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6 hours ago, henry4841 said:

Much like fishing lures. Many times they catch more fisherman then they do fish. Sure are pretty though. Klipsch now uses older technology polyester vs polypropylene caps because they are plenty good enough. Some even consider them better sounding. 

 

I have a stash of WIMA polycarbonate 4.7uf or 3.3uf (can't remember which one). 

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2 hours ago, captainbeefheart said:

 

I am going to have to steal that fishing lure quote from you!! So true.

 

 

We have been using Panasonic ECQE non-inductive metallized polyester capacitors for a while now. Very tight tolerances, low esr, low DF, no shock or audible noise or piezoelectric effects. Just fantastic capacitors really.

 

I think for the money, a good (reasonably priced, not boutique) polyester film cap is hard to beat in a crossover. I can't tell a difference between polypropylene and polyester for crossover use. However, in my tube amps, I like to use film & foil for coupling caps. 

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Good thread and a good question I can't answer--head-banging SET + Forte.  Never had Fortes but my problem with metalley stuff on CW & corner horns was more the recording quality.  I'd think PP would be the way to go for a steadier diet of metal, but honestly I've not had much luck with revealing horns and that sort of music on any amps.  Steve has always been quite a promoter but I also trust any amp that's continuously evolved for 20 years to be at least decent.  The product's options of output impedances is good/useful IMO.  I would think that if you are happy with the Fortes in your room with other power amps around that output that you will be fine, but measuring what you need first is a Fine idea.  If you get into the pops and in a bigger room playing thrashy things, all bets are off at 6w.  

 

Amp adds 10*log(watts) so 6W adds 7.8dB to whatever the (real) sensitivity is.  I use Paul Joppa's argument that boils to your target being 102 for most so with 6W you need 94.3dB speakers at 1m/1w to get enough headroom for most.  So it's probably fine that way, the question is the bass vs. the output transfomers.  I'd trust Steve on that because I know he doesn't settle for off-shelf and seems a relentless tweaker (if a bit hyperbolic at times).  Still, it all depends on what you're used-to and what your expectations are.  I've heard Zeppelin on horns that made me cringe and leave the room.  If it's me, I'd measure what I needed and check with some metalheads over there if you haven't (and let us know what you find/do). 

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Just remember what PWK said about music (Music lives in the mids) and tubes are excellent there. Better bass is to had with a PP tube amplifier but for reach out and touch performant presence a SET amplifier is where it's at. That said a proper designed PP SS amplifier will be an excellent choice as well but you as a rule have to spend much more on a SS PP amplifier then say with a modestly priced PP tube amplifier for that effect. Best performance is generally best achieved with class A with either one.  

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On 11/2/2021 at 2:45 PM, henry4841 said:

Just remember what PWK said about music (Music lives in the mids) and tubes are excellent there. Better bass is to had with a PP tube amplifier but for reach out and touch performant presence a SET amplifier is where it's at. That said a proper designed PP SS amplifier will be an excellent choice as well but you as a rule have to spend much more on a SS PP amplifier then say with a modestly priced PP tube amplifier for that effect. Best performance is generally best achieved with class A with either one.  

 

Not all SET amps lack in bass performance. Sadly many of the companies that make them market them towards people that have a fear of negative feedback, ironically the reason why triodes are so linear and have low impedance is because of internal negative feedback intrinsic to them. Many very nice push pull triode amps use negative feedback but for the reason I stated many SET amps on the market do not. It's funny because push pull cancels even harmonics so they naturally will have less distortion vs single ended so you would think they would use negative feedback with SE amps more than push pull amps. The best part about triode output stages is you do not need much extra negative feedback to get excellent results.

 

Single ended outputs need very high quality output transformers because of the standing DC current. The gap needed so the core doesn't saturate reduced inductance and so you end up needing larger core and more copper to get the inductance up to what is needed. Add to this that fact of having more copper you need very ingenious winding methods to keep good high frequency performance. Negative feedback will help increase bandwidth so even if some high frequency bandwidth is reduced from the gapped output transformer the negative feedback will bring it back to well above what is needed for audio.

