how to do the "pull a fuse" from RV50H

[zac_crocus]

wow i think all my preamp valves are in the wrong place.
I have ecc81's in the two clean/natural ports and 83's in everything else,
going by this diagram this is wrong?

help before I do damage.

Im pretty sure it came like that though


(I HAVE AN RV100)

[a.hun]

wow i think all my preamp valves are in the wrong place.
I have ecc81's in the two clean/natural ports and 83's in everything else,
going by this diagram this is wrong?

help before I do damage.

Im pretty sure it came like that though


(I HAVE AN RV100)

You are very unlikely to do any damage like that, but putting the right types in the right places will let the amp work more as it was designed to.


Andy.

[blackcloud45]

I have two Mk I RV50's. They each have 4 6V6's.

[zac_crocus]

what does the phase inverter valve do ?

[a.hun]

what does the phase inverter valve do ?

It passes the signal from your amps preamp section (tone and gain controls, amplifies the tiny signal voltages from your pickups) to the power amp section (volume control, produces enough current to drive speakers). Technically the PI is really part of the power amp circuitry though it does use a smaller preamp type valve.

In most amps the power section is what they call a 'push pull' design. (That includes all class AB amps and some true class A ones too.) The power amp has two halves or 'sides', each half running one or more valves. So the valves are always paired up, and while one of each pair 'pushes', the other 'pulls'.

What the PI really does is to split the signal into two halves, a positive and a negative half. If you picture your signal as a squiggly waveform going up and down from left to right, than it splits it right along an imaginary mid line, from left to right. This separates the upper and lower halves of the signal. The 'upper half' of the signal goes to one 'side' of the power amp, the lower half feeds the other 'side'. These two 'half signals' are then amplified and recombined at the output transformer stage which then feeds the whole signal to the speaker.

So the PI really just splits the signal in two to feed the two halves of the power amp.

A few amps don't need this though as all the power valves (one or more, not paired up) handle all the signal. These are called 'single ended' class A amps. Some class A amps though are 'push pull' designs and those do need a phase inverter.


Andy.

[jontheid]

Hi all, I am going to be a bit pedantic here but only to try to clarify things a little, I don't want to step on anybody's toes.

Regarding the phase inverter,

Andy states:

What the PI really does is to split the signal into two halves, a positive and a negative half. If you picture your signal as a squiggly waveform going up and down from left to right, than it splits it right along an imaginary mid line, from left to right. This separates the upper and lower halves of the signal. The 'upper half' of the signal goes to one 'side' of the power amp, the lower half feeds the other 'side'. These two 'half signals' are then amplified and recombined at the output transformer stage which then feeds the whole signal to the speaker.

This isn't actually true.

The phase inverter doesn't separate the signal into two halves.

What it does (ideally) is to take the signal from the preamp stage and to generate two complete output signals from this, both identical but totally out of phase with each other. One of these signals is in phase with the original signal, and the other signal is completely out of phase with it, hence phase inverter.

Please note: I am using the term 'original signal' to mean the input signal going into the phase inverter stage (not the input signal to the amplifier itself)

These two signals are then fed to the output stage of the amp.

The 'in-phase' signal then drives one output tube (or set of tubes)
These output tube(s) conduct more when the original signal goes positive, and less when it goes negative.
If the amp is biased to operate in class AB, these tubes will stop conducting completely once the original signal reaches a certain negative level.

The 'inverted' signal drives the other output tube (or set of tubes)
These output tube(s) conduct more when the original signal goes negative, and less when it goes positive.
If the amp is biased to operate in class AB, these tubes will stop conducting completely once the original signal reaches a certain positive level.

Hope this makes sense
Jon

ps

I think the term 'push-pull' output stage is really confusing when talking about tube amps (well, it is for me)
The way I like to think of it is this: the output tubes, when conducting, allow current to be 'pulled' down from the HV supply to the centre tap of the output transformer primary, though the respective halves of the output transformer primary, through the tubes and back down to ground.

The term 'push' to my mind sounds like the tubes are a current or voltage source driving something, which the tubes aren't really doing, they are just regulating the flow of current from a high voltage supply.

I like to think of it as a 'pull-pull' output stage - kind of like a tug of war if you like, with the two teams being the two opposing output tubes (or sets of tubes) pulling in opposite directions from the centre tap of the output transformer primary.

Even with no input signal to the amp, some DC current will still flow through each half of the output transformer primary (the bias current) which is why the output transformer gets warm even when the amp is idling (and why it gets warmer the hotter the amp is biased) - some 'pulling' is going on by both tug of war teams, but they are both pulling with exactly the same effort,
so no movement occurs, the teams just get a bit hot and sweaty.

When one team gets a momentary advantage and pulls more (say the input signal goes positive such that the corresponding set of output tubes conducts more) this also means that the opposing team gets a simultaneous corresponding disadvantage and pulls less (as the input signal going positive will make this team's output tubes conduct less).

Got to go pick up the wife from work, I'll come back to this later.

[a.hun]

Cool post jontheid, and don't worry, my toes don't feel even slightly trodden on. Anything that takes the story a bit closer to the truth is fine by me. (See my signature below!)

I also quite like your tug o' war idea. Makes sense as there is indeed always energy being expended even when there is no actual movement. Lot of sweating and heat production for no actual result. The DC bias current is a constant factor, but since DC is blocked to the OT secondary nothing gets passed until an AC signal component induces some actual action.


Andy.