Please explain rectifiers to me (schooling needed)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Motterpaul, Mar 17, 2015.

  1. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    I understand that rectifiers create DC from AC - and I know the rectifier section of a tube amp feeds the output tubes.

    But what exactly happens in the phase inverter mode? is it getting ac current and turning it into DC? if it is DC, why do we need two matched output tubes?

    Speakers are feed an "AC" signal - are they not?

    I am confused then, Why do we need a device that changes AC to DC in order to power output tubes and then speakers?

    I just don't get it - can you help me out?
     
  2. telenut62

    telenut62 Member

    Messages:
    60
    Joined:
    Sep 14, 2010
    Location:
    Australia
    Check some Uncle Doug vids....





     
  3. pdf64

    pdf64 Member

    Messages:
    5,147
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    Such a power amp is a balanced system, it uses two signals of opposite polarity, like a mic / pro audio http://en.wikipedia.org/wiki/Balanced_audio
    The phase splitter turns the unbalanced signal from the instrument and pre-amp into balanced signals.
    Tubes only operate on dc, the ac signal modulates the dc.
    Speakers only operate with ac signals, any dc will degrade performance, possible cause damage if significant.

    You may be getting mixed up with power inverters, which may take a dc power source (eg a car battery) and output ac, eg at line level.
     
  4. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    The rectifier converts AC to DC for the whole amp including the phase inverter and preamp.

    Your guitar, and all other audio sources, create an AC signal of varying frequency that carries the notes you're playing. Each note is a different frequency. The AC from your wall outlet is a single frequency, 60Hz in the USA. Two entirely different things.

    The AC voltage signal from your guitar gets amplified by every tube in the amp that it enters and the power tubes amplify current to drive the speakers.

    Oversimplification, but it clears up a few of your misunderstandings.
     
  5. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    NO replies yet - a complicated subject. One thing I have figured out is that tubes need DC to operate, and that the DC is taken out (or you could say converted back) to make the output to the speakers AC within the out transformer. It blocks DC and only allows AC to come out of the amp (to feed the speakers)
     
  6. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    Funny - all the nswers apopeared at once. Thank you, I will watch uncle doug's vids (tho I have seen them before).

    My problem is that I am not an electronics maven - so every time an explanation goes into talking about tube plate voltage controlled by this resistorX it goesd over my head, but I think I m starting to see it.

    Because tubes need DC, the audio must be converted to a DC signal (which is then modulated by the AC audio signal). This probably is how all the tubes work (pre and output) - then at the rectifier stage (which is what I am really trying to understand) I assume the audio signal is converted to a DC voltage for the output tubes. I had always assumed the output tubes pushed half & half of the AC output signal - but that is not right, They feed the OT and that is what changes the DC to AC audio signal.

    Am I getting closer?
     
  7. wyatt

    wyatt Member

    Messages:
    4,135
    Joined:
    Oct 11, 2005
    Not even close.

    The audio signal is always AC...it's a sine wave alternating between up and down.

    The job of preamp and power tubes is to amplify...but it's not magic, you can't create energy from nothing. Tubes use DC voltage (especially at their anodes/plates) as their energy source to amplify the AC signal. The power transformer only puts out AC power, so we rectify AC to DC for the tubes to use as a power source to do their job. It's part of the power supply.

    1) The power transformer supplies...
    ...AC power to all the tube filaments/heaters (tube cathodes need to be heated to work)
    ...AC power to the rectifier anodes ("plates" in tube rectifiers)
    2) The rectifier cathode supplies DC power to the plates of the tubes (and screens).
    2a) A separate smaller rectifier supplies DC power to the various tube cathodes for biasing in fixed bias amps

    To simplify...the AC guitar signal goes into the grid of the tube...and then a massive amount of DC power is applied to it to make a larger AC signal come out the Plate. THis happens again and again.

    All those capacitors are there to allow the AC to continue on while keeping the DC where it needs to be. The trick to a quiet amp is making sure the AC and DC all go where they are needed and don't bleed over.

    The PI, in push-pull amps, is indeed, different...it's job is to split the top half of the sine wave and send it to one half of the power amp and the bottom half of the sine wave and send it to the other half of the power amp--lternating positive..negative...positive...negative...--then the positive side of the power amp feeds one side of the OT and the negative half feeds the other--once again alternating positive...negative...positive...negative etc. Always AC. The OT itself is just there to correct the ratio of impedance between what the tubes would like to see and what the speaker is.
     
    Last edited: Mar 17, 2015
  8. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    I see what you are saying, we were in slightly different contexts, tubes DO need DC to run, so I understood the power transformer. where I made a mistake was in thinking the rectifier was somehow influenced by the audio signal. You say it is not, and I understand. It is merely to create DC.

