the triode's connected to the......

A

audiotang

New Member
Hey, I'm new and I'm ignorant.
I've been trying to edumacate myself on the "simplicity" of tube amp design ever since acquiring several cheapo amps (norh se9, consonance M99 Plus and jolida 202A).

Why so many amps? well, I got them pretty cheap and I wanted to compare Class A SE vs. class AB, EL34 vs KT66. Long story short they all sound different, but they each sound infinitely BETTER than my previous SS gear (NAD, Philips, Proton and Harman Kardon).

So here goes (deep breath)... I got AC from the wall into the amp, then after going through several stages out comes a signal from the output transformers that drives the speakers.

So what is happening in between?

I know it is different for each amp... the norh is class A and not push/pull, the consonance and the jolida are more similar, using class AB, some global feedback but the consonance has an SRPP drive stage whatever that means.

But where does the AC go first, the power transformer or the rectifier? is AC converted to DC when the current travels from the cathode to the anode in the power valves, or does it occur earlier? And is the drive tube role only to increase the voltage signal from the music source and send it to the power tubes for further gain increase?

Why does the norh only have one drive stage while the jolida and consonance appear to have 2? why does the norh use a tube rectifier, while the consonance and jolida don't?
I have sooo many basic questions... HELP!
 
I'll jump in with an initial response.

The AC meets the transformer first. the rectifier (tube or diode) is what does the AC to DC conversion. Tube rectification is often preferred for sonic qualities. It then goes through some filter capacitors to smooth out that DC signal that your amp is looking for.

I write in part to make sure I really understand what I think I understand so corrections by those who really have a clue are welcome.
 
Welcome,
Wall, cord, power supply transformer, rectifier, cap, choke, cap, output transformer, plate of power tube and through a voltage dropping resister to the grid of the power tube.
Back to cap, resister, cap to plate of driver tube which might be in a SRPP (triode amplifier driving a triode amplifier, most driver tubes are dual triodes)
Signal comes in through RCA plug and goes through preamp if integrated, volume pot (is often used especially with SRPPs) if "Passive" control is used then goes to grid if driver tube, comes out amplified and goes to grid of power tube controling the signal in the grid which is coonected to the output transformer.
Basically.
 
thanks db

that was fast... back to our story....

so then what... the now DC current from the rectifier is fed to the power tubes where it meets the amplified voltage signal from the drive tubes, which is fed into the grid on the power tube?

I'm just guessing at this point ?

and then what?

(I told you i didn't know much about this stuff.... but i want to learn!)
 
yikes...

thatch ear gets in there before i even post my reply to DB.... i'm impressed.

now all i have to do is decipher the sequence......

thanks.
 
"since acquiring several cheapo amps"

Hey Mister, watch what you call "cheapo". Before you know it the "amp police" will descend apon you with great fury!

MikE
 
Basically your typical tube amp the filtering has just begun when the juice runs off to the output transformer (OPT) primary. Without getting into taps and ultralinear (UL) and all that stuff the juice goes down 2 different wires to 2 different places of the power tube.
What happens between the RCA plug and the power tube is your amplifier.
What happens between the power supply (PS) transformer and the OPT, and after furthur filtering and lowering of voltages to run the driver tubes of the amplifier stage is the PS.
The OPTs and power tubes are running full tilt all the time. The signal after it is amplified tells the power tubes how to effect the OPT so that the secondaries tell your speakers what to do.
The simplest amp is a Single Ended (SE) and has low enough distortion that it needs no feedback loop from the OPT secondary back the the output of the driver tube like a Push Pull (PP) amp does. The purity of using less parts in the signal path and the lack of feedback is why SE sound is often prefered even though there is much less power to drive the speakers.
My amp is a SET or Single Ended Triode. It also has a separate transformer for the filament of the power triode (300B in this case) and the filament is what heats the tube up, so it is also directly heated and can be refered to as a Directly Heated Triode or DHT. Most DHTs are SETs but can be in PP.
An EL-34 is a power pentode and is usually in PP but can be found in SE and since a pentode is higher power output than a triode a SE pentode of medium power such as an EL-34 will have the output watts of a high power triode like a 300B.
So using a more common and less expensive tube you get the same watts as the less common and more expensive.
They sound different but not as different as a PP with feedback circuit does compared to a SE without feedback does.

And besides knowing how to follow a schematic and actually build an amp I truly have no idea in the world how tubes actually work.
 
You recieved some great information from the posters above ! I will add that you have now experience the bargain bin of tube audio and this pieces are all built to a price point. Think upgrades to your current gear the benefits will surprise you. The Jolida is the easiest and has the most information available about upgrading of the amps you have now.



Craig
 
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Tubino's analogy for tube amplification

Here's the analogy I use. Feel free to correct, or um, amplify.

An output tube (single-ended for simplicity) receives the audio signal at the grid, and this signal comes out bigger and "upside down" at the plate. How does this happen?

