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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Can someone explain why you need to re-adjust the balance on a tube amp every time

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Schematics! Awesome!
Ah, low voltage tube stage. Please please pleaeaeaease don't base your opinion on tube amps on this given amplifier. Cool if you like it, but it's not a reference. At all.

Any damage to the headphones won't be caused by the tube. So, your safe. If anything is damaged, it's be DC through the caps etc.

The dials should be set so the voltage between cathode and grid is about 0.4 volt.
 
Schematics! Awesome!
Ah, low voltage tube stage. Please please pleaeaeaease don't base your opinion on tube amps on this given amplifier. Cool if you like it, but it's not a reference. At all.

Any damage to the headphones won't be caused by the tube. So, your safe. If anything is damaged, it's be DC through the caps etc.

The dials should be set so the voltage between cathode and grid is about 0.4 volt.

Any way to explain how the dials should be set if someone doesn't have a voltage meter?

They really do sound good on the HD800. It mellows the HD800 out, gives them a bass boost, but doesn't really take away all that great detail and the treble is still there when it needs to be.

At least this thing taught me that I'm a tube guy. Still trying to figure out why though. Solid state sound now seems somewhat lifeless to me.
 
If you set the pot the the max (left or right), it will limit either the amplification of the up-going signal severely, or start limiting the amplification of the down-going signal, but with a much lesser effect while still adding distortion. If this is clockwise or counterclockwise depends on the board layout.
 
I think I found the perfect pot setting. I really like this amp hooked up to my chinese PCM5102 DAC. It has slight cracking (you can't hear it when music is playing but you can hear it in silence) and it clearly distorts the sound, but in a good way. lots of echoes and cool soundstage on the HD800. very fun.

Edit: Does anyone know how much power this has at 32 ohms? Just curious.
 
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Can someone explain why you need to re-adjust the balance on a tube amp every time you change the tube?

I purchased a cheap Little Bear P1 amp from china (which works pretty well actually) and I've been messing around by switching the tube (the tubes cost alas much as the amp lol) but every time I switch a tube I need to readjust these two screws circled in the attached picture in order to get the left-right balance close to right.

Does anyone understand why each tube re-adjusts the left-right balance?

Also, is there a precise method to ensure that you have the right left-right balance? One disturbing thing about this amp is that the left-right balance seems to vary slightly depending on the sound frequency and I'm never sure I have it right. I'll adjust it by ear for one song and then it will seem slightly off on another song.

Overall though I like the tube sound. I'm just not sure why.

It's because you don't have loop feedback to force the gain to be the same for each channel.
 
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Adjusting those pots should not be confused with "turning up the volume," although that's what it might seem like.

Imagine that you've got a constant flow of electrons in a tube. These electrons are flowing "from" the cathode (the bottom part of the tube) "to" the anode (the top part of the tube). These electrons are negatively charged. (Technically, the plate is producing a positive charge that is attracting negatively charged electrons from the cathode, these electrons being freed by way of heating the cathode. That's what makes tubes glow).

The screen in the middle is where you introduce an alternating current (i.e., a sound). This is a much smaller voltage than the current flowing within the tube, but it is enough to control the flow of electrons.

The result is that you can amplify a smaller sound (from your input) to a larger sound by controlling the flow of electrons through the grid. The sound you introduce forces the grid more positive or more negative. That's how tube amplification works, in most cases.

The knob that you're adjusting is the bias, and it controls the amount of current flowing through the tube. You can think of this as setting the operating point at which your tube can effectively control the flow of the electrons through the grid.

"A tube can only have current flow in one direction. An audio signal is 1/2 positive and 1/2 negative. So how does that work? What you do is set the zero of the audio signal midway of the tube. This is called bias. Look at it this way, the audio signal might go from -5 to +5. The tube can only handle -10 to 0. You bias the signal so that -5 is really -10 and +5 is really 0. That way you have the same range but it all stays within the limits of the tube." (credit)

Setting the bias too high leads to distortion. That's not a desirable effect, since it means that your tube isn't operating within the parameters of its design. That's not "the tube sound," that's just a poorly configured amplifier. (It's like saying you like the sound of a Honda Civic with no muffler because it gives you the "NASCAR sound." Well, yes, but it's a gross approximation of that sound...)

