How to test tubes

[ejb14]

Hi folks;

I found some 6FJ7's at thetubestore.com and decided to purchase 4 of them as they are claimed to be NOS, and I wanted to see if I can match power output closely on my amp's channels (rather than add a resistor inline on the input). My S.E.X. amp is otherwise functioning according to specification.

I have 8 other 6FJ7's, 2 came with the kit, one I purchased NOS from another site, and 5 others off ebay kind of pot luck. The ones from ebay are of varying quality, but do function. A couple of them appear to be pretty good - the others function but have minor issues (small occasional pops, static, or hum)

However, all 8 of them, when I feed my amp with 0.777V 60 hz signal, all produce about 3.75 to 4.5 Volts on the outputs with the volume control at full - great - all is working well with the amp and these tubes for power at least.

All 4 of the 6FJ7's sold to me by thetubestore.com, when presented with the same 0.777V 60 hz input signal, produce 0.3 V - about 15 times less output.

They are refunding my purchase, but say have a large stock of these and they said a random sampling test shows they are just fine; they said there must be something wrong on my end because I could not have received 4 bad tubes.

Am I missing something?
Thanks
-Ed

[Paul Birkeland]

The range of gain you are finding is 14.57dB to 20.7dB.  The 14.57dB isn't all that surprising, but the 20+dB of output is.

Did you measure DC voltages on the 4 6FJ7s you believe to be bad?  It would be statistically nearly impossible to get 4 bad tubes like that.  Did you double check the printing to ensure that they are 6FJ7s?

I would agree with the seller that something is more likely to be amiss on your end.

-PB

(PS - if you run the amp 24/7/365, you can expect about 5 years out of these tubes.  If you run them significantly less than that, an extra pair and the tubes provided could be good for a lifetime)

[Doc B.]

Are you sure you used a 0.777 volt signal and not a 0.077 volt signal when you tested the new tubes? That could create the result you are seeing.

Is the volume pot turned all the way up?

[ejb14]

That would be a good catch Doc! But no, it was the same signal..

Does this make any sense though -

I re-did my checks on the amp tonight - resistance check and voltage check with Kit supplied tubes - all within spec.

I then did the voltage check with all four tubes from thetubestore.com, and all within spec. I figured, must've been me last night - so I played some music through my DAC, almost no sound until turned the volume all the way up, at which point it was like a background music level - very low. I swapped back in the kit supplied tubes - and felt the power! Couldn't turn it up past 12:00 o'clock - too loud for my ears.

It was like I had a big resistor in there somewhere - so I re-did the resistance check again with kit supplied tubes installed - all measured within spec with what you have in the instructions.

I then installed the tubestore.com tubes - all resistances check out EXCEPT Terminal 4 and Terminal 18. Instead of 248 KOhms, I get 215 Ohms for T4 and T18. Off by a factor of 1000. A four tubes from them read like this on the resistance check. All four tubes from them the voltage check is within spec.

Does that mean anything?

[Paul Birkeland]

Man, that's odd.  That indicates a short from the grid of the second triode to ground.  This still has me wondering if you got something other than a 6FJ7 in the box!

[Doc B.]

Yeah, that was my thought too. Don't have my tube manuals unpacked yet. One could look for a pinout that might fit the scenario.

[ejb14]

Ok - here are some PICS -

[ejb14]

Are there two pins on the tubes that will show this short if it exists? I could measure a 'good' tube, then these 'bad' tubes with the DMM to see what the difference is and report it back to the vendor so they can do the same with their stock (assuming this is an issue with the tubes).

Thanks for you help as always!

[Paul Birkeland]

I suspect it would be pin 3 showing continuity to pin 1 and pin 12.

[ejb14]

Hmm -

Pin 3 does not have continuity to 1 or 12, but does have continuity to Pin 8 on all four of the 'bad' tubes.

On my 'good' tubes, Pin3 does not have continuity to any other pin.

That must say something right there..

[Chris65]

EIA Base 12EJ has pins 3 & 8 both connected to the grid of triode section 2. My guess would be they are 6FM7.

Hope you didn't pay the $9.95 Tubestore are asking? Should be no more than $5 a piece, ESRC tubes shows them for $3ea.
I bought some from ESRC a couple of years back for $1 a piece.

[ejb14]

Wow - if that is the case (if they are not a 6FJ7), then they are labeled wrong, and potentially the vendor's entire stock is labeled incorrectly. Glad it didn't wreck my amp. That might explain why the tubes 'tested' ok according to the vendor. Sure they test ok, they are just the wrong tube.

And, yes, I did pay $9.95, though they refunded it all. I don't mind paying $10 a tube for something that works and is high quality. I was not aware of ESRC - thanks will check that out.

[Paul Birkeland]

Yeah, pin 8 on the FJ7 goes to nothing, but in your tubes apparently it is arranged like an FM7.  I will have to check future tubes before they go out to be sure we don't supply any like this!

If you want to try these, a resistor and a wire connect at pin 8.  If you remove the resistor, then move the wire to pin 10 where the other end of the resistor was, you should get sound at least out of the amp with these tubes. 

I would be very interested in knowing whether the amp works with these tubes and this mod, just to see if these are some kind of mutant 6FJ7 or just mislabeled 6FM7s.  Your assistance would be greatly appreciated!

-PB

[Chris65]

And, yes, I did pay $9.95, though they refunded it all. I don't mind paying $10 a tube for something that works and is high quality. I was not aware of ESRC - thanks will check that out.

At least you got your money back. Other options are VacuumTubes.net & Findatube.com which both also list the 6FJ7 @$3.

[ejb14]

Yeah, pin 8 on the FJ7 goes to nothing, but in your tubes apparently it is arranged like an FM7.  I will have to check future tubes before they go out to be sure we don't supply any like this!

If you want to try these, a resistor and a wire connect at pin 8.  If you remove the resistor, then move the wire to pin 10 where the other end of the resistor was, you should get sound at least out of the amp with these tubes. 

I would be very interested in knowing whether the amp works with these tubes and this mod, just to see if these are some kind of mutant 6FJ7 or just mislabeled 6FM7s.  Your assistance would be greatly appreciated!

-PB

Sure - I will give this a go perhaps this weekend. I use my amp every day, so I will have to suffer with a regular solid state music while I do this work though ;

The folks at thetubestore.com said they removed the tubes from inventory as they all tested the same way (Pin3 continuity to Pin8); however, they also said the tubes did not successfully test as a 6FM7 either.