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Definition of Class A?
Definition of Class A?
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Old 7th December 2017, 10:57 PM   #101
smoking-amp is offline smoking-amp  United States
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A couple more crude gm sums, down to class B.

Class aB not looking too good for grid 1 drive. (not much flat gm region in the middle for the low overlap cases)

I hope Crazy/Twin Drive fixes this mess.

Have to get my gm "scope" rigged up for real tests. Really need to know what output scheme can get the most constant gm at what bias current (or % overlap). Pentode, Triode, UL, g2 drive, g2 g1 dual drive, g2 g1 Twin/Crazy Drive...
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Last edited by smoking-amp; 7th December 2017 at 11:09 PM.
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Old 7th December 2017, 11:25 PM   #102
SpreadSpectrum is offline SpreadSpectrum  United States
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Your curves look very similar to mine. One thing I have done that you didn't was a curve at 50mA (-3V bias). Huge hump on that one.
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Old 7th December 2017, 11:35 PM   #103
Wavebourn is offline Wavebourn  United States
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Definition of Class A?
Quote:
Originally Posted by smoking-amp View Post
Have to get my gm "scope" rigged up for real tests. Really need to know what output scheme can get the most constant gm at what bias current (or % overlap). Pentode, Triode, UL, g2 drive, g2 g1 dual drive, g2 g1 Twin/Crazy Drive...
AB2, g1 with nested feedback to it.

gm doubling it is only one problem. I prefer to solve all problems in complex. :-)
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Old 7th December 2017, 11:39 PM   #104
smoking-amp is offline smoking-amp  United States
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RE: SpreadSpectrum
That's interesting.
Any chance one could combine a humped up with a valley down set to get near constant gm? (4 tubes then)

RE: Wavebourn
Quote:
AB2, g1 with nested feedback
I've been thinking of a transitional local N feedback inside global N feedback. The feedback transitions from global to local mode at HF.

Last edited by smoking-amp; 7th December 2017 at 11:49 PM.
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Old 8th December 2017, 01:03 AM   #105
AmadeusMozart is offline AmadeusMozart  New Zealand
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I like the Audio Note kit 4 setup: A floating paraphase inverter with a small capacitor accross the anode resistor and feedback to the cathode. (controlled rolloff of HF, depending on value of the bypass capacitor about 2.5dB down at 20kHz - 11dB feedback at 50Hz, 3dB at 20kHz). Also make the PI balance adjustable and then adjust at the output after temporary disconnecting the feedback. That takes care of matching discrepancies in the following tubes and it is surprising how much difference it can make in overall distortion. (Matching is far from perfect). I do not like fixed bias and shared cathode bias resistors. Use individual bias resistors for the finals and when power output increases then the bias changes more into class AB. (6V6: 18.5 V bias at idle, 22.3V bias at full output). Even better is to make the resistors adjustable and adjust by measuring the anode current (which is as simple as measuring the voltage drop across the primary of the OPT) and not the cathode current (screen current can vary a lot from tube to tube). Just my opinion, ymmv. Peace, AM.
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Last edited by AmadeusMozart; 8th December 2017 at 01:18 AM.
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Old 8th December 2017, 01:48 AM   #106
smoking-amp is offline smoking-amp  United States
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Hmmm, screen current variation between tubes would make those automatic biasing circuit boards (Tent Labs or Audiamp.eu), which use cathode sense resistors, go out of balance for the OT.

Quote:
(controlled rolloff of HF
The transitional N Fdbk approach preserves the feedback correction at HF by going to local N Fdbk instead. Just rolling off all the N Fdbk at HF means no error correction there.

Last edited by smoking-amp; 8th December 2017 at 01:58 AM.
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Old 8th December 2017, 02:28 AM   #107
Wavebourn is offline Wavebourn  United States
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Definition of Class A?
This "transitional feedback" is nothing good; it is a dominant pole compensation, a necessary evil when using high loop gain feedback over more than 2 stages with transformer. Nothing is good in lower feedback on the upper frequency. It is how the majority of modern solid state amps are made. When you analyze misbehaviour of different stages in extreme conditions, you would see what I mean. Input stages with weak tubes like 12AX7 are getting overloaded by high frequencies riding on the waveform of an orchestra, and as the result you hear a porridge of sounds instead of distinct instruments.
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Old 8th December 2017, 02:29 AM   #108
AmadeusMozart is offline AmadeusMozart  New Zealand
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Quote:
Originally Posted by smoking-amp View Post
Hmmm, screen current variation between tubes would make those automatic biasing circuit boards (Tent Labs or Audiamp.eu), which use cathode sense resistors, go out of balance for the OT.....
Tetrodes have more screen current variation than pentodes. I do not like pentodes in PP due to the differences in harmonic content: tetrodes have more 2nd than 3rd and with pentodes it is the other way round. In PP the second harmonic gets minimized.

Quote:
Originally Posted by Wavebourn View Post
This "transitional feedback" is nothing good; it is a dominant pole compensation, a necessary evil when using high loop gain feedback over more than 2 stages with transformer. Nothing is good in lower feedback on the upper frequency. It is how the majority of modern solid state amps are made. When you analyze misbehaviour of different stages in extreme conditions, you would see what I mean. Input stages with weak tubes like 12AX7 are getting overloaded by high frequencies riding on the waveform of an orchestra, and as the result you hear a porridge of sounds instead of distinct instruments.
Let's agree to disagree - if the 12AX7 is used properly then it does its job properly. Unfortunately many belief in "less is more" and try to work with less stages rather than have each stage working in its optimum range and hence work with less overall distortion. The 12AX7 is a small signal triode and is driving a 6SN7 which is about the most linear triode around. As such mot much is required from the 12AX7.

Speaker impedance swings are the biggest at low frequencies and that is where you want the most feedback. Peace.
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Old 8th December 2017, 04:10 AM   #109
Wavebourn is offline Wavebourn  United States
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Definition of Class A?
Yes, agreed to disagree. ;-)
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Old 8th December 2017, 01:05 PM   #110
smoking-amp is offline smoking-amp  United States
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I think I'm seeing something interesting. We need a bumped up gm sum to combine with the valley'd down gm sum of class AB. Under biasing (the -3V 50 mA) can produce a bump, but will make more heat than typical class A.

The solution -may- be to use a class AB stage with a CCS tail (combined with the non-CCS class AB valley stage, so 2 + 2 tubes). The E55L CCS tailed gm curve is shown below (gm versus current). This should make nice bumped up gm sums at reduced current. Can we find a -fit- between the bumped up sum curves obtainable and the valley'd down sum curves obtainable?

Edit:
Looks like the canceling process will still leave two valleys to either side of the flattened center section. Now what to do?
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Last edited by smoking-amp; 8th December 2017 at 01:16 PM.
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