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| Pentode vs. ultra-linear vs. triode-connected pentodes http://dcaudiodiy.com/phpBB3/viewtopic.php?f=5&t=99 |
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| Author: | dberning [ August 8th, 2013, 8:00 pm ] |
| Post subject: | Pentode vs. ultra-linear vs. triode-connected pentodes |
To continue the discussion of Ultra Linear vs. pentode vs. triode, ultra linear and triode connection of a pentode is a technique that applies local feedback in the output stage. Feedback lowers output impedance and lowers distortion. When the plate of a pentode is connected to the screen, that applies the most feedback and the output stage has its lowest gain (triode connection). In ultra linear, only a portion of the plate voltage is applied to the screen, and this is part way between pentode where no voltage is fed back and triode where 100 % is fed back to the screen. The output impedance of the triode connection is the lowest, and the gain is the lowest. For pentode the gain is the highest and the output impedance is the highest. Distortion is the highest also. Ultra linear lies in between in all of these characteristics. Power output is highest in the pentode mode because the screed is maintained at the highest voltage during tube turn-on and the tube can pass more current at higher screen voltages. It should be noted that this is treating the output stage as an isolated entity. When an amplifier has global feedback, the overall gain of the amplifier may not change much when switching between ultra and triode. This is because the feedback network at the input of the amplifier is working to define the gain. But what happens is that the quantity of feedback shifts and is divided between the input error stage and the output stage, relatively more going to the output stage and less to the input stage for the triode connection. Generally, this is desirable as shorter feedback loops are better than long ones. But which one you prefer to listen to may not be as expected and is clearly up to subjective debate. David |
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| Author: | dberning [ August 8th, 2013, 8:55 pm ] |
| Post subject: | Re: Pentode vs. ultra-linear vs. triode-connected pentodes |
I thought I should further stress that the trends I mentioned apply to the output stage as an isolated entity. Other things can happen with an amplifier with global feedback. The measured harmonic distortion of the complete amplifier will likely increase if the output stage is switched from pentode to ultra-linear or triode mode, particularly the lower-order harmonics. This is because there is less loop gain. If the amplifier has what I call stability band aids, the frequency response can be adversely affected. Hum and noise can increase. Other changes or tube changes you make within the amplifier will likely be more pronounced, both sonically and measurement wise. So when you say you like the sound of the amplifier better in one mode or another, consider all of these possible reasons. I am sure that there are other reasons that I can't think of right now. David |
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| Author: | TubeDriver [ August 8th, 2013, 9:02 pm ] |
| Post subject: | Re: Pentode vs. ultra-linear vs. triode-connected pentodes |
Thanks David. Lots of different ways to set up a PP pentode amp. I don't see any specifications for the Hasimoto transformer regarding the percentage or number of turns that the UL tap runs of. I guess this is another factor. So I general terms, a pentode output running in triode modeshould require less global feedback since it has lower output resistance than in pure pentode mode? |
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| Author: | David McGown [ August 8th, 2013, 9:49 pm ] |
| Post subject: | Re: Pentode vs. ultra-linear vs. triode-connected pentodes |
Pete, I will have to dig out the Hashimoto HWC-30-8 datasheet for you. I am willing to bet that the UL tap is 40%. David |
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