Tubes

[Steve Conner]

I guess, but it only diminishes it in so far as the tube can no longer pass high plate currents, so it can't have values of gm corresponding to the values of plate current that it can't pass any more.

I bet that if you measured gm at a current below the worn tube's saturation current, it would measure the same as it did when the tube was new.

Hence why I recommend doing an emission test to check that the saturation current is enough for the job. You should keep the test short because oxide coated cathodes are damaged by running in saturation. If the cathode is already tired, testing it will just wear it out more.

[JC@]

So, in short, this is what we have

The coating eventually erodes away, and the electron emission is compromised.

...anything that diminishes usable plate current ends up reducing the tubes' usable gm

…the vacuum within the glass tube can get weak. More air molecules get inside and interfere with electron flow.
The tubes can't make as much power when they wear out. Because guitar amps are treble boosted, you probably notice the lack of power in the treble first.

As a tube ages, the transconductance goes down and the plate resistance goes up. This has the effect of pushing the plate curves downward on the chart… … Since you haven't changed the load in your amp, this is like hooking up a lower-than-optimal load to a healthy amp. Running a lower-than-optimal load generally results in a slightly darker/smoother tone.

I'm prepared to believe that the measured gm doesn't change, provided it's always measured at a current that the worn cathode can still emit without saturation. Above this current, though, the effective gm would fall. And lower gm means less gain inside the power amp's NFB loop, which means the presence control will boost less, another possible reason for the dulling.

[Rhodesplyr]

I know that as they age, tubes can get noisy, gassy, weak, shorted...
But why really do tubes change their sound quality over the time?
I mean, why do they become dull sounding, for example?
This is not the transistor vs tube dilemma or something like that, as everybody knows how "crisp" they sound when new.

Here are two observations for the discussion.

1) I have a few high-hours Mullard EL34s used in Dynaco ST-70s that test good in a tube tester (not the ultimate test, I know), but when run in the ST-70, the sound is murky and muffled.

2) I was bench-testing a rebuilt Leslie 147 amp to see what its output was with various pairs of output tubes. A pair of used Tung-Sol 6550s put out the *highest* power before clipping of any tubes I tried at 1kHz. However, if I try to play the organ using those tubes, it sounds like someone threw a blanket over the Leslie.

#2 is the oddest result to me. I'm not sure why they would be putting out a robust signal at 1kHz but not at other frequencies. Is it possible for tubes to be matched well at part of the frequency range, but not at others?

[Old Tele man]

...wonder how they'd simultaneously "test" at a higher frequency, say 10KHz?

...probably not as 'close' nor as well as they did at 1KHz.

[tedmich]

so its true that tube testers have little to do with how a tube performs at their correct voltage and with a real signal? No wonder those used Telefunkin el84's I bought sound so weak...

[ThermionicScott]

so its true that tube testers have little to do with how a tube performs at their correct voltage and with a real signal? No wonder those used Telefunkin el84's I bought sound so weak...

They're good at checking for shorts, leakage, and vague "goodness", but yeah. New JJ preamp tubes also test weak sometimes, but perform alright in amps.

- Scott

[Robert M. Martinelli]

so its true that tube testers have little to do with how a tube performs at their correct voltage and with a real signal?

Yup,
OTOH they have A LOT to do with the ever-growing bank accounts of the people who sell'em

Cheers

Bob

[Steve Conner]

2) I was bench-testing a rebuilt Leslie 147 amp to see what its output was with various pairs of output tubes. A pair of used Tung-Sol 6550s put out the *highest* power before clipping of any tubes I tried at 1kHz. However, if I try to play the organ using those tubes, it sounds like someone threw a blanket over the Leslie. ...
Is it possible for tubes to be matched well at part of the frequency range, but not at others?

No, it's not. Tubes don't have any frequency-dependent behaviour: they're amplifiers, not filters. Except in so far as they generate harmonics: maybe these tubes sounded dull because they were too clean, and weren't adding enough grit to the sound.

[Robert M. Martinelli]

Another thing worth mentioning ( IMHO ) is that perfectly symmetrical P-P stages cancel even order harmonics, so the "better" the output tubes are matched, the more the stage symmetry gets close to "perfection", so less even harmonics make it to the speaker, making the sound "poorer" to our ears.

JM2CW

Best regards

Bob

[Diablo]

No, it's not. Tubes don't have any frequency-dependent behaviour: they're amplifiers, not filters. Except in so far as they generate harmonics: maybe these tubes sounded dull because they were too clean, and weren't adding enough grit to the sound.

Since tubes don't have a frequency dependent behavior, can you explain why an EL34 sounds so different from a 6L6?

[Steve Conner]

Broadly speaking, it has to do with the tubes' different transconductances, and how that interacts with the NFB loop, presence control, and speaker impedance vs. frequency.

This explains the difference at low levels, and differences in the clipping behaviour also come into play at high levels.

I've written about it at length in the past, try searching the forum to see more.

[Rhodesplyr]

No, it's not. Tubes don't have any frequency-dependent behaviour: they're amplifiers, not filters. Except in so far as they generate harmonics: maybe these tubes sounded dull because they were too clean, and weren't adding enough grit to the sound.

Ummm......I've been playing Hammonds & Leslies since 1991, and I know the difference between clean/dirty and high frequencies that are simply missing

When I wrote that it sounded like someone had thrown a blanket over the Leslie, I mean a HEAVY blanket--or a sleeping bag. This was not a subtle difference. Other known-good 6550s (Winged-C) and EI KT90s were all in the same ballpark in terms of performance, both objective and subjective.

I can't explain the result, but if you have better diagnostic equipment than I do, I'll send you the pair of Tung-Sol 6550s for closer examination. Based on getter condition, I'd say both are high-hours tubes.

[Steve Conner]

Ummm......I've been playing Hammonds & Leslies since 1991, and I know the difference between clean/dirty and high frequencies that are simply missing

Do you, though? What if every Leslie you ever played had 5% THD, but there's something magic about this pair of tubes, that means that when you put them in, your THD goes down to 1% or 0.5%. The missing harmonics would make it sound dull compared to what you're used to.

You're better placed to answer the question than me, since you have the actual equipment that exhibits the behaviour. All you have to do is run frequency response and THD tests on the Leslie amp with different tubes.

If I'm right, then the frequency response won't change, except for second-order effects associated with loop gain. (The test should be done on a dummy load to eliminate the effect of loop gain on speaker damping. This can cause huge changes in frequency response, but only on a speaker load.) But the tubes under question will have less HF THD.

If you're right, then your dull sounding tubes will show a drop in the HF frequency response. But I don't see how that can happen.

It's the difference between subtractive and additive synthesis, if you like. People normally talk about tubes as if they somehow act as filters or EQs, changing the amplitudes of already existing harmonics in a musical note.

But I can't see any physical mechanism in a tube that could do that: no part of it has a time constant in the audio range. To my mind, tubes can only change the timbre of a note by generating new harmonics.

The third possibility, which I hinted at above, is that maybe the magic tubes have lots of transconductance, so they lower the amp's output impedance, damping your Leslie's HF unit better. You can explore this by seeing how the frequency response changes when you replace the dummy load with the speakers.

[Old Tele man]

...tubes have interelectrode capacitances that ARE affected by frequency, and although they should be constant, in reality, they aren't always so.

...and, because of the Miller Effect, gain and those capacitances vary as (a) the tube transconductance (gm) changes and (b) the total circuit gain (AV) changes with the inductive reflected load changes due to frequency.

...and, then, as you mentioned, there's also the NFB!