 

Another HUGE benefit of adding negative feedback is you can now start to also add some positive current feedback from the speaker to actually create a negative output resistance and go from a damping factor of 8 all the way up to 100!!! Seriously just +2db of positive feedback can have huge advantage into lowering the output impedance to control speakers. Nelson Pass uses this trick with some of his amps like the F7. The first generation people complained of boomy poor bass control so he added just +2db of positive feedback and drastically improved performance. Sadly far too many tube amp manufacturers just do not desire to improve their designs that are basically just recycled old designs with fancy capacitors that do nothing.

 

I have listened to 150 watts per channel SET amplifier that had extremely tight bass performance. From Outkast to Opera this amp was amazing, lush midrange you would expect from a SET amp but clean and tight enough to play difficult complex music like a SS amp. It's all in the engineering, it took many years to design that amp and the output transformers were $10k a pair!!

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30 minutes ago, captainbeefheart said:

Another HUGE benefit of adding negative feedback is you can now start to also add some positive current feedback from the speaker to actually create a negative output resistance and go from a damping factor of 8 all the way up to 100!!! Seriously just +2db of positive feedback can have huge advantage into lowering the output impedance to control speakers. Nelson Pass uses this trick with some of his amps like the F7. The first generation people complained of boomy poor bass control so he added just +2db of positive feedback and drastically improved performance. Sadly far too many tube amp manufacturers just do not desire to improve their designs that are basically just recycled old designs with fancy capacitors that do nothing.

 

If I am not mistaken the F7 uses those lateral mosfets where degeneration resistors were not necessary. That positive current feedback is old tube technology Nelson resorted to to improve the amplifier. Nothing wrong with either feedback systems if used with discretion but you will find negative feedback more common. Again if not mistaken positive current feedback in tubes was to make a pentode sound like a triode. Forgive me if I am wrong and I will blame it on age. 

 

I find a good SET tube amplifier has more than adequate bass and does not need to apologize to anyone but I have found SS to have better bass tighter bass but again what is most important for serious listening are the mids. If it does not sound good better bass will not matter and that is where SET's shine. 

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This has been discussed before but I thought to list it again. This SE tube amplifier has positive reviews from a number of talented reviewers on youtube and is moderately priced even with a price increase this year. Not a lot of money for the sound you will get if one wants to experience the sound of a SET tube amplifier.  

https://www.amazon.com/BOYUU-Hi-Fi-Stereo-Amplifier-Single-end/dp/B07L4GL63N/ref=sr_1_9?crid=25UQUN38C3ZCY&keywords=tube+amplifier+stereo&qid=1636029815&sprefix=tube+amplifier%2Caps%2C209&sr=8-9

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5 hours ago, henry4841 said:

If I am not mistaken the F7 uses those lateral mosfets where degeneration resistors were not necessary. That positive current feedback is old tube technology Nelson resorted to to improve the amplifier. Nothing wrong with either feedback systems if used with discretion but you will find negative feedback more common. Again if not mistaken positive current feedback in tubes was to make a pentode sound like a triode. Forgive me if I am wrong and I will blame it on age. 

 

I find a good SET tube amplifier has more than adequate bass and does not need to apologize to anyone but I have found SS to have better bass tighter bass but again what is most important for serious listening are the mids. If it does not sound good better bass will not matter and that is where SET's shine. 

 

Positive current feedback is much different than what you are talking about. I am not sure if you are speaking about adding resistance to emitters or leaving the cathode resistor unbypassed which is current degeneration negative feedback. You also mentioned making pentodes behave like triodes, there are two ways of doing that, one way is to just tie the screen grid to the plate and the tube actually is a triode at that point. Or, there is what has become known as "Schade" feedback by Otto Schade who didn't really discover this where you bring plate/collector/drain signal through a resistor back to the grid/base/gate of the device and you end up with triode like curves and distortion. Schade feedback is negative voltage feedback.

 

Positive current feedback is rare to see in amps and needs to be done carefully in conjunction with negative feedback. All the other feedback methods I mentioned do not take error information from the speaker, the current feedback I am talking about senses the current through the speaker and processes it somewhere back into the amp at an earlier stage.

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36 minutes ago, captainbeefheart said:

 

Positive current feedback is much different than what you are talking about. I am not sure if you are speaking about adding resistance to emitters or leaving the cathode resistor unbypassed which is current degeneration negative feedback. You also mentioned making pentodes behave like triodes, there are two ways of doing that, one way is to just tie the screen grid to the plate and the tube actually is a triode at that point. Or, there is what has become known as "Schade" feedback by Otto Schade who didn't really discover this where you bring plate/collector/drain signal through a resistor back to the grid/base/gate of the device and you end up with triode like curves and distortion. Schade feedback is negative voltage feedback.