    In fact the PPIMV actually works on the power to the grids in the output tubes - giving them a little more power to drive the OT a little harder (or scale it back) - but it isn't a great tone in my opinion.

    I was watching uncle Doug - so I assume the first Marshalls (JTM-45) were split load phase inverters? Do you know? They were based on the 60's bassman.
     
  9. wyatt

    wyatt Member

    Messages:
    4,135
    Joined:
    Oct 11, 2005
    The JTM-45 is based on the '59 Bassman, and both use a long-tail pair PI.
     
  10. Avatar Tech

    Avatar Tech Member

    Messages:
    455
    Joined:
    Dec 13, 2011
    You're getting closer. Starting with the input stage, the tube plate idles at a relatively high DC voltage, say 100v. A very small (milivolts) AC signal hits the control grid of the tube, and that causes big voltage changes to appear on the plate. It's an AC signal sitting on top of a DC voltage. So a simple example would be to just assume that a signal voltage of 2v peak to peak is applied to the control grid. It goes from +1 to -1. If that causes a 10x change to the plate voltage, the plate voltage fluctuates between 110 and 90. A 20 volt AC signal sitting on top of that original 100v where the plate was idling with no input. The DC component is then removed, or decoupled with a capacitor, before being sent to the next gain stage. So the next control grid gets a true AC signal applied to it.
    A phase inverter has one input and two outputs. One output wave form is just an amplified version of the input, so output A is the same wave form as the input. Output B is a mirror image of output A. If output A has a peak voltage of + 10, then output B is simultaneously peaking at -10. The voltage difference is what gets amplified by the power tubes in a push pull amp. Each phase inverter output is creating only a 10v signal on it's own but there is a 20 volt difference between the power tube grids.
    Colin
     
  11. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    Wyatt - thanks. Uncle Doug covers long-tail that so I can look it up.

    Avatar - thanks, too. I saw that, too. Only four pins of the phase inverter get used and it is actually like two separate parts.

    I like the OT transformer is "like two horse with their tails tied together" image. I never would have gotten that any other way.
     
  12. VaughnC

    VaughnC Supporting Member

    Messages:
    15,451
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    The signal from your guitar is a tiny AC signal, which has + & - voltage excursions above and below a 0 volt reference.

    The amp's preamp stage basically makes said + & - voltage excursions larger by using the preamp tube(s) and the lower voltage DC from the amp's power supply.

    The phase inverter stage now seperates the + & - voltage excursions and sends the + excursions to one output tube and the - voltage excursions to the other output tube. This is commonly called a push/pull circuit, which is more efficient than having a single output tube amplify the whole +/- signal. The output tubes need the highest DC voltage from the amp's power supply as they are supplying the necessary power to drive the speaker.

    However, to get the amplified version of the original guitar signal to drive the speaker, the + & - voltage excursions now need reassembled to accurately reproduce the original tiny AC guitar signal....which is one job of the output transformer. Its other job is to match the output impedance of the tubes to the impedance of the speaker while blocking the amp's high voltage power supply DC from the speaker.

    The net effect is the AC output signal applied to the speaker is now a much larger version of the original tiny AC guitar signal we started with. This is a bit oversimplified, but its the basic idea of a how a common push/pull guitar amp works.
     
    Last edited: Mar 19, 2015
  13. RiftAmps

    RiftAmps Member

    Messages:
    204
    Joined:
    Mar 18, 2014
    Location:
    Brackley, UK
    To boil it down.....

    An amplifier needs fuel to run (think about a car), this fuel is called DC voltage. We get the DC voltage by converting AC voltage from the wall socket into DC by a process called 'rectification'. This happens inside the amplifier and can be done with either a valve or a diode.

    Without a good supply of 'fuel', the amplifier simply will not work correctly, if at all.

    Once the amplifier is up and running (the car is running at idle) it will sit there until it's fuel supply is switched off.

    The amp/car will not do anything else until a human comes along and 'drives' it. We do this by plugging our guitar in and start playing.

    The signal from the guitar is also AC voltage, however it is very small and weak compared to the AC voltage that comes out of a wall socket.

    The amplifiers job is to take that very small signal and amplify it many times to a point where it is powerful enough to drive a speaker.

    The hardest thing for me to get my head around was that both AC and DC voltage can exist in the same wire at the same time, either going in the same or different directions, without mixing together - think oil and water in a pipe. Once I understood that concept it made understanding amplifier circuits much easier.
     
    Last edited: Mar 23, 2015
  14. VaughnC

    VaughnC Supporting Member

    Messages:
    15,451
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    Yup, AC is defined by a varying voltage swinging + & - on a 0V reference. However, if said varying voltage only varies + (or -) and never crosses the zero reference, or if the reference voltage is other than 0v, it would be called pulsating DC.

