Tube burning station

[Mars Amp Repair]

Say,
I am in the process of building a chassis for the purpose of burning in preamp & power tubes. For some reason the QC on tubes (JJ's in particular) has been abominable as of late. I've been experiencing a failure rate of 2 or 3 out of ten or so. I as particularly discouraged when Mesa started using JJ preamp tubes. I have already experienced bad new 12AX7's from Mesa...and they are supposed to be burning these tubes.

Anyway, I've taken to purchasing (an a rate that is impossible to make a profit on) burned & pretested tubes in order to reduce the reworks that result from tubes going bad.

I think I have a pretty good idea of what plate voltages to make for my power supply & bias and have a monsterous power transformer from an old 50's Tectronix scope to feed the filaments, but wonder if anyone has any links or has actually built a station like this.

I intend to purchase the Hickok 539C tube tester for rating the tubes too. I've been reading a lot about them and am impressed at the versitility.

Any inside info? thanx, glen

[Steve Conner]

Hi Glen

I'm sure there are a lot of things like these around. The only one I've seen is John Chambers' valve warmer: http://www.chambonino.com/construct/const6.html

It doesn't apply plate voltage though, it just warms the heaters.

For burning preamp tubes, I'd just make 20 copies of the standard RC coupled amp circuit, and hook them all up to a power amp and speaker, so I could hear if any tubes started making nasty noises.

However, the CF and PI positions in guitar amps put different stresses on the tubes, and it might be worth having different circuits to test tubes for these positions.

Before investing in a tube tester (audiophools have pushed the prices up) you ought to read this:
http://www.jacmusic.com/Tube-testers/index.html

Basically a tube tester is worthless unless it's calibrated, and nobody remembers how to calibrate them any more, except maybe the guy who wrote that.

[Euthymia]

I built such a thing a few years ago for my own tube supplier. 500V B+ adjustable with a Variac, and I made an adjustable C- supply to go with it.

12 octal sockets, so he can really load 'em up.

The one I made only works with octal power tubes of the 6V6/6L6/EL34/6550 etc. family, but with more sockets it could easily do triodes, EL84's, etc.

We borrowed heavily from this concept: http://priceamp.home.comcast.net/~pr...ubematcher.htm

One caution is that the schematic linked to that page fails to include grid stoppers, which caused us a pile of hassle when we first got it going. Couldn't figure out why we'd load it up with known good tubes and then one or more of them would start redplating, and no amount of C- would stop it.

I messed with it for hours, night after night until I made the connection that my radio would go staticky every time the redplating would happen.

The rack was turning into an RF (or maybe even microwave) oscillator!

Simple addition of 1K grid stoppers fixed it.

I later read that Sovtek/EH ran into a similar problem when they built their burn-in racks, but implemented a more complicated solution.

Years later, it's still in use. I did put in the 10R cathode resistors with a rotary so that he could check current draw of each individual tube.

If you're going to leave it on overnight, as he does, putting a 1A fuse in the cathode line will let one tube fail without disrupting the rest of your burn-in.

[Mars Amp Repair]

Hello Erik,
Yeah, I thought it wouldn't be the most complicated thing in the world. Thanx for the tip on the grid stoppers, too.

I have this monsterous power transformer from an old 50's Tektronix scope that should have all the power I would possibly need...I just have to figure out which leads are the ac input. It's all on posts & I neglected to mark the wires when I tore the scope apart :-[

I'll also probably build it to accommodate about 6-8 octal types. thanx again, glen

[Mars Amp Repair]

thanx Steve.
There is a guy in town here who specializes in rebuilding the old Hickok tube testers & claims to have the calibration down.

I'll give that link a look for sure. If I can save some dough or at least get a more modern version, that's ok by me, too. I've been reading how the meters on the Hickok's lose their magnetism or something & have to be replaced & recalibrated.

Thanx, glen

[tim]

Glen,

Those failures you've been noticing as of late with J/J's...is that on their preamp tubes or power tubes (or both)?

It concerns me because I use J/J's only when retubing amps and have not noticed this (yet). I have always found J/J's quality to be some of the best in the industry but if their quality is starting to slip, I'm a bit concerned.

