Single Ended Amps - Opinions

[roberto]

Will try 470K, thanks. Strange that this late 60’s Bassman is the only Fender using this kind of feedback.

There are many kind of feedback that have been imported from Hi-Fi and then forgotten (even in Hi-Fi). They will sound with the input capacitance of a pentode, the available power of a pentode and the loadline of a triode (more or less depending on the amount of feedback): implement it badly once, and no one will use it again.

Also, consider that it will clip differently than an amp with the standard feedback from secondary to the previous stage, and differently means that people are not used to that sound, so again a good idea can have troubles finding its way in a certain specific field.

Another further point: Shade proposed originally his idea for the 6L6 family of tubes and SE applications in Hi-Fi, so a (known as) 6L6GC based clean amp experimented it. Like the Fender UL experiment (most probably there to save one choke and get a cleaner power amp).

Just one point: Shade idea is to have local feedback on an output tube to linearize it using parallel feedback (and so more current to drive it), but you can also swing more voltage with a series feedback (it gives the opportunity to squeeze more power with a particular configuration): there are really many many ways to get same results.

[Tobyk]

Single ended
Pros: Preserves harmonics better than Push pull
Closer, more organic, direct feel. Often less bassy
Cons: Often harsher overdrive. More trebly.
More one-dimensional tone.
Agreed? But the cons can be remedied, I believe!

I don't agree with this as I read it.
Doesn't "preserves harmonics" mean something like "doesn't reduce the harmonics you already have"? Both SE and PP stages can have low distortion, preserving whatever harmonics exist in the signal coming in.
Can you expand on "closer, more organic feel"? What exactly does that mean? I have some guesses, but I don't think I understand what you really meant.
As to "Often less bassy", SE amps have a problem with bass response all built in, but it's an economic problem, not inherent in the setup. SE amps need much larger OTs than PP amps for the same bass response because they can only use 1/4 of the magnetic field swing in the core. Economics (... "MBA Disease") tells the makers to provide the least bass response they can get away with to save some money. To expand on that, hifi gear in the early 60s often had SE output stages, and did quite well with bass, as they needed to cover well under the typical guitar tuning's 82Hz lowest note. in my earlier post in this thread I mentioned my favorite amp for tone of all time. It used an SE output from a then-high-end home hifi. No lack of bass there. I would class the concept of SE= less bassy as the amp designer shooting themselves or their customers in the foot.
Same for "often harsher overdrive". There isn't any particular reason this has to be so. Overdrive can be controlled and set in the design from little to more to mind blowing. Just like plastic bobbins on OTs do not necessarily mean worse OTs, SE does not necessarily mean harsher overdrive. The trick is in being careful about which stages distort and how much.
"More trebly"? It's all in the frequency response. If the designer cheaped out on the OT, there won't be much bass and it will sound more trebly. Of course, a few caps in the preamp and the treble from the preamp can be cut down to match.
"More one-dimensional tone"? Can you describe that in more detail? I don't understand what you mean in a way I can relate to the actual amplifier circuit.

My net on this is that as the song said, you gotta know when to hold 'em, know when to fold 'em in terms of amp design. It's good that you're trying to not hold firmly to generalizations from few examples. There are many examples of SE amps like what I think you may mean; but I can peg a number of PP to those observations too.

I can tell you don’t recognize my descripton. And I wrote that to get the discussion going. So in your mind, are there any general differences, or can a SE/PP amps be made to sound just about the same? Is it just a matter of a bit of added feedback to the SE and then there’s no difference, everything else being as equal as possible?

[dorrisant]

This may be a stretch but I feel like it is a valid suggestion. Look into the Beltone AP-12 (or AP-14). Really great clean sounding amp and gets better with the gain cranked up. Yes, this is a Tiesco amp... I had one here for a while and regret that I sold it. I think I may have the owner bring it back for some voltage checks. It really did sound quite good, enough that I'd build one from scratch.

[bepone]

PP stage has far better overdrive. tested by myself in countless applications, low and high wattage. not only topology, there is also a inductance in OPT like a factor (average PP vs average SE - which is not heavilly oversized , has maybe a magnitude beter inductance), so better control of the speaker in PP side.
but it is a challenge to make SE good , so probably with some strong power tube can pass , some 50 watt like GU50, or 40Watt KT88.. with large and quality transformer (5-8 kg)

[Colossal]

Bepone,

Did you do your SE tests with high current, low plate voltage conditions or high plate voltage, lower screens, and about 40-50% current?

