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The point of an 845, 211, 805 amp?
The point of an 845, 211, 805 amp?
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Old 8th July 2019, 11:58 PM   #121
gabdx is offline gabdx  Canada
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well, good graphic man, my triode amps are more linear despite this, they have better classA and Class AB, I cannot explain except in the sound....

I built 2 amps with same transfo, same power supply, the only difference is the drive.
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Old 9th July 2019, 12:03 AM   #122
Wavebourn is offline Wavebourn  United States
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The point of an 845, 211, 805 amp?
Apples and potatoes must be cooked differently to get best meals from each.

I am getting better result from pentodes with nested feedbacks than from best linear triodes. Well; if the sound quality from EL34 is the same as no-feedback paralleled 4P1L, still my EL34 amps have better bass control due to servo damping, and amplification of channels does not depend on particular tubes, i.e. no need to select and match so close to get the same loudness from both channels. It is defined by resistors.
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Old 9th July 2019, 12:49 AM   #123
Bigun is offline Bigun  Canada
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The point of an 845, 211, 805 amp?
Just imagine if you were to try transistors
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Old 9th July 2019, 01:53 AM   #124
gabdx is offline gabdx  Canada
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all my amps have almost 0 bass control, only feedback cures it, like a bandage on a bleeding wound , this is why I am building a new kt77 with cathode feedback (8-9 db) and apply another 8-9 db of GNF instead of the actual 13 db of GNF. The regulator should not let the voltage go down 0.1V at 10Vrms of output, which should take all the electrons that the tube can give, (AC heaters all the way, separate transformer lhahha0)....

i am still ignoring absolutely what is the servo damping....
defined by feedback resistors?

feedback equalize the gain between channels, for volume, we have to resort to black magic... (L pad averaging lol)
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Old 9th July 2019, 04:37 AM   #125
SpreadSpectrum is offline SpreadSpectrum  United States
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Despite what? Physics?

I'm so confused by your writing style sometimes...
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Old 9th July 2019, 05:48 AM   #126
dcbingaman is offline dcbingaman  United States
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Originally Posted by PRR View Post
Without NFB, yes, in almost all useful cases. Resistive loads and usual tube and stray capacitances.

Inside a NFB loop, if you drive hard past the low-pass corner, the input stage sees a square-wave and the output of the low-pass becomes a ramp-wave.

Tube amps are "usually" low-NFB and "usually" have enough raw bandwidth to cover real speech/music signals without too much strain.

High NFB can make a poor amp look "good" for small signals but larger signals will behave badly.

Wait and see if I am wrong. (*Someone* will pop-up with a counter-example.)
My earlier point. Good tube amplifiers don't require a lot of NFB because tubes, particularly triodes, are inherently more linear than other amplifying devices, i.e. semiconductors, beam pentodes etc. When you get into higher NFB, usually to correct for non-linearity in the amplifying devices themselves, then you run into a "slew" (pardon my pun) of other issues, including differences in device bandwidth, the ability to deliver current or charge into complex impedances downstream, etc.

There are usually more problems that can pop up than there are passive and active circuit components. In fact, there is a direct correlation between the number of components and the "Q" of the system, which, in the limit, is essentially, a complex bandpass filter with (hopefully) some voltage and / or current gain. High "Q" filters make lousy amplifiers for music signals.
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Old 9th July 2019, 06:09 AM   #127
Hanze Khronye is offline Hanze Khronye  Australia
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Originally Posted by daqvin_carter View Post
You can also use the 845 for lower power amplifiers and just expect really long tube life. Beats pushing a 300B towards its ratings if you need 8 watts or more.

I agree. And to the same end with other scaled-down situations - IME the speaker efficiency and active LF supplement can change things quite some.

Higher power SE, if wanted sure. At some point (and if you can find a good SE:PP xfmr) PP triodes sound pretty good too (IME - better image width and depth at the expense of vividness and forward projection), and mitigate the need for higher B supply unless you really need more than 15W.

HK
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Old 9th July 2019, 06:22 AM   #128
Wavebourn is offline Wavebourn  United States
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The point of an 845, 211, 805 amp?
Quote:
Originally Posted by gabdx View Post
i am still ignoring absolutely what is the servo damping....
Negative output resistance
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Old 9th July 2019, 06:56 AM   #129
Chris Hornbeck is online now Chris Hornbeck  United States
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Quote:
Originally Posted by SpreadSpectrum View Post
Can you explain this some? I have built 3 tube amps (two push-pull and one SE) and my measurements all show distortion that increases monotonically with level.

Do you have results that show that standard tube amp topologies are not monotonic? I'd love to see them and learn more on this.
This is an excellent point, and difficult to argue against. Decent vacuum valve amplifiers are typically biased hot enough to remain in class A over almost all of their operating range. A good balance between a truly non-switching below clipping signal transfer and a switching-only-on-peak-levels transfer was worked out in early days, and really hasn't changed much since the classic era.
The seldom highlighted virtue of vacuum valves is that they run hot! Not a fashionable virtue, but very valuable for audio.

There is one other non-monotonic mechanism in conventional valve amplifiers. The output transformer's core has a permeability vs. signal curve (B/H curve) that passes through a large change of slope through the zero crossing region. Push-pull amplifiers' output valves, properly DC bias balanced, see this as a variation in the parasitic (primary) inductance. Low source impedances, triodes or feedback from the anodes, minimizes but can't banish this factor.

Single-ended designs bias the B/H curve somewhere away from the zero crossing, improving the low-level performance in that respect, but at other significant design costs. Everything on the Goddess's Good Green Earth is horses for courses.

All good fortune,
Chris
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Old 9th July 2019, 07:07 AM   #130
Zoran is offline Zoran  Serbia
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Once I used additional transformer after output transformer. But with very "fat" wire diameter. (No gap, 1:1, induction for max voltage p-p with respect to amplifier small power)
So the DC resistance of the secondary, from that added transformer, was much smaller than the tube OT. less than 0.5ohms. Somehow the tightness, low end control and low mid def. was significantly better? .
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