Hopefully it's an anomaly.

[Euthymia]

I have this monsterous power transformer from an old 50's Tektronix scope that should have all the power I would possibly need...I just have to figure out which leads are the ac input. It's all on posts & I neglected to mark the wires when I tore the scope apart :-[

An ohmmeter, a sheet of grid paper, and thou!

Although if you can find a manual for the scope you took it out of, you might get some faster clues. The Boatanchor Manual Archive has most of the old Tek manuals.

http://bama.sbc.edu/

[icefloe01]

If you'd like to make your own PCB's for it, I've been heavily considering this system for the things I'm wanting to build:

PCB Fab in a Box. You need access to a fine resolution LASER printer though.

Eric.

[Mars Amp Repair]

Hello Tim,
I have noticed with the preamp as well as the power tubes for JJ's lately have just sucked. I've talked to other servicers as well as end users & everyone seems to be experiencing the same thing.
JJ's used to be so reliable is one of the reasons I was using them for the past few years. Only once in a while would you have a bad tube.

But now, I'm hard pressed to get a preamp tube that isn't either noisy or just plain shorted. My last batch I bought 8ea 12AX7's & 3 out of them was bad.

I'm disappointed that Mesa has moved to using them exclusively, but they claim to burn them in & give them extensive testing. I hope that's true.

glen

[tim]

Hmmmm, maybe I can find an email address for JJ and drop them an email and see if they're aware of it. It can't hurt.

[Enzo]

Where are you buying these bad tubes?

[Mars Amp Repair]

Hello Enzo,
I have been getting them from either CE Distributing or a local vendor that gets them also from CE Dist. Recently I purchased 8 12AX7's (spiral variety) JJ's & 3 out of the 8 were either shorted or after a short time one triode just plain died.

Then I've had my fair share of power tubes that just short after burn in. They seem to have quite a bit of crap floating around inside the tube. I'm thinking impurities.

glen

[Mars Amp Repair]

yeah,
I'd probably just hard wire the chassis. Not going to be that ambitious..thanx glen

[Bruce / Mission Amps]

thanx Steve.
There is a guy in town here who specializes in rebuilding the old Hickok tube testers & claims to have the calibration down.

I'll give that link a look for sure. If I can save some dough or at least get a more modern version, that's ok by me, too. I've been reading how the meters on the Hickok's lose their magnetism or something & have to be replaced & recalibrated.

Thanx, glen

Hey Glen don't waste your time paying anyone to "calibrate" a Hickok 500-750 series tube tester.
After proper contact cleaning, the calibration is stupid simple and after you do it, all you need to do is use known good rectifier tubes and one good 6L6GC.
Also, I have a bunch of tube testers (I use to collect vintage tube testers as a hobby) and still have an H 539... like myself many others think the H 750 is better then a 539 because the mu meter also has an English scale... which is much more like a real tube tester once you figure out what known good tubes look like in the test mode.
None of the under $1000 tube testers run the tubes at very high voltage (around 160v-180v) so a bench built amp where you can monitor voltages.... like a SE Champ with big transformers and an adjustable bias supply... is much more realistic in seeing what a good or weak guitar amp tube(s) looks/sounds like.

[Mars Amp Repair]

Hey Bruce,
thanx for the info...I'll have to contact you once I get to that point. I have also read about the Hickok 750.

As far as I can determine, the actual transconductance and current draw is relative to the particular tester you're using and the parameters under which the tubes are being tested...esp the current draw...there don't seem to be any standards for plate voltage, grid bias, etc between testers...BUT

From what I can also ascertain for purposes in guitar amps, the actual transconductance or idle current are really not that significant...what is most important is that the numbers match, esp for the quad where you have 2 tubes that are essentially in parallel.

Am I correct in thinking the 'hotter' rated tubes like Groove has, really only refers to the transconductance...and that the higher the transconductance, the hotter they rate the tubes? (hotter meaning breaks up sooner)...sometimes I think those types of ratings are all smoke & mirrors anyway. glen

glen