Cheers

[bepone]

better sound is at lower voltages... more clean possible.
so at lower voltages higher current , little bit steeper load line, you get better linearity.. until one side of sinusoide hits the end, then start the distortion.

higher voltages, lower currents, output stage must be accompanied with more laid load line to keep distortion still acceptable. if you are going with the same load line like in the first case (lower voltage) you can get early cliping, less clean.

so graphical method, from the datasheet, drawing the load lines in pentode circuits to see what is possible to get. and then ordering dedicated output transformer... or take some working point and recommended load (OPT) from the datasheet..

current you choose in class A like 80% of max power of the tube, as high as possible, becouse linearity of the tube is increasing with the current through (see transfer curve). so for 40W max power tube, and 350V.. current = 0.8*40/350=91.4mA. If you go with 100%, this is 114mA, but this will cause excessive overheating and shorten the life of the (expensive) output tube..forced cooling is also a option for increasing the life..or reducing the bias to 70-80%..sensitive to bias. reducing the bias too much will cause early clipping and reduced output power..opposite from PP stage.

[roberto]

better control of the speaker in PP side

Agree, also because the PP has a symmetrical control of the speaker, being one tube has always maximum current at every end of the sinusoid, whilst SE has one side with maximum current and one side with almost no current. It depends on the applications, it can be part of its beauty.

[roberto]

If you go with 100%, this is 114mA, but this will cause excessive overheating and shorten the life of the (expensive) output tube..forced cooling is also a option for increasing the life..or reducing the bias to 70-80%..

Downside of the forced cooling is the added noise, another option is apply heat sinks to tubes (did we talk about it on this forum too?). I usually keep around 85% for SE.

[bepone]

Downside of the forced cooling is the added noise, another option is apply heat sinks to tubes (did we talk about it on this forum too?). I usually keep around 85% for SE.

yes, i have at home F2a telecom special tube with the radiator ! nowadays unfortunatelly , F2a cost 1 kidney ..

[roberto]

Here it is, from 1959 Brimar data manual:

“The use of close-fitting screening cans of high thermal conductivity in intimate thermal contact with a large area of the bulb, in conjunction with an adequate heatsink can materially reduce the operating bulb temperature and very considerably improve the life of the valve.”

Developed and commercialised here:
https://www.pearl-hifi.com/03_Prod_Serv ... olers.html

[dorrisant]

PP stage has far better overdrive. tested by myself in countless applications, low and high wattage. not only topology, there is also a inductance in OPT like a factor (average PP vs average SE - which is not heavilly oversized , has maybe a magnitude beter inductance), so better control of the speaker in PP side.
but it is a challenge to make SE good , so probably with some strong power tube can pass , some 50 watt like GU50, or 40Watt KT88.. with large and quality transformer (5-8 kg)

Whatever... I strongly disagree!!

https://www.youtube.com/watch?v=-L6WkalTxfQ

https://www.youtube.com/watch?v=kFOQ2JMwUe8

No offense intended at all, but please review and keep an open mind.

Go ahead... ask me how I did it. I dare ya!

[bepone]

good good! you have put a little bit feedback
i have in plan of 10yr anniversary of trainwreck style in KT88 SE. she was very very very good amp.. after i disasembled and use PT for some other amp..
with time i completelly turned to PP, which made overdrive a lot easier and "groovy".... but i never closed the door for SE

[roberto]

I don't think that having a preference means being closed minded, especially in the case of bepone that showed to have tried many approaches, so to have developed a reasoned preference. But I agree that there are always new ways to play around circuits, so our own opinion can morph into a new nuance.

I would say that PP has a better overdrive because most of the famous recorded overdrive tones come from PP amps, so our mind is more used to a certain character of overdriven sound.

I absolutely agree that there are SE amps that overdrive very nicely, but there are more factors that can affect this: two speakers/cabs that have different impedance plots will cause the same SE amp to overdrive differenctly at different frequencies, so to sound more or less pleasantly, whilst a PP amp is generally less affected by this. ...and this is for sure one of the reasons of the diffusion of the PP, together with its higher power, lower weight per watt, longer tube life.

[Mac Dillard]

Late getting in on this discussion but if you want to look at some really good SE amps go to https://ax84.com/archive/ax84.com/classicprojects.html
The Hi- Octane amp will give you a lot of distortion. The Single ended lead amp is one of the best sounding amps I have ever built. JMO

[Colossal]

Late getting in on this discussion but if you want to look at some really good SE amps go to https://ax84.com/archive/ax84.com/classicprojects.html
The Hi- Octane amp will give you a lot of distortion. The Single ended lead amp is one of the best sounding amps I have ever built. JMO

The HO is very good sounding SE amp and capable of some great high gain tones.

Here's an HO built by Chris Hurley from AX84:
https://www.youtube.com/watch?v=u_MBHfKI3cw