    So, for instance, if a signal is swinging from +1v to -1v on a 0v reference (or 2v peak to peak total), it would be an AC signal. However, if that same signal was on a 100vdc reference, the signal would be swinging between 99v & 101v (still 2v peak to peak), but would be called pulsating DC. But, if you applied said pulsating DC signal to a coupling capacitor, the 100v DC reference voltage will be removed, leaving only the 2v peak to peak AC componant of the signal, on a 0v reference, to be passed on to the next stage in the circuit. The DC componant of the signal is needed by the tube to amplify the AC (guitar) signal so, once the amplification is completed, the DC componant can be removed...usually by a coupling capacitor or transformer.
     
    Last edited: Mar 18, 2015
  15. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    Great post - thank you (and others also). I have always understood that "audio" signal is, "by definition" an AV waveform - even as a soundwave in the air it requires 360-degrees of + and -.

    I also understand that that AC waveform as created by a guitar pickup needs to be "preserved" in something that resembles its original form, although it can be "translated" to other forms as long as there is an "encode" and "decode" process (something like Dolby).

    I also get it that the "AC" of an audio signal has nothing to do with what comes out the wall - which is pure 60-cycle, high voltage electricity. So, the AC that comes from the wall is purely the fuel. While the AC that is the audio signal is (basically) the control instructions for what the amp needs to do in order to make that tiny voltage audio signal strong enough to drive speakers.

    I admit, I still do not fully understand what happens inside an amp, and that is MY problem. I know the key to learning something new is to fully understand the terminology.

    What I could use is a picture of nothing but where in the amp we see both audio AC and DC, and a brief explanation of how they interact at each point. First, though, I need to MEMORIZE tubes. I know how they work, and I can understand most articles, but I can't fully phrase questions (and I get lost in some articles) because I just can't fully remember the parts of a tube.

    Uncle Doug is pretty good. I think I will go back to those.
     
  16. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    OKAY - using pictures and mnemonic associations I figured out a way to memorize how tubes work (this is what works for me)..

    The cathode is where electrons 'cook' - meaning they are sitting there waiting to heat up ( from the heater) and they will soon "lift off" (sort of like steam emanating)....

    They will land on the plate which collects them (because a plate is where cooked things end up).

    The serving size is controlled by the grid - which is like a sifter or vent - and how hard it is "shaken" (how much gets through) is determined by how much AC current goes through is. The more current, the larger the number of electrons that collect on the plate.

    Don't worry about the picture so much, it is getting the parts of tube and their functions that I worry about. I use word pictures to remember what things do. This was inspired by Uncle Doug.

    I know also see how rectifiers work, and how the DC is turned back to AC by the Output transformer (center tap). The entire picture is starting to come together here.
     
  17. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    No. I think a better "visualization" is that the DC is the fuel that allows the various "engines" (preamp and power tubes) to increase the amplitude/size (preamp tubes) of the guitar's AC signal and the force (power tubes) to take that amplified guitar AC signal to drive the speakers.
     
  18. wyatt

    wyatt Member

    Messages:
    4,135
    Joined:
    Oct 11, 2005
    Nope.

    Audio output transformers do NOT convert DC to AC. They do several jobs.

    1. They take the high-voltage, low-current AC signal from the tubes and convert it to a low-voltage, high-current AC signal for the speaker
    2. They block any DC from reaching the speaker while allowing AC to pass
    3. They "impedance correct" the resistance offered from the speaker to what the amp designers want the tubes to see
    But they do NOT convert DC to AC...the DC voltage going through the OT is going to the power tube plates, it is not (directly) used by the OT.
     
    Last edited: Mar 23, 2015
  19. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    ^^^ That too!

    Don't feel bad though, electronics isn't something you learn by intuition or reading web forums. That's probably why there are 2, 4, 6 year, and longer, college programs in electronics. It's not like changing brake pads. ;)
     
  20. Motterpaul

    Motterpaul Tone is in the Ears

    Messages:
    10,400
    Joined:
    Dec 16, 2010
    Location:
    Encinitas, SoCal
    Phraseology is an acquired talent that takes time - when I said "convert" of course I was wrong - they filter out the DC. But the point what was both AC & DC current is now just AC - and I am pretty sure that does happen in the OT.

    The applications of the OT I do get - but my focus was on the tubes. Thanks for reminding of the things I need to remember about the OT, however.

    This is absolutely right. I am a person who is pretty good at teaching myself new things, because I have learned the first step to understanding anything new is to understand the terminology first.

    Fortunately, I already have a pretty knowledge of things like gain structure, waveforms, impedance, etc. I am not ready to design a new guitar amp, but I can often get a dead one working again. Especially since MOST technical problems are simpler than they appear (loose connections, etc.).

    Lately I have also been learning how to build effects pedals - so voltage dividers, RC tone stacks, LPFs and gain boost (thru opamps or fets) are all getting familiar.
     

Share This Page