So all that being said, I guess the answer is a bit of a yes.
When there is negative feedback involved, it's handy to double check a few things beforehand.
They probably won't be shocking, but better safe than sorry.
Btw, to make the story complete.
You can also make a diode from any triode or pentode as well 🙂
(not saying it will be a very good performing diode in all cases, but it is a diode nonetheless)
Well I guess that depends on whether you consider triode operation as a form of feedback. But even if so, I am talking about local and global feedback loop stability. Since even you admit that switching from triode to UL increases gain, this will increase loop gain and hence the feedback factor (which is just loop gain plus 1). And because the plate impedance also rises, the loop gain frequency response will likely reduce at both frequency extremes (depending on poles formed by coupling caps, transformer primary inductance/resonances etc.) Combine that with the rise in loop gain and an amplifier that was stable in triode mode could develop peaking and even instability in UL mode. This is elementary feedback theory, not sure why it doesn't make sense. You can even observe it in some Stereophile measurements of amps with switchable triode/UL operation.
I was talking about the entire system feedback (global).
That only counts when you apply a ton of feedback.
If that is your design goal, that's fine. (wouldn't be my advice)
But between the two, it's not that difficult to find a compromise that works fine with both.
The difference in gain between the two modes isn't that shocking to begin with.
Therefore the expected behavior in loop stability isn't either.
That doesn't have to do with elementary feedback theory, but just the practicality of it.
Unless you design right on the edge, but those couple of dB is not gonna make your amplifier oscillate all of a sudden.
For tube amps just a parallel capacitor on the feedback resistor will work just fine in most cases.
Well that was my whole initial point, it is a compromise, which you admit. We're just arguing how bad the compromise is.
I have the Rogue Audio Cronus Magnum 3 powering homemade speakers. (2x SS Revelator 18w,12mu,Ellipticor). Impedance measurement is at 4 ohms. Dips to 2.9ohms in the 300-600hz range. It's unlistenable in triode mode. Huge peak in the response in this region. I am using the 4 ohm tap on the transformer. Still sounds honky in this region listening to music. Measures well with a impulse response measurement.
It was gifted to me from my cousin. He was using it with Focal Sopra 3s. Was absolutely unlistenable due to large swings in the impedance measurement.
Could this be due to worn tubes?
I've tracked down a Adcom GFA-5550 to compare with the Rogue this weekend.
It was gifted to me from my cousin. He was using it with Focal Sopra 3s. Was absolutely unlistenable due to large swings in the impedance measurement.
Could this be due to worn tubes?
I've tracked down a Adcom GFA-5550 to compare with the Rogue this weekend.
Could be tired tubes. Are they biased properly, or are you able to bias them properly? How efficient are your homemade speakers?
I've never found Focals particularly hard to drive or hard to mate with amplifiers, but I've never heard the Sopra 3's. Maybe there's something wrong with the amp. The Cronus Magnum is pretty highly reagrded. It shouldn't sound that bad!
I've never found Focals particularly hard to drive or hard to mate with amplifiers, but I've never heard the Sopra 3's. Maybe there's something wrong with the amp. The Cronus Magnum is pretty highly reagrded. It shouldn't sound that bad!
It has nothing to do with the tube complement. The triodes 12AX7, 12AU7 are not involved. They are the pre-amp and driver stages. The KT120 Screen grid is switched between pentode (tetrode) and triode mode. It seems that author is a bit confused.
The Focal's impedance varies greatly. 2khz + was 3-6db above the midrange. Measured frequency response myself. Focal told him the tweeters needed to be "broken in". Yes they are hard to drive. With the 4 ohm tap the bass was bloated. The 8 ohm. Too much treble.
The amp sounded great on my 8 ohm Paradigms. I have no doubts it's a fantastic amplifier. I'm aware it's the interaction between the two that's not great.
I can see why people like the triode mode. It sounds really huge and soft. But, All precise imaging is gone. Vocals just seem to come from nowhere.
My current speakers would be low sensitivity. But, They're electronically filtered at 80hz. My power conditioner has an ammeter and the amp draws the same at idle as it does at high volume. Likely not running out of power and still in Class A.
I have the Sopra 3s, and rive them with my pentode-connected monoblocks, that have about 0.55 ohm output impedance (simulated, not yet measured) and they sound fine. They don't need a huge amount of power but do benefit from low output impedance.
Well, as I say, I'm not familiar with the Sopra 3s. I used to sell the Focal line at a shop in Northern Virginia and I always found them pretty amenable to a variety of amps.
I currently use homebrew versions of the classic Williamson "Musician's Amplifier" from 1950, with Heyboer's copy of the Peerless S-265-Q. And a very good copy it is. The "screen taps" (actually the 2500 ohms taps) are at 50% and I like getting twice the power with very little difference between triode and screen-tap mode. So I'm not arguing in favor of triode operation. That said, the original triode version is excellent, but you have to adjust the feedback for optimal performance.