Think of the flow of electrons between cathode and anode as the beam of light from a projector, or one of those old "batman" searchlights. Energy is flowing in one direction, with a visible result on a screen, or on a cloud on the sky.

Now someone comes along and makes handshadows in the beam of light, causing a shadow version on the wall, and it could be much bigger. Or the batman symbol on the searchlight, and there's a giant batman on the cloud.

The audio signal on the grid is the hand making shadows. The AC modulation is both adding and subtracting from the DC, whereas the hand in front of the projector is only subtracting (blocking) the light, so that's a key difference. The amplitude is varying, so it's kinda like making your hand bigger and smaller thousands of times per second. At 10 decades of frequences.

Okay, the analogy falls apart there. But I still think it can be a useful way to START visualizing what a tube does. By the time it falls apart as an analogy, you're ready to discard it anyway because you "get it."

Does that work for anybody?
 
Pretty much all triode tubes work on a fairly simple principle. The plate is highly charged with positive potential (voltage). The cathode is at a very low positive potential. The actual physical gap between the two is a vacuum. BETWEEN the positively charged plate, and the much less positively charged cathode, sits the control grid - which is essentially at "0" potential. So to paint a picture, let's assign relative numbers. The plate is +300V, the cathode is +2, and the control grid is "0".

Two important aspects: The plate is VERY positive relative to the cathode. And, the Grid is a LITTLE negative relative to the cathode. Now, when the filament (or the cathode directly) is heated (the product of current through a small resistance - "toaster wire") it "boils" electrons off the cathode - literally. A CLOUD of electrons forms around the cathode. These electrons want to cross the vacuum and rush to the plate which is attracting them with the very POSITIVE voltage. However, the control grid is a NEGATIVE field very near the cathode and it "repells" the electrons and keeps MOST of them from reaching the plate. The few electrons which ARE allowed to reach the plate create the "bias current" in the tube. A steady current from cathode to plate (regardless of signal).

Now, the AC signal from your source is connected to this control grid. When there is no signal - only the bias current is flowing in the tube. When the signal goes POSITIVE it ADDS algebraically to the grid potential making the grid LESS NEGATIVE and thus the grid has LESS REPELLING force and thus MORE electrons will flow to the plate - think "gate" or "valve" (hence the name). When the opposite occurs, and the signal goes negative, it makes the control grid EVEN MORE NEGATIVE and thus even less electrons flow to the plate.

So, what we have is a CURRENT FLOW which can be controlled by the signal grid. Increasing with positive going signal, decreasing with negative going signal. But so what? How does it AMPLIFY? The plate is connected to the supply voltage through a plate resistor. And as resistors do, when the current through them changes a voltage drop is created. The more current, the larger the voltage drop. The less current, the smaller the voltage drop. Convenient! Now we have a flowing current, a resistance to run it through which will create a rising and falling voltage, and a way to control the amount of current. Our "output signal" will be taken off this resistor.

So, as a small AC signal wiggles up and down on the grid, it acts like a gate controlling a fairly small current through the tube, but that little current is running through a resistor connected to a VERY HIGH VOLTAGE. So, "small changes in signal, create equal changes in current, which produce LARGE SWINGS OF OUTPUT VOLTAGE across the plate resistor. Voila - amplification. The "property" that determines what amount of signal will control how much current in the tube is called "transconductance." A measure if you will of the amplifying multiplier of the tube. A 12AX7 has very high transconductance and will provide up to about 70X of amplification. A 12AU7 has low transconductance and will provide about 7X amplification.

So in a rough example, using a 12AX7 with 300V on the plate, a 100K plate resistor, and a 1.5k cathode resistor. The bias current (no signal) will be in the range of 1 milli-amp. This means 1 ma will be flowing through the plate resistor. And this means that the 300V supply will be reduced to 200V at the plate. (I*R). Now we apply an AC signal of +/- 0.1V to the control grid. This will swing the plate voltage from a low of about 195 to a high of about 205. That means we now have a 10V signal off the 100k plate resistor. Or, we just raised the little 0.1V input by 50X. If you then feed that into another tube with say a gain of 10X - suddenly you have 100V of signal after starting with just 1/10th of a volt.

Hope that helps a bit.
Mark
JuicyMusic
 
I'm not sure if most or any of the AK forum members know who Mark is in relation to me but I would like everyone on Ak to know that he is basically my mentor and taught me all of what I know about tube audio repair and design and hopefully some day all of what he knows ! He is really a great guy that has years of experience with tube gear design and production. Its just great to see him back producing his creations.

Craig
 
thanks... i'm chewing on all the info here

and digesting as much as I can. this is interesting.
now, what advice do you have as far as learning to read schematics?

why do some amps require that tubes be manually biased, and others don't require it?

yes, i am still full of questions. thanks for all your time so far.

