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Old 28th October 2010, 03:25 PM   #31
Anchan is offline Anchan  United States
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Default High Frequency Extension in Triode vs UL

I've been switching a lot back and forth between UL and triode. On one hand, I feel that triode has a much more even presentation. And as many people have noted, UL can be much punchier, with lower lows and higher highs. In many ways, I miss that high frequency sparkle when listening in triode, but prefer triode for other reasons.

My question is very specific regarding the high frequency part.
Is the HF rolloff due to the miller effect of the output tube? So in this case, perhaps a tube with the lowest possible Cg1 will give me the best high frequency detail?

Or, is it that much of the HF detail is lost in the transformer, and is not 'making it through'? I am using the 25W Edcors, which maybe are a little large?

I suspect it is a combination of both, or perhaps some other factors I am not aware of.
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Old 28th October 2010, 11:07 PM   #32
Ian444 is offline Ian444  Australia
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I cannot answer your question but have observed similar, regarding PP amps. I have a Simple PP and a 6P1P PP amp and found that triode mode seemed to have less HF and less "sparkle" overall. I would be interested to hear an answer to your question too...

Just some food for thought. I don't have a very technical background but have done a bit of experimenting and listening the last couple of years. I believe you don't have any GFB (global feedback) in your amp due to the balanced input drive system. Generally I have found best results using pentode mode without GFB. With no GFB, the amp needs to run as clean as possible, which requires some experimentation. Some tweaks that have worked very well for me: (in no particular order)

1. Varying the size of the screen grid resistors for best sound.

2. Varying the screen grid voltage by using a regulator. Some amps benefit from a reg, some don't.

3. Mosfet source followers to drive the output tubes (ZVN0545 as per Baby Huey schem or the Tubelab SE method)

4. Use of LED's as cathode loads for the output tubes.

5. I have found that it seems the bigger the tube, the harder they are to "tame". You might get a nice surprise with those 6V6's you ordered.

What I like about this hobby is that a person can tune the amp to suit their own ears, system, speakers, room. All good clean fun...
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Old 29th October 2010, 11:26 AM   #33
Anchan is offline Anchan  United States
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Ian- You mention you have observed similar in PP amps. Does this mean that in a straight up SET design, the lack of HF in triode mode is not an issue?
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Old 29th October 2010, 10:04 PM   #34
Ian444 is offline Ian444  Australia
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I can't answer that since I have very little experience with SET amps. It might be worth posting your question in the Amplifiers > Tubes/Valves section of this forum. You will get a lot more response there.
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Old 30th October 2010, 02:54 AM   #35
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There are several things at work here. Some make for a measurable difference in frequency response, and there are some psychoacoustics at work here too. Generalizations don't hold up well.

Miller is working overtime in triode mode, and has the day off in pentode mode. This can reduce the HF response. On the other hand, the tube has the lowest dynamic resistance in triode mode. This can tame a marginal OPT. These affect the frequency response in opposite directions with the absolute result depending on the characteristics of the tube and the OPT.

The characteristics of the amp itself will change with the mode. In triode mode the output impedance and the distortion and gain are the lowest. This usually results in the cleanest and most accurate sound. In many cases triode mode can be clean enough to run without any feedback at all.

Pentode mode is just the opposite with a high output impedance more distortion and gain. UL is somewhere in between. Very few pentode amps can function without any feedback. The feedback will lower the distortion and output impedance back towards the triode levels, with some exceptions. The obvious is the increase in output power, but the trade off is an increased level of higher order harmonics. Often the increased harmonic content is the source of the high end sparkle that some users report. Take this too far, and you get "pentode nastiness". Apply too much feedback, and your pentode amp starts to sound like a solid state amp.

These effects are very much dependent on the individual amp, and of course the speakers. Some speakers require a low output impedance to control the woofer cone movement. This means triodes or feedback, sometimes even both.

There is one big difference between an SE amp and a P-P amp. The P-P amp inherently cancel much of the second harmonics generated in the output stage. This means that the distortion produced with a small amount of feedback will be mostly third harmonic. Third harmonic distortion in reasonable amounts doesn't sound bad and adds high end sparkle. It also makes the bass sound fuller. Apply more feedback and the 5th and 7th start to appear and things get nasty.

The SE amp has a lot of second harmonic. In triode mode without feedback, the second harmonic will be dominant, often as much as 5%. This provides the "euphony" that hooks the SE amp user. A SE pentode amp with feedback may have a lower overall distortion level, but the makeup may no longer be predominantly second harmonic. Here the trade off takes a bit more tweaking to get right, and the type of speaker is more important. UL again falls in between.

I have a Simple SE with a switch for triode or UL. It has another switch to engage about 5 db of local cathode feedback on the output stage. I can also use several different output tubes. I have found that the optimum settings depend on the speakers and the type of music being played. Big inefficient speakers and dynamic music demand KT88's in UL mode with cathode feedback. More mellow music works in triode mode without feedback, but the KT88's still sound best. Take the same amp to some efficient full range speakers and it sounds absolutely gross in UL mode. Triode with EL34's are the ticket here, on all music.
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Old 30th October 2010, 06:55 PM   #36
Anchan is offline Anchan  United States
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Thanks for the response. I am learning a lot with this project. There certainly are a lot of balancing factors at work as I am learning with my ears.

BTW- I popped in the 6V6's, biased at about 11W idle current. These are sounding the best to me so far. The amp is running a hell of a lot cooler, which is a major plus as well.
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Old 5th January 2011, 01:35 AM   #37
Anchan is offline Anchan  United States
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Default Amp Finished

I thought this would be a fitting post to wrap up this thread.

I finally put this amp in a chassis, so here are a few pics. Also, I did a writeup on it, with some wiring diagrams, so please check it out.
Click here for the link, and go to the electronics tab and click on 6v6 amp. I hope the web page will help whoever may decide to take a similar route.

All in all, it was fun project, and thanks for the help along the way.
Attached Images
File Type: jpg 6v6Amp1.jpg (52.9 KB, 193 views)
File Type: jpg 6v6amp4.jpg (56.6 KB, 186 views)
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Old 5th January 2011, 02:16 AM   #38
Anchan is offline Anchan  United States
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here's the direct link. Could get it to post before for some reason.
6v6 amp
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Old 5th January 2011, 06:15 AM   #39
Ian444 is offline Ian444  Australia
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Absolutely excellent job Anchan, very good write-up too. Thanks for sharing.
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Old 5th January 2011, 03:10 PM   #40
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Excellent craftsmanship and an informative website. Thank you!
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