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Old 12th December 2012, 04:48 PM   #31
rongon is offline rongon  United States
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I would say that it is much easier for an audio amateur to design a triode amp to sound acceptably good than to design a pentode or UL (or distributed load) amp to sound as acceptably good.

I say this because the triode will need less feedback around it to be acceptably linear than the pentode, and applying feedback is pretty tricky. If I understand the problem correctly, it's that OPT and its resonances, etc. that can cause problems once feedback is applied, and also whatever roll-offs there might be in the power supply and audio coupling circuits.

I am assuming that the operating points and output transformers chosen are reasonably well-specified for the job at hand, whichever that might be.

Last edited by rongon; 12th December 2012 at 04:51 PM.
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Old 12th December 2012, 06:43 PM   #32
DF96 is offline DF96  England
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Yes, if you don't do feedback loop stability then triode-mode is a quick and easy way to reduce distortion and output impedance - but it also reduces power.
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Old 13th December 2012, 03:31 AM   #33
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Quote:
Originally Posted by kevinkuang View Post
Does anyone know what's the idea behind the triode mode, technically? and according to the sonic difference, is this idea successful?
It's basically to make up for the lack of audio power triodes. Back in "the day", the pentode revolution happened early on. Most power triodes were intended for RF designs: Class *2 operation, frequently "zero bias" types that don't draw very much Q-Point current, and with high Rp's.

For audio, you don't have much choice: 45, 50, 2A3 (or its 6.3V version: 6A3) 300B or 845. There are some TV vertical deflection types like the 6CK4, but these are in 2A3 territory for Pd ratings. So if you need something that falls between 300B and 845 territory, you need to pseudotriode one of the audio pents if you want triode finals.

As for UL v. triode, it's just a matter of degree. UL restores some of the inherent NFB that triodes have by operating the screen(s) at some AC level. With pseudotriode, you restore all the NFB by basically removing the plate from the circuit. UL is still local NFB and no different from cathode feedback methods, or parallel (anode-to-grid) NFB. You get back some of the triode like characteristic as opposed to pure pentode operation where the Vk2 doesn't vary.

Inherent Feedback in Triodes.
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Old 13th December 2012, 09:59 AM   #34
oshifis is offline oshifis  Hungary
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I read somewhere that triodes have "nice" distortion spectrum with 2nd harmonic dominating, while pentodes have "nasty" spectrum with 3rd harmonic content. How do pseudotriodes behave? Is UL somewhere in between? Is there a linear relation between UL tap and 2nd/3rd harmonic ratio? Assume the same output power in each case.
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Old 13th December 2012, 10:05 AM   #35
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In push-pull circuit the 2nd harmonic is very low with triode, UL and pentode connection.
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Old 13th December 2012, 01:36 PM   #36
DF96 is offline DF96  England
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Virtually all active devices have 2nd dominating. It may be true that pentodes have more 3rd than triodes, but 2nd usually dominates in both. P-P reduces even-order, including 2nd. If pentodes had more 3rd than 2nd, as often seems to be said, then pentodes would never be used in P-P because there would be no point.
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Old 13th December 2012, 01:57 PM   #37
rongon is offline rongon  United States
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Quote:
Originally Posted by DF96 View Post
Virtually all active devices have 2nd dominating. It may be true that pentodes have more 3rd than triodes, but 2nd usually dominates in both. P-P reduces even-order, including 2nd. If pentodes had more 3rd than 2nd, as often seems to be said, then pentodes would never be used in P-P because there would be no point.
I thought the reason for using pentodes was higher efficiency. Yes, a pentode will generate more 3rd harmonic distortion, but that -- and the low damping factor resulting from the high plate resistance of pentodes -- can be corrected with negative feedback. The limiting factor is usually the stability of the resulting amplifier. The PP pentode (or UL) amp will get you more power output than the same tubes strapped as triodes, but will require more negative feedback to get the distortion low and damping factor high.

If you triode-wire a beam tube or power pentode, you get something so close to a triode that it acts just like one. Now you have low plate resistance (yielding better damping factor) and lower 3rd harmonic distortion level, so less need for negative feedback. This makes stability much less of an issue. The price to be paid, as you mentioned, is that your 35 watt pentode amp will now only make 10 watts (or something like that).

There are some, like me, who find something more enjoyable in the sound of an amp that uses less (or no) global negative feedback. There's a nice looseness, a sort of ease to the sound. I know the distortion is higher, but I like it anyway. You may not, and that is just fine. But I have yet to like the sound of any PP pentode or beam tube amp I've heard that didn't have quite a bit of negative feedback. (Not to say that there isn't an amp like that out there that I would like -- it's possible. I just haven't heard it.) I have heard quite a few PP triode amps with no global NFB that I've liked quite a lot.

I guess it comes down to how much power you actually need.

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Old 13th December 2012, 02:15 PM   #38
DF96 is offline DF96  England
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When I had almost finished building my UL EL34 amp I listened to it with feedback removed, just to check that the feedback was going to improve a good amp rather than paper over a bad one. It sounded OK to me, although I didn't leave it in this state for long.

To hear a pentode with no feedback listen to most 1950's radio receivers. Distortion may be low at reasonable volumes, but the speaker resonances are not well controlled because there is almost no damping.
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Old 13th December 2012, 02:25 PM   #39
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Here is an example at 1KHz of a push-pull 6CA7 in UL mode. The 2nd and 3rd are about equal, although pretty low.
Actually that is labeled wrong.
The top plot is without feedback, the bottom plot is with -5dB GNFB applied.
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File Type: jpg 6CA7thdplot.jpg (466.2 KB, 228 views)

Last edited by scott17; 13th December 2012 at 02:28 PM.
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Old 13th December 2012, 02:32 PM   #40
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by oshifis View Post
I read somewhere that triodes have "nice" distortion spectrum with 2nd harmonic dominating, while pentodes have "nasty" spectrum with 3rd harmonic content.
Pentodes only show higher levels of odd harmonics on large signals (and only then under certain conditions), that is, when operating right up to the ends of their transfer curve. On small signals, however, they show a decaying series of harmonics much like a triode. In fact, if you operate a pentode with a small-ish load impedance then the distortion is triode-like all the time. You can see this by looking at load lines.

Quote:
How do pseudotriodes behave?
A triode-strapped pentode is a triode, and so bahaves exactly as a triode. 'Pseudotriode' is just a silly name the audiophiles invented so they can talk loftily about 'real' triodes like they're extra special. Ultimately you still have three chunks of metal in a glass bulb- they're just triodes.

Quote:
Is UL somewhere in between? Is there a linear relation between UL tap and 2nd/3rd harmonic ratio?
I would guess the relationship is not linear, since you're cancelling out one non-linear transfer characteristic with a different non-linear characteristic. I suspect you could get all manner of harmonic signatures depending on the exact operating conditions. Just guessing though.
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