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the unstable sound from tube pre-amp !
the unstable sound from tube pre-amp !
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Old 21st October 2002, 01:36 PM   #11
Circlotron is offline Circlotron  Australia
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Default Lots of variac pictures.

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Old 21st October 2002, 01:37 PM   #12
Gabevee is offline Gabevee
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More like a variable inductor. (VARIable reACtor. Or is it simply Vary AC?)

It is a variable voltage source for AC. You pretty much vary the AC line voltage up to full manually, slowly, so that old components that may be susceptible to sudden full current and voltage power ons won't blow up. Instead of being a variable resistor, thoguh it is a variable inductance.

Hope this helps

Hope its right!

Gabe CGV Electronics
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Old 21st October 2002, 04:29 PM   #13
up4 is offline up4
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totally understand !!!

thanks folks ~

I think it's good for building a new amp, right?

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Old 22nd October 2002, 12:51 PM   #14
Gabevee is offline Gabevee
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thanks for the kind words, BTW.


While I agree that the sound of a tube amp will change a little over the time it is on, until it stablilizes heatwise, if you are suffering from noises and coloration more than just a little, then I would suspect that the circuit is probably designed to squeeze too much from the tubes.

I can think of two things that would cause that. One is using a fraction of the current that the tube was designed to handle. For instance, I have a couple of amps that bias the tubes at about 0.25mA. For a 12AX7 or 6EU7 that isn't much of a big deal (2 mA max). But for a 12AT7, with a max current handling of about 10mA, that is a bit extreme. Yet the 12AT7 is a medium mu tube with a relatively low transconductance (heretofore known as Gm). So slight current changes won't affect its sonic character.

OTOH the 12AX7 and 6EU7 will be a little more affected by the changes in current due to their high mu.

Now, there are some preamps that use small signal audio pentodes. Pentodes have very high Gm. As I presume you know, but I will explain it here for those who may not, Gm is the change in plate current for the change in grid voltage. So a pentode stage designed for high gain (can be up to 1000 per tube stage!!) will be susceptible to heat changes.

The other factor is the use of NFB. Some design circuits with the overall gain way up, so when they apply NFB the THD, etc., is way low. But, sometimes due to capacitances, microphonics, etc., the circuit can become unstable and oscillate. This can happen with a slight change in current, induced by heat.

BTW, the circuits I mentioned in the amps I play with (not my amps, but Magnavox) are extremely stable. I hear very little change from 10 minutes to 10 hours after power up.

Gabe CGV Electronics
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Old 23rd October 2002, 12:19 AM   #15
fdegrove is offline fdegrove  Europe
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Default 12B4

Hi Up4,

A full circuit diagram would help,include your psu.
Did you include a mute switch?
I adopted the habit of always muting the preamp before I turn on anything else.
All things considered a warm up time of 20 to 30 secs for tube gear is quite normal,you're probably spoiled by the almost instant on service of solid state gear?

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