B&K Model 747 Tube Tester Demonstration and Importance

[@soapboxearth2]

I've got a 707 model. I bought it along with 500 or so vintage tubes for my guitar amp building. 226 bucks for the lot. What an incredible deal. I had to mod it to test 12ax7 properly. I'm told it's recently calibrated. Seems to work great, but lots of the sockets are worn . I have to find a good replacement for the octals . They're smaller than the belton I usually use.

[@TheCleaner6969]

I'm wondering is it ok to just press the Test button? I heard you need to check for shorts & then grid before hand & if the shorts lights up don't test any further. I tested a 12AX7 that had crackle & it lit the shorts light up. I forgot not to go any further & I pressed the grid button for a few seconds & it went into the bad range then realized I shouldn't go further. Now the tube doesn't light up the shorts light & the Grid Emission doesn't fail.. Do you think I damaged my tester?

[@TheCleaner6969]

The other question I had was I had an EL34 tested in an Orange VT1000 & it failed 2x's but in this tester it still showed good.

[@proampsolutions]

The Orange VT1000 tester has very limited capability and only gives you a general idea what is going on. I do not use one for this very reason.

[@TheCleaner6969]

@@proampsolutions Any thoughts as to why my B&K would show a short after I calibrated it & I press Grid Emission (which I know I shouldn't do) the needle went up 3/4 of the scale & when I pressed the shorts button again it showed no shorts?

[@proampsolutions]

Grid emission is a catastrophic failure in a tube. The tube is probably destroyed to the point where the short that existed may no longer exist. If your B&K is properly calibrated, you should test for shorts first, if you have a short test no further the tube is bad - period. You should never persist with a shorted tube, bad things happen. Also, good is relative! That is why there is a scale on the meter. A tube may sound just fine to the listener but in fact may only transconduct 65% of what a new tube does, but it still "works". A tired power tube will likely still work fairly well, it just might take less negative bias to run it at ideal idle dissipation. A calibrated meter is a good tool and should help you quantify results, not leave you with questions.

[@TheCleaner6969]

@@proampsolutions I calibrated my meter correctly & your explanation explains exactly why the Shorts isn't lighting up & I'll throw the tube out. As for the one Power Tube I'm a bit unsure as you stated previously the Orange tester isn't as good so the PT that fail in the orange tested good in the B&K but I heard crackling & the volume cut out so I guess the Orange found something the B&K didn't?

[@DarrenForbes]

Do you always test both plates on those tubes?

[@proampsolutions]

Yes I always test both plates on a dual triode tubes. One can be bad and the other good, or they can be significantly out of balance which may be important, depending on the application.

[@James_Bowie]

That's actually a 747B (B for breaker)

[@Wil_Bloodworth]

Don't bother messing with those Shorts and Grid Emission buttons. No one needs those. LOL

[@James_Bowie]

Shorts? Bah! Just crank up the voltages. :-)

[@jakenikolaus]

👇 *PromoSM*