The only way to effectively set your bias point is with a digital multimeter measuring the voltages, and with a reference from the manufacturer telling you what their suggested bias point is. I can get a cheap multimeter for $10 that will measure voltages up to 1000VDC (not that I would!).

However, as funk1980 said, you're operating this tube so close to it *not* operating that you're going to get all kinds of weird behaviour, and adjusting the bias outside of a very specific region can either make it amplify poorly (low volume), or distort your signal. (The 6922 seems to be designed to handle about 200V on the plate; your amp is feeding it 13V.) The only way to find the "sweet spot" (if it exists at this voltage) is to find out what the manufacturer designed the circuit for.

EDIT: I just noticed that funk1980 said you should adjust it so that the voltage from cathode to grid is 0.4V
 
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I have set in the middle right now. Hopefully that's decent. The only thing I noticed was that if I turned both dials all the way down, it didn't sound so great (but now I wonder if that might have just been loudness effect, i.e. quieter doesn't sound as well). Hopefully middle is a decent amount of distortion.

I'm getting a proper amp soon any way, probably a schiit valhalla, so this won't be getting much more use in a month or so.
 
p.s.s. I just turned both pots all the way up again (up meaning to the level that puts out more volume, all else being equal) and I kind of think they sound best this way even though funk said that you should set them around midway.

Maybe I like the distortion. I don't know.
 
EDIT: I just noticed that funk1980 said you should adjust it so that the voltage from cathode to grid is 0.4V
Around 0.4V. I looked at the grid curves for the given values in the schematic at 0.4V gives a center-ish biaspoint. But at that low a voltage, things will vary a lot.

I have set in the middle right now. Hopefully that's decent. The only thing I noticed was that if I turned both dials all the way down, it didn't sound so great (but now I wonder if that might have just been loudness effect, i.e. quieter doesn't sound as well). Hopefully middle is a decent amount of distortion.

I'm getting a proper amp soon any way, probably a schiit valhalla, so this won't be getting much more use in a month or so.

Loudness is a frequency filter to boost mid frequencies and give a 'loud' impression that way. Changing the bias doesn't do that.
The Schiit amps are nice little proper tube boxes!

p.s. I noticed the schiit doesn't have any of these dials on it, so I assume I won't have to worry about setting the pot right if I buy that. Please correct me if I'm wrong.

Correct. Higher tube voltages and other surrounding circuitry can minimise any variations in the triode's audible characteristics.

p.s.s. I just turned both pots all the way up again (up meaning to the level that puts out more volume, all else being equal) and I kind of think they sound best this way even though funk said that you should set them around midway.

Maybe I like the distortion. I don't know.

The center point is just a guideline. If you like it better another way, that's fine too!. Taste can't be argued.
 
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In another forum, someone told me that the dials adjust voltage from 11 to 14 volts and -- since this is a 12v tube -- it should be set at around 12 (so around 1/3 of the way between the two extremes).

Not sure if there's any truth to this. any way.

Depends on where you're measuring. A tube has to work with lots of different voltages. The 6922 is not a 12V tube. Not at all. It works best with a B+ of around 150V to 200V, which is a relatively low voltage when it comes to tubes.

In the other forum's case, the '11 to 14' volt is the quiescent voltage measured at the anode. The suggested 0.4V is measured a the cathode, relative to the grid. That's what sets the bias or IOW the headroom. You'll notice that a 0.4V bias will give around 12V anode voltage :)
 
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diyAudio Senior Member
Joined 2002
Hi,

At such low anode voltages the valve is working in "space charge" mode.
Valves such as the ECC88 can handle that but naturally that comes with a set of consequences.

Don't expect it to drive a heavy load such as modern headphones properly. Ideally the load should be higher than a couple of KOhm.

Ciao, ;)
 
The tube is driving a IRF630 output so no worries about overloading it. IMHO a pretty poor circuit design technically, but will almost certainly provide that euphonic "tube sound" distortion that many people seem to prefer and enjoy.

Mike

Do you think its poor design will cause it to give more of the distorted tube sound, thus making it a better amp for those that like that tube distortion? Or does it not work that way?
 
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