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VFD vs traditional vacuum tube
VFD vs traditional vacuum tube
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Old 26th October 2017, 07:45 PM   #31
JMFahey is offline JMFahey  Argentina
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Originally Posted by DF96 View Post
A swing from +8.7V to -6.3V is 8% distortion.
Thanks DF
Care to post/link how´s that calculated? , I forgot and my Time Machine is out of fuel
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
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Old 26th October 2017, 08:55 PM   #32
PRR is offline PRR  United States
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Originally Posted by ubza1234 View Post
.....under sized power supply and output transformers?.....
A large number of classic guitar amps offer considerable (and adjustable) distortion in preamp stages between a Gain and a Master knob. You can moosh the end of the preamp yet play the power section so soft it is nearly clean.

This is not the same as flogging a pair of 6V6 into a cheap OT. But for many players it is musically useful.

Nelson Pass in the other section posted a plot showing up to ~~10% at clipping. This can be tweaked with bias and loading. A conventional general purpose amplifier triode shows 5%--- so this shows what a "bad" tube the NuTube is. It really is optimized for good glow at low-low current and on/off operation, not linearity.

Process (and ruler) for estimating 2nd Harmonic from loadline asymmetry is in RDH (3rd or 4th).
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Old 27th October 2017, 03:58 AM   #33
ubza1234 is offline ubza1234  Australia
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Wow! Thanks for all that!

Merlinb, I read that too cute costs on early guitar amps, they would typically use vastly underrated transformers. The power transformer would sag it's voltage allot adding a kind of compression to it. The power supply just couldn't cut it at full tilt.

Also, main output transformers where typically fairly underrated aswell. These would reach magnetic saturation and add a weird compression effect that would dynamically compress the bass and treble ends more so than the mid range.

They also mentioned non linear biasing, which would clip one half now than the other.

Finally, they mentioned how the output transformer would filter out allot of the really harsh clipping, especially once the transformers reach magnetic saturation.

I read this over on peavey's tech note section, which to be fair is largely marketing, but interesting none the less. Maybe they are exaggerating? What are your thoughts?
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Old 27th October 2017, 03:59 AM   #34
ubza1234 is offline ubza1234  Australia
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In saying that, I think they where referring to really really early amplifiers that didn't have a distortion setting. That the baskets thought, "who would ever play it that loud anyway? Well get away with it!" lol
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Old 27th October 2017, 10:21 AM   #35
DF96 is offline DF96  England
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+8.7V to -6.3V is an average swing of +- 7.5V, with 1.2V of distortion. Assuming this is second (it will be almost entirely second) then this will be 0.6V of second harmonic and 0.6V of DC. At the signal peak you get 0.6V 2nd plus 0.6V DC. At the zero crossing you get 0.6V DC minus 0.6V 2nd. So there is 7.5V signal plus 0.6V 2nd, which is 8%.
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