Last question for now... what do i do with all the crappy solid state and home theater gear i've accumulated over the years?

:confused:
 
As for learning to read schematics, a good bet is to find one of those "Electronics for Beginners" books at Radio Shack or similar places. A few dollars.

Once you know what the "symbols" are, just download some free audio amplifier schematics from a source such as triodeel.com (there are many others) and begin to "trace" them out a little at time. Start at the INPUT JACK and see if you can work your way to the output terminals.

Of course, there will be parts you may not understand, but then ask questions. Something I advise beginners interested in this, is to grab something like a Dynaco Mark III schematic and learn what EACH AND EVERY part does, and why it is in the circuit. Since this is a very simple amplifier, you will end up with a reasonably sound understanding of how they work.

Why are some tubes biased manually and others are automatic? Good question. There are two basic kinds or types of biasing schemes. One is called "fixed bias" the other is called "self bias" or sometimes just "cathode bias."

In "fixed bias" amps, the biasing voltage for the grids is obtained from a "fixed voltage source" - hence the name. This means a tap on the power transformer is fed through a diode to get a negative voltage of around -70V or so. This is then filtered and run through a potentiometers to the grids to supply biasing voltage. The power tube cathode are at "close to ground potential."

In "self bias" or "cathode bias" amplifiers, a simple dropping resistor is placed in the cathode of the output tube so that when plate current flows, a voltage drop will appear across the resistor placing the cathode POSITIVE (perhaps 50 volts or so) with respect to the grid. Since the resistor is fixed, there is no adjustment and the tube "self biases" once power is applied.

In these self biased schemes, you will generally see a large value capacitor placed across this resistor to short the AC signal on the cathode to ground, leaving the cathode DC positive, but AC grounded. This is because these resistors have to be on the order 330 - 680 ohms or even higher, and if they are left unbypassed, there will be too much degenerative feedback in the cathode.

Is one scheme better than the other? Well, it depends (of course!). For any given power supply B+ such as say, 500V, you can obtain more output power from a fixed bias than from a cathode bias. This is because the drop across the cathode effectively reduces the B+ by that amount (The key measurement being the DIFFERENCE from cathode to plate). Also, self biased amps are harder to balance (one side of pp to the other side) unless you add several more parts, in which case it might be better to just use a fixed bias with normal adjustments for bias and balance.

On the other hand, self biased rigs are simpler, easier to maintain and require fewer parts.

As for what you do with all that HT gear - - - well, you're on your own there!
 
I somehow missed your triode operation description from 04-16 Mark and wow! that is the best description of tube operation I've read - I think it's getting much clearer now.

Thanks for cutting through the theory haze for me.
 
Thanks for the kind words guys. Took me a whille to figure out what "CRAFT" meant! HAHA

Have a nice weekend.
 
thanks ya'll

i appreciate all the info... folks on this site are generally friendly and helpful.
Even when it is a "newbie" asking dumb questions.
i'm enjoying the sound of my 3 tube amps by the way.
The ability to buy a tube amp at a reasonable price is a very welcome development and has re-ignited my dormant interest in sound gear.
i can't afford to go overboard with this stuff so I am keeping my systems pretty moderate.
I think the real challenge is putting together a good sounding system w/o going broke ! The most important thing is to spend the time on audiokarma and get enough info to cut through the BS.... otherwise one can spend gobs of money at a stereo store and come away with an overpriced evil sounding pile of junk. Yes, I did canvas the best buy and circuit city stores and the proliferation of high priced "junk" systems is astounding... I didn't like anything I heard there.
I've also got the WAF to deal with, so I can't go with the big box surround sound stuff anyway.
Right now I have 3 basic but good enough for me systems as follows:
1. jolida 202 tube amp/el cheapo emerson cd changer/HTD in-wall speakers.
2. nOrh se9 tube amp/Proton 440 tuner/yamaha direct drive turntable/Pioneer 100 cd changer/B&W dm110 speakers.
3. Consonance M99Plus tube amp/tv-dvd/nOrh 4.0 speakers.
i am pretty satisfied but will probably do some tube rolling starting with the Jolida in the near future.


My previous solid state and home theater crap has been relegated to the garage. Which reminds me: Anyone interested in a Philips stereo receiver? Acoustic Research integrated amp? JBL passive pedestal-style subwoofer?


Again, thanks everyone... I'll be around.
 
philips receiver

FR-50 or something like that? will have to check when I get home.... it is circa '91 or '92. High current model w/ remote control. Very clean euro design, kinda looks like a Harman Kardon, not too cluttered looking. I don't think it has any dings or scratches. I do not have the original box but I think I have the manual.
 
re philips receiver

thanks for letting me know the model #unfortunately its not the philips receiver(s)that im after.the ones i want are from the late 70's and are from the hi fi labs series of equiptment.regards,
chris
 
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