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How does a tube's need for bias change with age?
How does a tube's need for bias change with age?
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Old 21st November 2019, 12:26 PM   #31
mondogenerator is offline mondogenerator  England
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Quote:
Originally Posted by Chris Hornbeck View Post
Actually, this isn't even remotely true.


All good fortune,

Chris
I believe the OP picked up this gem of misinformation, from V. Capell book on loudspeaker design
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Old 21st November 2019, 02:42 PM   #32
6vheater is offline 6vheater
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Originally Posted by Wavebourn View Post
Wavelength in the room does not matter. It matters in terms of resonances in the room, that's it. What matters, the volume of displaced air.
Sorry Mr Wavebourn but on this point you are wrong,
You don't work in recording music or acoustics,

I will give an extreme example.
Put this big cabinet in your toilet, and see if it will reproduce the 30hz note...
IT WON'T. You might hear 60hz , 90hz, 120hz overtones because there is enough space for that, but 30hz NO....you just THINK you hearing the fundamental.

You "think" it is, but in reality all you are hearing is harmonics.
It's got zilch to do with displaced air, everything to do with wavelength.

Even a 1/4 wavelength will not find its space in there, below which the ear cannot hear it.
You can put as much sound pressure and displacement in the toilet as you like, until the walls shake and the toilet door blows off, but you will NOT hear the fundamental.

You have the classic confusion of amp & speaker makers, not knowing the difference between displacement of air, SPL, and development of waveforms.
They require space, exactly as the transmitter aerial requires space and the right length of wire to transmit a radio wave properly.

Put the wrong tuned circuit on there, you have a huge mismatch and nothing comes out!
You can put as much power in as you like, but nothing will transmit at the right frequency,but you might make your neighbours TVs go completely haywire.

A speaker is not a receiver it's a transmitter and exactly the same physics holds.
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Old 21st November 2019, 02:53 PM   #33
6vheater is offline 6vheater
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Quote:
Originally Posted by mondogenerator View Post
6vHeater

The 4" can compress the air volume twice as easily, and assuming Fs and sensitivity are equal, in a purely hypothetical example, the 4" car speaker would go louder.

.. explains why my eyeballs vibrate in some Bassheads cars, but have never vibrates at a rock concert (at higher SPL), where my liver does!
Doesn't explain anything at all.

A 4"
Quote:
woofer
makes me laugh hearing such a term used.

Have you ever actually MEASURED anything?
I mean, properly with Bruel & Kjaer Microphones using proper calibration, or a proper calibration microphone used to measure SPL.
They are quite expensive toys.

Some of us actually work in the industry.

Actually if you like to know because there are laws in Europe making it illegal willingly to damage the hearing of people permanently by some of the idiotic tests I see discussed here.
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Old 21st November 2019, 04:39 PM   #34
DF96 is offline DF96  England
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Quote:
Originally Posted by 6vheater
shall we say 10mhz?
Maybe you meant to say 10MHz?

Quote:
The way you can tune into that radio station is taking a wacking great long length of wire and coil it up to make a tuning coil.
It's not 2km long, but it's some multiple of a 1/4 wave, then you need an aerial, usually made of ferrite which multiplies the signal quite a bit in a certain freq band.
No. You don't need a great long length of wire. It is not some multiple of a 1/4 wave, coiled up.

Quote:
So yes you do need quite a LONG compact bit of wire to listen to a radio on 10mhz, get it,
10mhz yes. 10MHz (which is 29.9792458m wavelength, as you said) only needs a short piece of wire. 2 or 3m will be fine, and you can get away with less.

Quote:
Originally Posted by mandu
The cathode material is pulled by the plate, which in turn is the plate voltage applied. Higher plate voltage pull more material. As the cathode slowly looses material, the maximum plate current and power handling capacity is reduced.
In most cases the emission wearout mechanism is not the cathode surface having material physically pulled off it by electrostatic forces, but metal ions diffusing back into the oxide material.
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Old 21st November 2019, 04:45 PM   #35
DF96 is offline DF96  England
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Quote:
Originally Posted by 6vheater
They require space, exactly as the transmitter aerial requires space and the right length of wire to transmit a radio wave properly.
Not really true. A transmitter antenna needs to be a sufficient size to get a high enough radiation resistance to allow for efficient coupling to it. A smaller antenna will still radiate just fine, but you will lose power in the matching network.

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Some of us actually work in the industry.
Your point is? In some industries there is poor correlation between experience and understanding. Audio is one such industry.
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Old 21st November 2019, 04:49 PM   #36
kodabmx is offline kodabmx  Canada
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How does a tube's need for bias change with age?
A speaker is a transducer, not a transmitter. "Loudspeakers, earphones converts electrical signals into sound (amplified signal → magnetic field → motion → air pressure)"
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Old 21st November 2019, 09:08 PM   #37
Tubelab_com is offline Tubelab_com  United States
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The OP asked about tubes, and which way the bias would shift over time. There were several posts discussing the decline in emission and Gm over time requiring less negative bias over time to produce the same current. This the normal case with a tube which was manufactured with pure materials and a good vacuum.

