Update my power tube knowledge

[RyanThomas13]

I'm going thru my pile of old tubes with the goal of determining the life and strength of what I have, many of them are EL34's probably about 20 or so. It is mostly old stuff I've collected off ebay over the last 10-15 years. I checked them all with my Hickok 600A this week, they test in the "good" range between 8250 & 10,000 micro mhos. This reading I know doesn't mean everything (or anything depending on who you talk to), so I also checked them all for current in my Hiwatt DR504 (adjustable bias added for each of the power tubes). This is where I scratch my head a bit. This week I got a new production JJ EL34 from tube depot, I wanted this tube to be my test tube for my Hickok so that I can write down the measurement and check it in the future to see if my tester has drifted. It measured 9800, which would put it in the top 3 of the tubes I have. Likewise I also wanted a new production tube so that I had something to compare all my old tubes to, as far as current is concerned.

Right now my bias was set to roughly 35mA @ 475 plate volts and this was with a pair of old Brimar tubes I've had for a long time (one tube has been used long enough that all the lettering has faded). They both have identical current and measured 35mA regardless of the position I had them in, this made it possible to measure my other tubes in pairs knowing the adjustable bias was set identical on each side.

A quad of groove tubes GT-EL34-M, a minimum ten years old and of unknown usage measured 35, 35, 36, 39mA
Three Japanese Matsushita's 35, 35, 36mA
A pair of German Siemens measured 24, 29mA
A pair of Mullard XF2's measured 37mA, 19mA
(other misc. stuff)

The highest current pair I have is some fat bottle G.E. 6CA7's that measured 42 and 63mA!! (huh)

And the new production JJ measured a whopping 22mA, the second lowest one I have.

Now having all this data I'm left feeling a little bit like I don't fully understand the criteria for which tubes are strong and which are weak? And how is it possible that a US made 6CA7/EL34 has triple the current of a new production tube?

[mhuss]

6CA7s are not true pentodes, they are beam tubes like KTs or 6L6s. They are generally equivalent to EL34s, but I'm sot surprised there's some differences.

The JJs I have no explanation for.

[RyanThomas13]

Thanks Mark,

So then those 6CA7 measurements mean about as much as my KT77 measurements

So what I want to do is use my pair of Mullard XF2's for this, knowing most of the other old stock tubes are in the mid 30 mA range, do you have any feelings on using the one that measured 19mA, too weak? My tester said it was still in the middle of the good range, like 8750 I think. Or is it a "if it sounds good than it is" situation.

[jabguit]

6CA7s are not true pentodes, they are beam tubes like KTs or 6L6s. They are generally equivalent to EL34s, but I'm sot surprised there's some differences.

Always suspected this.
I've tried finding data sheet(s) for any US-made 6CA7s like Sylvania or GE, but cannot find any. Any help?


cheers,

[pdf64]

I’ve got an old worn pair of GE 6CA7 and can confirm that they’re almost certainly a beam, rather than suppressor grid, pentode.
The weird thing is that they draw as much screen grid current as regular EL34. They don’t sound or perform any differently, that I can tell.
eg open loop output impedance works out pretty much the same whether the amp is fitted with them or EL34.

I think the unusual thing about post #1 is how many EL34 are idling at a closely similar level. My collection of them is a bit more spread out.

[Stevem]

One thing you might want to do to your 600a to level the playing field more like I did with mine is to wire in a voltmeter to monitor the filament setting on that 8 pin socket so as to keep it right at 6.3 volts.

I wired mine into a socket saver.

That analog meter in the Hickok is crude for this!

I found from resistance testing that tubes heaters are not all the same.

Here’s a small sampling.

GE 6CA7. .7 ohms

Mullard EL34 xf2. .8 ohms

Soctec EL34. 1.1 ohms

JJ EL34S. .6 ohms.

I have also found that during the Summer when I use my tester plugged into the same circuit that my AC is on that the filament voltage will drop when the units compressor kicks on.

Have you calibrated your tester with a known 6L6?

[RyanThomas13]

Thanks Steve,

Thanks for the tip on heater voltage, this had never occurred to me. And just to clarify you are not using an external power supply (other than the tester) for the heater voltage correct? You are just watching the meter so as to fill in the gaps about differences between tubes.

And no I don't have a check tube for the 600A, this new JJ EL34 was suppose to become my check tube ...but I'm not sure how I feel about using a check tube with such low current.

Maybe I'll get in touch with tube depot and see if they'll swap it out for something with a bit more juice.

Like I said in the original post, getting a new production tube and checking both with the 600A and in circuit was intended to paint a more clear picture of what I had stocked but now I feel like its all gray area. Knowing that a new EL34 can be as low as 22mA, that would mean in this particular amp from 15?mA-45mA would/should all be considered good healthy tubes. That range just feels too wide to me, but what do I know.

[pdf64]

For a given set of electrode voltages, it seems reasonable for a random functional valve to have an anode current of around +/-50% of a typical / nominal / bogey level.
Your results seem to align with that range pretty well.

[mhuss]

6CA7s are not true pentodes, they are beam tubes like KTs or 6L6s.

Always suspected this.
I've tried finding data sheet(s) for any US-made 6CA7s like Sylvania or GE, but cannot find any. Any help?

To confuse things further, European manufacturers sold EL34s (true pentodes) marked "6CA7" for the U.S. market. Unfortunately, I don't have any data sheets that reflect the internal construction of these tubes, but as pdf64 said, you can clearly see the beam-forming plates (as opposed to a suppressor grid) on lots of US-made 6CA7s.

[Stevem]

No, I still run the tube by means of the Hickok heater supply.

Have you tested out the two rectifier tubes in that tester?

That big Mercury filled 83 rectifier is getting hard to find.

You can convert these to SS recto which is something I may yet do to mine, that is if I live long enough to use up my last two 83 rectifiers, lol!

Do you have the needed info and gear to check the calibration of that tester?

I test alot of tubes with mine and I check it’s cal twice a year.

If you get into working on Ampeg amps and need to test the 12 pin compactron tubes some of them have I can tell you how to make a adapter like I did for my Hickok to test those tripple triodes out.

The Fender super champ of the 80s even used a tripple triode, but a different type then the two that Ampeg used.

[RyanThomas13]

Just got back from a mini-vacation, 4 kids screaming at each other and now I need a vacation after my vacation

Steve, in reference to the Hickok, I got the tester a couple years ago from someone who works on these, claimed to be fully calibrated and ready to go. I have not calibrated it since but it sounds like it's a good idea to do so. To be honest I have never opened up the guts to look around and see how its made, components etc.

Depending on how much I estimate using it in the future, converting to a solid state rectifier seems like a good idea. And no I have not done any research on how one goes about calibrating these, so any information on this is much appreciated.

I did hear back from TubeDepot and they will swap out the JJ EL34 for a higher output tube no questions asked. They also said all tubes are checked and sorted for low distortion/high output before they are shipped out, and a preference for either can be made at the time of purchase.

[RyanThomas13]

One last thing, TubeDepot did send back a new JJ EL34 that measured 35mA, obviously a much stronger tube. 5 stars on their customer service.