Is a tube tester a worthwhile investment?

[mike stehr]

 

Around 12-13 years ago, I bought a sleeve of Mullard EZ81/6CA4 from that auction site. Listing said they were NOS, and never a mention of testing them. I wanted some Mullard EZ81s for my Magnavox single-ended 6BQ5 console amplifier. I had no tube tester at the time, so I tried them in the amplifier. Put in the first EZ81, hit the power switch...maybe a second goes by...FLASH! Like a camera bulb...heater to cathode short that blew a hole through the cathode tube. Took out my power fuse. So I went bought another fuse, stuck in another EZ81, hit the power switch....FLASH! So this is getting stupid...I went and visited a audio friend, who had tube testers. We found a couple more bad ones, and the good ones tested like new. I contacted the seller for the listing, told him of the situation, and asked if he had tested the rectifier tubes. He said no, because they were new old stock. Argh... Nonetheless, he replaced the bad tubes and everything was fine. But it's one reason why I bought a emission and transconductance tester.

 

 

"Hello Mike, maybe it was the amplifier that was the problem and not the rectifier tubes? What was the measured capacitance of the input filter cap?"

 

No, it was junk rectifier tubes. The little amplifier still uses the original Mallory multi-sectional capacitor, 40uF for the input filter. I've never measured the capacitance, but the sectional cap has been used for years in the amplifier with no issues. I recently went through and freshened up the amplifier around 9 months ago, I've had no issue after that. I've had the console amp since 2003-2006 or some such.

 

I posted a thread with the Maggotbox amplifier. It's white, and a forum member Sheltie Dave has it...I need to get it back from him...

 

 

"I think it should be 40uF but I have seen and measured up to 100uF."

 

I'm pretty sure for the old Magnavox SEP console amplifiers, it was 40uF. I'm sure capacitance-wise, it measures higher, but like I said I've never had any issues.

 

 

"Also what was the power transformer secondary winding source resistance?"

 

If you mean the DCR of the secondary, it's within spec from what the Sam's schematics of Magnavox, and other Magnavox schematics I have found claim. I can't recall the number off my head, it wasn't the exact resistance number, but exact enough to be a non-issue.

The first thing I do with rebuilding/modding or whatever with a vintage tube amplifier, is check and test the power and output transformers for shorts, DCR, maybe even set up a resistive primary/secondary load and run some square waves for OPTs. If the Magnavox PS xformer secondary had a short or voltages problems, I would have known about it years ago. And once I got a strong Mullard EZ81 in the amplifier, the tube stayed in the amplifier the whole time.

(though I did some tube rolling with rectifiers for a time, and never ruined any in the process.)

 

 

 

"The 6CA4 datasheet specifies 50uF for typical capacitor input filter value, I would use a 47uF. BUT you can still exceed the max peak current rating for the rectifier if the source resistance is to low, so it's a good idea to add 50-100 ohm resistors in series with the plates of the rectifier tubes, this will limit the inrush current so no flash over in the rectifier tubes. No worries in this amp for "sag" in the power supply with the added resistance because it's single ended and the current should not vary very much."

 

I would stick with 40uF, and use a CRC and go higher with capacitance on the second/following caps, or CLC with a choke...there's many ways...

It's a good idea, and I'll keep it in mind, (series Rs on the plates) but I really doubt there is any problem.

 

"Just saying it might be a good idea to go back and look at the amp."

 

I did. About 9-10 months ago.   

 

https://community.klipsch.com/index.php?/topic/147066-homebrew-6bq5el84-sep-amplifier-based-on-rh84-circuit/

 

10th post down...

 

 

Edited September 1, 2015 by mike stehr

[mark1101]

If you don't have a tester how do you know when your tubes need replacing?  They degrade so slowly over time...........as does the sound quality, it is hard to know when replacing them makes sense or is required.

 

I recently had a channel in my LK-72 go bad on me.  Low output, bad sound, etc.  I actually found a bad 12AX7 with my tester and replaced it and all was well.

 

In all my years I never experienced a bad 12AX7 (a failed one)............so my tester definitely helped me out, saved me time and a possible eventual smoke job.

[Wolfbane]

This is a controversial subject which comes up frequently.  My answer is that, for most users of tube equipment, it is not needed (unless a person gets large quantities of tubes and wants to weed out those which are dead, shorted, or otherwise obviously defective in which case a lowly emissions type tester is quite adequate).  Regardless of how a tube measures, its performance in a particular circuit cannot be predicted since designers often use operating parameters which are vastly different from those of the tester.  I've had many tubes which don't test well but perform flawlessly in some circuits.  The reverse is also true where a tube tests "better than spec" but turns out to be noisy or microphonic (particularly evident with our favorite Klipsch speakers).  So, post your opinions on this!  It will be an informative discussion.

Maynard  

 

I own a tube tester, or two, and have tested thousands of tubes for matching purposes mostly with a friend's Amplitrex AT-1000 (he sold significant numbers of audio tubes but he passed away last year). Before purchasing the 1st tester I bought for my own use I should have read Alan Douglas' book 'Tube Testers and Classic Electronic Test Gear'. After reading Alan's book I purchased a couple of tube testers from him that are pretty hard to find today as most businesses that sell tubes acquire these and keep them forever.

 

If you have lots of tube equipment and lots, and lots of tubes it makes sense to purchase a good quality tester, if not find someone with a quality tester and either have them test for you or show you how.