http://www.vac-amps.com/page0011.html
This has me pretty confused. It says it's PP, which means 2 tubes paralleled for each half, right? That should be 8*4 = 32W, or at most 40W. Is there something I'm missing here, or do they just push the tubes that hard, and get 16W out of each 300B? How long would the tubes last at that operating point?
Does anyone have any experience with these amps?
This has me pretty confused. It says it's PP, which means 2 tubes paralleled for each half, right? That should be 8*4 = 32W, or at most 40W. Is there something I'm missing here, or do they just push the tubes that hard, and get 16W out of each 300B? How long would the tubes last at that operating point?
Does anyone have any experience with these amps?
Class Struggle
With tubes, the classes are pretty easy. You know what class A is- the idle current is the same as the max current. (180 degree operating angle)
Class B has an operating angle near zero degrees; in other words, its idle current is negligable compared to max current.
Class AB is in the middle. Its idle current is below the max current, but is still significant.
AB is subdivided into AB1 and AB2: in AB1, no grid current is drawn. In AB2, the grid is allowed to draw current. This requires a heftier driver stage.
By and large, you don't see AB2 that often.
There's also class C, but that's never used for linear amplification in audio. It's very efficient, very useful if you can bandwidth limit it severely, and is commonly found in RF transmitter circuits.
With tubes, the classes are pretty easy. You know what class A is- the idle current is the same as the max current. (180 degree operating angle)
Class B has an operating angle near zero degrees; in other words, its idle current is negligable compared to max current.
Class AB is in the middle. Its idle current is below the max current, but is still significant.
AB is subdivided into AB1 and AB2: in AB1, no grid current is drawn. In AB2, the grid is allowed to draw current. This requires a heftier driver stage.
By and large, you don't see AB2 that often.
There's also class C, but that's never used for linear amplification in audio. It's very efficient, very useful if you can bandwidth limit it severely, and is commonly found in RF transmitter circuits.
This is probably a really dumb question, but I never understood that. I've understood class A as a tube that's conducting for the entire sine wave cycle. For PP, I can see how this means idle current = max current, because as one half conducts more the other half conducts less. But for an SE stage, is the idle current still the same as the max current? How does that work? The tube moves up and down the load line, right, so the current should be changing, shouldn't it? So idle current should be 50% of peak current.
Hm. If that's true, then a PP stage not only cancels out even order harmonics, a class A PP stage also presents an easier load to the power supply. It's probably really funny that I just realized that.
My wife's response to the current version of my amp: (CCS'd ECC99 driver, current regulated 2A3 heater, UBT-3 OPTs, teflon caps) - "it sounds pretty good". Her response when I completed the first iteration of my 2A3 SET (125ESE OPTs, AC heaters, straight from the Angela website) - "it sounds... off".
I've been eyeing your amp lately... I'm pretty sure mine doesn't sound as good, but I want to know if I'm any closer than I was before
What would be great would be to bring my amp down to your place some day and get a lesson or two on chasing (RF, and other) gremlins, troubleshooting, improvements, just anything
Normally (but not "officially"), people use the term "pure" class A to distinguish a true silicon furnace from one of those Japanese sliding-bias units. People DO cheat the term a bit, as you imply; if you see an amp that's claimed to be 200 watts of "Class A," it had better be throwing off enough heat to roast a wildebeest.
That's what one would suppose, but what text book mentions the term?fdegrove said:Hi,
For our D'Haen:
In CLASS A OPERATION, current flows for 100% (360º) of the input signal.
My guess is that the "pure" part refers to the 100% current flow although in most so-called amps it's more often than not not the case.
Cheers,
And if I marketed a P-P amp that ran in limiting class A, and called it "Pure Class A", who could contradict me?
By "limiting class A" I mean including the last non-linear part of the transfer characteristic.
Cheers,
Hi,
If you like I could doublecheck it and give you page #s frm the RDH and McGrawhill.
To my mind the description I posted is correct for all intends and purposes.
If you deviate from that you shouldn't call it a pure Class A amp.
I wouldn't. But then again I don't buy marketing hype either.
I know what you mean though...
My main quibble is mostly with the fact that the term is often used to market amps where, when you take a closer look it's not even close to Class A operation.
Big A small b if you're lucky.
Cheers,
If you like I could doublecheck it and give you page #s frm the RDH and McGrawhill.
To my mind the description I posted is correct for all intends and purposes.
If you deviate from that you shouldn't call it a pure Class A amp.
I wouldn't. But then again I don't buy marketing hype either.
I know what you mean though...
My main quibble is mostly with the fact that the term is often used to market amps where, when you take a closer look it's not even close to Class A operation.
Big A small b if you're lucky.
Cheers,
I like the ones which call out "single ended" and "class A" as separate virtues, as in, "not only is it single-ended, it is *also* class A". And that too, in a linestage. How many push-pull class B linestages are there? PP maybe (or more likely, balanced or differential, neither of which would be single ended, right?), but I really wonder if there are many non-class A linestages.
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