Matched tube testing on Hickok 6000A

[Mars Amp Repair]

Hey guys,
This might be the dumb question of the week, but here it goes.
I have a Laney GH100S amp with 2ea EL34 Genalex tubes & 2ea EL34 JJ tubes (That's the way I received it).
I tested the idle current on both sets of tubes and the Genalex tubes idle current was about 20ma & the JJ's were 62ma. Plate voltage was about 440V. (Screen voltages & plate voltages are very close).
SO, what am I missing that made the Genalex tubes so much less idle current/wattage than the JJ's?
I've tested the tubes an find the idle current and gm on all 4 tubes all are very well matched.
SO, what facet of the tube with both the current draw & gm on the hickok matched, is causing the mismatch when placed into this amp.
BTW, I installed a new EL34 MQ in the amp and all the idle currents were matched around 38ma after I calculated the idle wattage.
Again, what facet of the tube would be so different that the tube tester wouldn't pick up. BTW, I have a current tap adaptor I made and use to measure the idle current on the Hickok 6000a.
Just when you think you've figured it out, along comes the monkey wrench that makes ya question everything!
Thanx, Glen

[Helmholtz]

What screen voltage does the tester use?

[g1]

Are the JJ's definitely EL34 and not their E34L ?
I recall the E34L needing more negative bias to run on par with EL34.

[Mars Amp Repair]

Actually, the original tubes were all KT77's with no suffix letters. I replaced them with EL34's.
Tube testers are weird & use unfiltered DC and AC in some instances to feed the tube. Honestly, I've never really understood how they work. I tried to measure from the K to the Screen or Plate & the readings don't make sense.
Either way I cannot quickly measure the voltages on the tube.
g

[g1]

What I was trying to say is that JJ sells 2 different types of tubes. One is EL34, one is E34L. They are not the same, and if you are not aware of this, it can be overlooked.

[Mars Amp Repair]

What I was trying to say is that JJ sells 2 different types of tubes. One is EL34, one is E34L. They are not the same, and if you are not aware of this, it can be overlooked.

Got it...Thanx g1.

[nevetslab]

It was April 2022 when I dragged out my Hickock 539A Tube Tester to have a go at getting it back up and running. Prior to that, all my tube testing was done on / in the chassis of the amps being serviced, but now we all had Putin's Invasion of Ukraine to deal with, when the source of vacuum tubes dried up around the planet for some time.

But, my efforts in getting that instrument completed and back in shape was unsuccessful. So, looking up on the top shelf in the shop, I pulled down the Rack-Mount Yamaha T100 Tube Guitar Amp and had a look at it inside and out.





Besides having a Compu-Bias Probe Meter and four Octal Test Probes, this amp came to the rescue to assist in sorting tubes from the ever-growing collection of pulls, as well as matching new tubes back then when I could find them, as well as being able to listen to them at normal operating conditions, which you CAN'T do with the Compu-Bias Probes, which can't handle the load currents in that mode.

I will confess NOT having come upon the E34L power tubes.

After modifying this dusty Yamaha T100 and making it a handy test instrument, I turned my attention to the two Blackstar Model 200 Bass Amps that I had been given as Stage Props, and after some digging, found I had a suitable chassis, along with a suitable Power and Output Xfmr that had the beginnings of creating a Hiwatt DR201 Bass Amp/Power Tube Test Fixture for sorting/matching KT88's and 6550 Power tubes. That project came out very nice and yielded a really nice sounding bass amp to boot!



Still one of these days I'd like to get back to the Hickock 539A Tube Tester and find what I've missed or need to complete THAT task.

[Greg Robinson]

Are the JJ's definitely EL34 and not their E34L ?
I recall the E34L needing more negative bias to run on par with EL34.

To add to the confusion: Groove Tubes, back before they were bought by Fender and were still run by Aspen Pittman, offered a tube they designated E34L, with added "heat wings" attached to the top of the plates. They claimed a plate dissipation rating of 30W, compared to the standard 25W of EL34's. I don't recall where they were manufactured and my google-fu is failing me today. They otherwise behaved as regular EL34's given identical operating conditions.

JJ at some point began offering a tube they similarly designated E34L. This, unlike the Groove Tubes version, was an EL34 modified with a wider control grid spacing to require a greater negative bias voltage, so they should bias up appropriately in amps designed for 6L6GC's without needing to modify the bias circuitry. Of course, you'll still need to be sure that the power transformer can handle the extra heater current requirements, and that the octal sockets are wired appropriately for EL34's (grounded pin 1, not used as a jumper point for screen or control grid stoppers). It still has a plate dissipation rating of 25W. Unfortunately, many websites selling the JJ E34L STILL list it as a "more powerful EL34". Even the datasheet from JJ is not immediately clear, as both EL34 and E34L share the same datasheet, and only show plate curves for the EL34. The fine print under typical characteristics reveals the diffence though.

So don't go installing E34L's in an amp designed for EL34's, you likely won't have a bias range negative enough to avoid overdissipation or outright redplating. And they're not really a direct drop-in-replacement for 6L6GC amps either, so I don't really know who this product is for...

PS: does anyone else remember the fad of "cryogenically treated" tubes oh, say 10-15 years ago? Thinking back made me recall them.

[mozz]

I worked for lucent back 25+ years ago, we thermal cycled parts. I don't remember the temps but it went from very hot to very cold many times. It weeded out the weaker parts, whether it helped the parts settle I don't know.

I would think it would help somewhat with tubes, how much I can't say. Was it worth it for longevity purposes I can't say either. I bet some engineers who work with temp cycling could offer some insight.

[Mars Amp Repair]

All very cool using the Yamaha amp to match tubes as well as check for microphonics. How do you determine the gm or gain of the tubes for that facet of matching? Are you scoping the grid Vp-p and calculating gm from the plate current?
I guess I'll not have an answer to why these 2 pairs of EL34's (one pair the Genalex and JJ's) that draw the same current & have very similar gm on the tester, would draw such different currents when in the amp no matter what position they were in. I suppose thinking about it that the tubes load line at a higher voltage may not be linear (esp with older used tubes) as their older tubes creating a very different at different Plate voltage.
Just guess.
I also of course test the tubes in the amp I'm testing for microphonics and shorts (nice lightning storm in some ), but that wouldn't help if you're testing a batch of used tubes for quality and matching.
Thanx. All very interesting.