Many modern production tubes, and many from the last days of tube manufacturing were not always well manufactured. Dirty materials, and minimum cost manufacturing were often the norm. Some NOS tubes are now 50 to 100 years old. Their vacuum isn't what it once was.

A contaminated vacuum will contain material (including air) which can ionize. This produces the blue / purple glow often seen INSIDE the plate structure on some tubes. A deep blue glow ON THE GLASS itself is a different and totally harmless phenomenon. Running a tube harder (hotter) will INCREASE the rate at which trapped ions are released from the materials inside the tube. Obviously a poorly constructed tube has more junk inside to begin with, so heating them up starts the inevitable march toward a red plate runaway failure.

This ionization will create a path for grid current to flow which will REDUCE the bias on the tube causing an increase in plate current. There is a reason why most tubes have a "minimum grid circuit resistance" spec. That resistance forms the path to bleed off excess grid current before it can start a runaway event. Did Leo Fender and others observe this spec?????? Do old guitar amps eat tubes????

Even in properly constructed amps that are operated within (but close to) the tube manufacturers ratings, some of today's new production tubes will start to exhibit bias creep, a slow but noticeable increase in tube current over time. This is more common in class A amps since their average dissipation is higher than other amps. Once a tube starts exhibiting this phenomenon, it should be replaced, or it will eventually go into a red plate runaway possibly damaging other parts.

As for big bass from small speakers....Yes, crank that small speaker to somewhere near Xmax, and it will distort. Keep it well out of that range and you can produce mostly fundamental tones. Running small speakers in a near field situation can make substantial bass. Can you get 95 db at 1 watt from a 4 inch woofer at 20 Hz, no, the physics won't allow it.

Quote:
I test my own stuff with a DAW, studio microphones, hi res 24bit sound,
So do I. I started recording live music, mostly rock, in the mid 90's with Cakewalk software and a Pro Audio Spectrum 16 card. Been doing it ever since. My 90's vintage Yamaha NS-10M Studio near field monitors can produce 40 Hz with minimal distortion....at a low level. Yes, the response drops like a rock below 70 Hz, but they are still clean at 40 Hz. My daughter played drums back then and the Yamahas did a better job on faithfully reproducing her drums than all the others I tried, so I bought a pair....still have them.

Today I have a pair of IK Multimedia iLOUD micro monitors on my DAW in addition to the Yamahas. They use tiny 3 INCH woofers that are flat to 45 Hz thanks to modern DSP tricks. No, they are not going to shake the walls, or crank out a 27 Hz organ note, but they do very well despite their size. They DO produce real clean bass at close range. Their specs show a maximum SPL of 88 db at 45 Hz at 10% distortion from a distance of about 2 feet (near field). I have no reason to dispute their data.

I have not tried their new MTM version, which should do much better due to the 4 INCH WOOFERS, nor will I until I can find them for far less than list price.

IK Multimedia - iLoud Micro Monitor
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Old 21st November 2019, 11:02 PM   #38
petertub is offline petertub  Sweden
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How does a tube's need for bias change with age?
To throw som figures in this thread :
Sometime ago a replaced a quad of 6550 in my amp, they have been active 4324 hours.
They still kept bias, adjusted about 10%, that still measures useful in maximatcher.
Tubes are kept as proof of evidence.
During this time 2 GZ34 is replaced, 3 12au7 clones ( RCA something) as one of them started
to hum from filament<>cathode leakage.I replced all 3 as "service procedure".



( time measured with a digital hour-meter installed when i built the amp)
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Last edited by petertub; 21st November 2019 at 11:04 PM. Reason: spelling
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Old 21st November 2019, 11:46 PM   #39
kodabmx is offline kodabmx  Canada
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How does a tube's need for bias change with age?
I got 15000H ish usable life from a quad of 6P3S tubes in a classic datasheet example (Metal 6L6) class A pentode amp. Rare power cycling, rather I ran it all the time.
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Old 23rd November 2019, 03:23 PM   #40
Tubelab_com is offline Tubelab_com  United States
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One of my SSE amps runs Electro Harmonix 6550's. It is usually biased at about 80 ma with 440 volts B+ for about 30 to 32 watts of dissipation.

For nearly 3 years it was the amp that went between my PC and the Yamaha NS-10's, so it got used whenever the PC was on (several hours a day). It's now 12 years old and has been used off and on since then and still has the same tubes in it. The pretty blue glow is gone, but they still work fine.

Some SSE users have had red plate runaway occur in new production Tung Sol KT88's after a couple of years.

These are both New Sensor tubes. Some batches are good, some not so good.
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