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Old 1st January 2013, 01:34 AM   #11
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Quote:
Originally Posted by Antoinel View Post
Surprisingly [or not] the modulated heater current [while driving a loudspeaker] reaches the cathode, grid and plate as a varying voltage.
Antoinel,
Not surprising--but I have wondered what the effects are with plate and screen voltages in place with their bypasses, and sufficient bias applied to grid via cathode resistor or LED are. I will post the scheme of my amp soon, just having trouble finding the time anymore! I am making an assumption of course, that you observed voltages on the tubes pins without any of the above mentioned voltages in place---or perhaps you had applied voltages?
This is interesting to me as it will be instructive to learn the nature of what any feedback is, whether it be degenerative or regenerative, or possibly some wacky combination of both. All I know is, the amp sounds better than I thought it should. What amp? Well, I will try to post the schematic tomorrow.
What type of speaker are you using and how loud will it go? Is it driven well enough, or do you feel some pre-amplification is in order?
Thanks and Happy New Year!
Terry
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Old 1st January 2013, 02:06 AM   #12
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Many Fisher tube stereo sets used the 12AX7 heaters in the preamp as cathode resistors in the power amp.
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Old 1st January 2013, 04:00 AM   #13
cotdt is offline cotdt  United States
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You know what's cool? I loaded my tube amp with a resistor, and the tubes produced sound by itself, without a loudspeaker!
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Old 1st January 2013, 06:14 AM   #14
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Glowing tubes will make nice looking cosmetics above your stereo setup.

Gajanan Phadte
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Old 1st January 2013, 06:32 AM   #15
freax is offline freax  Australia
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I once used a 7 pin signal tube and a 6v lantern battery to keep my hands warm as a teenager.

The idea was that I would put the battery on my belt and keep the tube in my top pocket for warmth.

Never did do that, I just kept connecting the leads to the battery and holding the tube in my hands to keep warm.
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Last edited by freax; 1st January 2013 at 06:37 AM.
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Old 1st January 2013, 12:52 PM   #16
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Quote:
and the tubes produced sound by itself, without a loudspeaker!
The sound heard coming from your amp is most likely produced by the OPT. I have some OPT's that are rather loud and some totally quiet. Loud ones can sometimes be quieted by jamming something between the bobbin and the core. Sometimes the windings themselves produce sound.

It is common for tubes to produce random sounds associated with thermal expansion. Sometimes these sounds wind up in the speakers at a low volume.

I have never heard music come from a tube, but if one had something inside that was free enough to vibrate, it would be a better microphone than speaker. I have a Motorola branded 6SN7 that is microphonic enough that you can talk to it and hear yourself in the speaker. Turn the volume loude enough and acoustic feedback occurs.
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Old 1st January 2013, 02:47 PM   #17
jrenkin is offline jrenkin  United States
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Originally Posted by tubelab.com View Post
I have never heard music come from a tube, but if one had something inside that was free enough to vibrate, it would be a better microphone than speaker. I have a Motorola branded 6SN7 that is microphonic enough that you can talk to it and hear yourself in the speaker. Turn the volume loude enough and acoustic feedback occurs.
I have a EL34 PP amp that plays music with no input signal. Some local radio station....I gotta get inside the thing and fix it.
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Old 1st January 2013, 02:52 PM   #18
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Thank you all for your valuable input. You did have "Other Apps" for vacuum tubes. A hand warmer will be useful for a snow-stranded motorist. Cosmetic apps; like those of Post #1. A vacuum tube as a small loudspeaker; I wonder about its frequency range and relative loudness. An app which keeps showing up is using independent tube heaters as cathode resistors in vacuum tube preamps and power amps.
A happy and safe 2013 to you all.
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Old 1st January 2013, 03:38 PM   #19
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Quote:
Originally Posted by tubelab.com View Post
The sound heard coming from your amp is most likely produced by the OPT. I have some OPT's that are rather loud and some totally quiet. Loud ones can sometimes be quieted by jamming something between the bobbin and the core. Sometimes the windings themselves produce sound.

It is common for tubes to produce random sounds associated with thermal expansion. Sometimes these sounds wind up in the speakers at a low volume.

I have never heard music come from a tube, but if one had something inside that was free enough to vibrate, it would be a better microphone than speaker. I have a Motorola branded 6SN7 that is microphonic enough that you can talk to it and hear yourself in the speaker. Turn the volume loude enough and acoustic feedback occurs.
Thanks for your input. You described an omnidirectional microphone.
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Old 1st January 2013, 03:40 PM   #20
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I have a EL34 PP amp that plays music with no input signal. Some local radio station....I gotta get inside the thing and fix it.
Interesting. The LM386 chip in my app has done that too.
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