Would someone please clarify, in a push-pull tube amplifier, two pairs of power tubes operate in opposition of phase.
The audio signal is applied to two power tubes—one with a positive polarity and the other with a negative polarity.
One pushes the speaker cone the other pulls it.
In a SET amplifier does one tube only push the cone forwards and backwards or only forwards?
Many thanks
EZ
The audio signal is applied to two power tubes—one with a positive polarity and the other with a negative polarity.
One pushes the speaker cone the other pulls it.
In a SET amplifier does one tube only push the cone forwards and backwards or only forwards?
Many thanks
EZ
Any single-ended tube will amplify the full wave form, that's what tubes are meant to do. ;-) The difference with a PP amp is that you split the wave form with a phase inverter stage so that each output tube only amplifies half the wave form.
Not quite. In a PP transformer the current through both halves is in the same direction, creating the magnetic flux that builds then collapses when the current stops building at the peak of the signal and then changes direction. Do some quick research on the way magnetic flux builds and collapses and transfers that energy to the secondary as a full sine wave.
A PP amp can also be class A where both output tubes amplify the entire waveform with going into cutoff.
Does this mean that a Class A PP amp would produce audio with just one valve per chanel operating?
Probably but wouldn't sound too great I suspect. A PP output transformer isn't designed to handle this asymetrical current load. A SE transformer has an air gap in the laminations to deal with this. A PP transformer usually will have a small air gap but it's only designed to handle a small DC current mismatch between the tubes.
Yes, and it would sound almost normal except for a drop in power. It happens a lot that one tube isn't driven for some reason and the amp sounds fine. As long as the tube is still conducting its idle current, there won't be any additional hum from an idle current imbalance.
In Class A both tubes are working all the time, but with opposite phase.
As one moves intpo Class AB one of the tubes stops being used above the transition point designed in (usually only Class A for very low power, and when you get to Class B, only one tube is working at one time, with potentiual problems with notch distrotiomn at the hand-over.
A Class A PP will produce less power than CLass AB. An example, the EL84 PP we started with is about 15w Class AB in Ultralinear, redine as Class A triode it output 3.2 w at clipping. Boy does it sound good.
dave
Start with a working Class A push pull amplifier, and then . . .
Either remove one output tube, or when one output tube dies:
With plate current in only one half of the primary windings;
And no plate current in the other half of the primary windings;
There will be No cancellation of the magnetic field. With both tubes working, the opposing dual DC currents cancels magnetic fields in the the laminations.
As a result of only one plate current, the laminations will be magnetically saturated.
. . . Perhaps nice for a Fuzz Electric Bass Guitar.
But Terrible Sounding for a Hi Fi Stereo.
Either remove one output tube, or when one output tube dies:
With plate current in only one half of the primary windings;
And no plate current in the other half of the primary windings;
There will be No cancellation of the magnetic field. With both tubes working, the opposing dual DC currents cancels magnetic fields in the the laminations.
As a result of only one plate current, the laminations will be magnetically saturated.
. . . Perhaps nice for a Fuzz Electric Bass Guitar.
But Terrible Sounding for a Hi Fi Stereo.
Last edited:
Yes. And let me tell you it is quite humbling when you realize your audiophile golden ears couldn’t figure out one of my output tubes wasn’t working at all. Felt like a damn fool once I figured it out.
Was nearly there - one tube worked for both tubes, the other worked with minimal current.
I knew something was wrong, but not that and not by the sound.
Thought that basically one tube of the pair is lot weaker than the other but at service measurements showed that one tube barely worked and other worked like crazy for both.... and yet i couldn't tell that by listening music.
I knew something was wrong, but not that and not by the sound.
Thought that basically one tube of the pair is lot weaker than the other but at service measurements showed that one tube barely worked and other worked like crazy for both.... and yet i couldn't tell that by listening music.
In PP both tube support the primary current in the output transformer. The classs of operation is about across how much of the cycle each tube conducts. The simplest operation is class A where both tubes conduct across the entire 360 degree cycle. Here is a PP load line which also shows the paths of the individual tubes operation.
Last edited by a moderator:
Thank you for taking the time to answer my question.
As a corollary can a PP amplifier be modified to work as an SET without a complete redesign?
As a corollary can a PP amplifier be modified to work as an SET without a complete redesign?
If you change the output transformers. But you still have to disconnect and/or parallel half of the channel circuit components and it's just not worth the trouble. To do it right you also have to consider the power transformer's capability. Just let a PP amp be a PP amp and maybe change the bias point to make it run only in class A. But even doing that you need to know how much current the OPT's can handle and the PT rating for that role. It's not worth the trouble if you are interested in the quality of the music as your goal. Look at a SET circuit and a PP circuit. You decide what a complete redesign means.
There have been some attempts to use PP transformers in an SE amp, but they are really just compromises. Manley is the only company I know that makes a PP/SE switchable amp, the Neo-Classic 300B. They use a specially made output transformer that can work either way. But for the rest of us it's better to choose one or the other. ;-)
Another solution is take a look on Beltain by Klimo
It use a pp trafos with 300B and EL34 to cancel the flux inside the iron
W
Also, understand that your term SE(T) in your questions is really SE vs. PP. But we know what you mean.