 

Positive current feedback is rare to see in amps and needs to be done carefully in conjunction with negative feedback. All the other feedback methods I mentioned do not take error information from the speaker, the current feedback I am talking about senses the current through the speaker and processes it somewhere back into the amp at an earlier stage.

Positive current feedback is used in the F7 along with a touch of negative feedback. I built a version of the F7 using the lateral mosfets and positive current feedback using some members at diyaudio guess at how Nelson designed the circuit. The original schematic of the F7 has not been released. Being such a simple circuit it was not that hard to do. Wasn't my cup of tea so I tried it for awhile and removed the boards and replaced with some others. Page 4 of this article describes some about Nelson's choice of positive current feedback. Same as used in old tube circuits. https://firstwatt.com/pdf/prod_f7_man.pdf

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15 minutes ago, henry4841 said:

Positive current feedback is used in the F7 along with a touch of negative feedback. I built a version of the F7 using the lateral mosfets and positive current feedback using some members at diyaudio guess at how Nelson designed the circuit. The original schematic of the F7 has not been released. Being such a simple circuit it was not that hard to do. Wasn't my cup of tea so I tried it for awhile and removed the boards and replaced with some others. Page 4 of this article describes some about Nelson's choice of positive current feedback. Same as used in old tube circuits. https://firstwatt.com/pdf/prod_f7_man.pdf

 

I read nothing on page 4 about this coming from old tube amplifiers. I do know there were a handful of tube amps that did utilize this approach but it was extremely uncommon. Grommes and Bogen for example used current feedback from the speaker, in the case of the Bogen you could adjust the current feedback from positive to negative for variable damping control. The Grommes used a set amount of positive feedback.

 

19 minutes ago, henry4841 said:

Positive current feedback is used in the F7 along with a touch of negative feedback.

 

It is the opposite. No engineer would ever dream of having a summed amount to positive feedback. What I mean by that is like the F7, Nelson says he uses a moderate amount of negative feedback with a touch of positive feedback. So the amp doesn't sum to an oscillator, the positive feedback adds to the negative feedback making the total feedback still negative. For example moderate amount of negative feedback for Nelson is usually around -20db and if he uses like he does in the F7 +2db of positive feedback the sum is -18db and we still have an amplifier not an oscillator.

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I'm old and forgetful and you are determined to be right so I will not comment further. Chances are in your favor that I confused the big mosfet 50 watt class A project of Nelson's with shade feedback with the F7 with current feedback. I still think Nelson said current feedback was old tube technology but I could be wrong again. I would have to watch some of his old BAF conferences to refresh my memory and to me it is not worth the trouble trying to be right. How one treats another person is more important then being right. 

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20 hours ago, henry4841 said:

I'm old and forgetful and you are determined to be right so I will not comment further. Chances are in your favor that I confused the big mosfet 50 watt class A project of Nelson's with shade feedback with the F7 with current feedback. I still think Nelson said current feedback was old tube technology but I could be wrong again. I would have to watch some of his old BAF conferences to refresh my memory and to me it is not worth the trouble trying to be right. How one treats another person is more important then being right. 

 

I know which BAF conference you speak of. He was talking about "Schade" feedback being old tube technology, this is to connect the plate with the grid via a resistor which is negative voltage feedback. This takes the pentode curves and makes them into triode curves. He was discussing the static induction transistors or SIT's used in the Sony amps. Those transistors naturally have the triode like curves so Nelson wanted to make an amp that everyone could build with regular Mosfets and by using "Schade" feedback will give the same curves as the SIT transistors. He made the SE 50 watt Class A amp but instead of directly connecting the drain to gate with a resistor he used an input transformer to process the feedback increasing input impedance instead of lowering it by just connecting the drain to gate. I actually built this amplifier which is why I know so much about it and remember the discussion very well.

 

There is nothing negative here and I am not "determined to be right", I just want to get all the facts straight for all our sake's as anyone in the world can read this and I feel it is important to have the facts of the matter in order.

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