Looking for tube reliability input

[heisthl]

I always burn amps in for at least 10 hours and usually put them on a 4 hour gig test before shipping. I tap on the tubes looking for microphonics at different times along the way of the burn in process. Recently I got bit by a bad tube in a newly sold amp that was doing a high profile gig. The problem was a JJ 12AX7 in the PI position. The performer was basically running the amp on 10 for 2 hours when the problem occured. I have attached a video of the symptom. Anyone have a bright idea on how to avoid this type of failure in the future?

[ayan]

I always burn amps in for at least 10 hours and usually put them on a 4 hour gig test before shipping. I tap on the tubes looking for microphonics at different times along the way of the burn in process. Recently I got bit by a bad tube in a newly sold amp that was doing a high profile gig. The problem was a JJ 12AX7 in the PI position. The performer was basically running the amp on 10 for 2 hours when the problem occured. I have attached a video of the symptom. Anyone have a bright idea on how to avoid this type of failure in the future?

Warning: babbling ahead and take everything with a grain of salt, as I am an engineer and not a tech (and those of us in industry know what that means). Anyway, if the heater electrons go to the cathode -- due to their physical proximity, because the tubes are built sloppily -- one thing you can do is change the voltage reference for the 6.3 VAC winding so that the PI's cathode doesn't look so positive to those electrons anymore. Makes sense? I have never done this, but referring the 6.3 VAC supply "ground" point to a higher (or lower) DC voltage may solve your problems for you.

Gil

[Fischerman]

I thought that JJ ECC83S tubes were some of the more reliable preamp tubes but that's based as much on what others (somewhat 'in the know') have reported as my own experience (I have a bunch and all seem fine as far as reliability...I'm usually not crazy for the tone but the PI is a great spot for them...or so I thought).

I have also read that the Sovtek LPS tubes don't like high cathode voltages...as Gil said it's usually the heater-cathode voltage that's the constraint. And FWIW, a high-voltage Marshall has way, way more voltage on the cathode in the cathode follower triode than any PI I've ever seen. IIRC, Soldano specifically said not to use LPS preamp tubes in the sockets that have cathode followers in their amps (maybe Google that...you might get something).

I've done the raised heaters thing (usually creating a 40-60vdc node and connecting the heater CT to it) in several amps but I built them that way from the get go so I can't make any claims on reduced noise or increased reliability or anything. They're high-gain and quiet though.

[VacuumVoodoo]

Warning: babbling ahead and take everything with a grain of salt, as I am an engineer and not a tech (and those of us in industry know what that means). Anyway, if the heater electrons go to the cathode -- due to their physical proximity, because the tubes are built sloppily -- one thing you can do is change the voltage reference for the 6.3 VAC winding so that the PI's cathode doesn't look so positive to those electrons anymore. Makes sense? I have never done this, but referring the 6.3 VAC supply "ground" point to a higher (or lower) DC voltage may solve your problems for you.

Gil

This is a good advice but wouldn't have helped in this case.
The failure was due to sloppily assembled tube slipping through QC at the factory: heater wire in close proximity to and making contact with cathode wire inside the tube.
Seen this happen with "reliable and better than best NOS" tubes too. No reason to bash JJ, just be aware it's something that can happen. Too bad when it happens during a gig.

[heisthl]

I'm sure others on this Forum have been haunted by bad tubes shortly in the performance cycle of a new build. Short of hitting this tube with a hammer when it was hot I don't see how I could have caught this type of tube failure because normal "microphonic tapping" didn't bring it out until the tube really cooked for awhile. I was hoping for a magic solution to avoid this in the future. I'm not ready to bash JJ tubes yet but I probably will not use them in future builds.

[VacuumVoodoo]

I was hoping for a magic solution to avoid this in the future.

Well, short of putting the amp on shaker rig during burn in there's not much else you can do to catch this type of failure

[IMG:498:664]http://i26.photobucket.com/albums/c149/ ... hake_2.jpg[/img]

[heisthl]

That might do it - great idea, beats my coffecan under a board intermittants finder. I'll have to drive through the neighborhood looking for the old washing mashine in someone's yard to rig up for an imbalanced spin cycle test platform or maybe just a hinged board on a pitman connected motor. It looks like yours is an offset wooden disc forcing a side to side jarring motion.

[talbany]

You can always ship it to yourself... I recommend UPS.
If it comes off one of their trucks and works ,its indestructable!!!!!





Tony VVT Amps

[heisthl]

UPS is a good test but not as good as DHL (Drop Hard and Lob).

[talbany]

I don't know about DHL but Ive shipped some of our 100 watt Overdrives with Marshall style Z mounts and have had both transformers leaning at about 15 degrees. serious enertia......





Tony VVT Amps

[heisthl]

I shipped a twin reverb to Dumble comversion in a AWA Road Case via DHL and they flattend one of the raised rounded knob type corners of the case, the amp inside had cabinet splits at both top corners and ripped the tolex . Later I did a test with a sledge hammer on one of the same style corners and as near as I can tell it would have had to have dropped 10 feet and landed on that corner to flatten it that much. Maybe they pushed it out of the plane onto the runway.

[VacuumVoodoo]

It's getting OT but...
Don't you guys use these shock indicators when shipping amps? I put one or two on the outside of box (so shipper knows) and one inside on the amp itself.
I also use independent freight brokers, never a problem. Had an amp arrive in Japan with a slightly damaged box corner. Shipper sent me a photo and asked the customer to unpack the amp with their agent present to see if any damage was done the amp. Everything was ok.

About the shaker rig. It's not mine This was a test done at a certain car manufacturers factory in my town as part of CE certification process.

[talbany]

heisthl

WOW!!! that's a good one!!!!
I use DHL for my overseas shipments..
and so far have not had to file a claim (knock on wood).
We use the instapak forming bags work great but cost's us $50.00 in packing material to ship our Fralin Model amp.. .




Tony VVT

[Fischerman]

FWIW, my understanding is that most of these obvious 'drops' are from elevated conveyors. And as of several years ago, UPS used way more elevated conveyors than FedEx (that may have changed since). What bugs me is...if you drop a big-heavy box 10 feet from an elevated conveyor...you broke it. But they'll deny claims based on some technicality of the box (eg - "the box wasn't brand new") or packaging (which in some cases is terrible...but in many of those cases they never should have accepted it).

[Buschman]

Anyone playing a tube amp professionally should always have an extra set of tubes with them & know how to deal with it. They just go bad. Odd that it showed up in the PI & not a gain stage. I will sure be glad when some of the new 12ax7s wind up bullet proof.