the design...
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Thanks Crooner i didnt know that, but now i have to retest about 100 tubes :( the tube did jump up to 21.6 after retest but the mMhos is still under min at 3.81. the other three went up to 21 range as well but still low on mMhos 2 of them. My house voltage is high 123-127v, i do have a variac should i run the tester at 120v? at least now i know the seller wasnt try to misrepresent them.


mA
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mMhos
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gadget73 said:
hm, pic didn't load previously, the info I was wondering about is right on the label. 400 volts, probably plate and screen, and -36v of bias.

anyway, datasheet conditions for 400/400 and -36v should be 34ma per this

https://www.vtadiy.com/loadline-calculators/loadline-calculator/#calculator

Not sure what the official +/- tolerance is but 50% of normal I'd say is whipped. I wouldn't really think the other 3 are all that healthy either based on that data.
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Thanks thats the kind of number i was looking for. All my other EL34 except a couple test above 30 (mostly NOS though) but those low numbers gave me concern as ive never had any readings like that.
 
Chrisxo55441 said:
Thanks Crooner i didnt know that, but now i have to retest about 100 tubes :( the tube did jump up to 21.6 after retest but the mMhos is still under min at 3.81. the other three went up to 21 range as well but still low on mMhos 2 of them. My house voltage is high 123-127v, i do have a variac should i run the tester at 120v? at least now i know the seller wasnt try to misrepresent them.


mA
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mMhos
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That sounds like the one in question, probably has heater sensitivity.

Bringing up the heater voltage (by running it by itself in the tester) makes it closer to the performance of the others- but it HAS to have the higher heater voltage to get enough cathode emission. That's likely a sign of a worn-out cathode. Not much life left in that one...

Regards,
Gordon.
 
Yeah, that's my experience exactly with the Maximatcher 2. In your case, the line voltage should not be an issue. If it was lower than 120V, it would be a different matter.

The Maximatcher 2 manufacturer is not clear on this, but I have also found that preconditioning the tubes (let them run on the tester for about 10 minutes, and then applying high voltage to each for at least five minutes), also makes a difference in the readings, specially Gm. They do offer a "MaxiBurn" rack as an option for "power users".

Now, preconditioning the tubes before taking a reading make sense since this is exactly how you would do when setting up bias on a tube amp. Wait at least 20 minutes to perform an initial reading/adjustment and then wait an hour for the amplifier to stabilize and make final adjustments.




Chrisxo55441 said:
Thanks Crooner i didnt know that, but now i have to retest about 100 tubes :( the tube did jump up to 21.6 after retest but the mMhos is still under min at 3.81. the other three went up to 21 range as well but still low on mMhos 2 of them. My house voltage is high 123-127v, i do have a variac should i run the tester at 120v? at least now i know the seller wasnt try to misrepresent them.


mA
View attachment 2334041

mMhos
View attachment 2334042
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might be an interesting experiment to pop a voltmeter into pins 2 and 7 to see what the heater supply actually is
 
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