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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Apr 2012, 8:36 am   #101
glowinganode
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Default Re: My first experience with electronic tubes

Granted, I wasn't thinking about the DC conditions though.
I guess as long as both the valves are run class A it should be ok, should reduce even harmonic distortion.
Just thinking out loud really.
Cheers,
Rob.
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Old 4th May 2012, 2:57 pm   #102
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Default Re: My first experience with electronic tubes

Hi all

I made some changes in the amp configuration. I connected the triodes in cascade and kept the pentodes in parallel with the output in series. The second triode feeds both pentodes. I'm not interested in a PP right now.

More gain (really more gain). Now I will try to introduce some feedback to improve linearity as I have enough gain to sacrifice. I will show a schematic soon.
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Old 7th May 2012, 7:49 pm   #103
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Default Re: My first experience with electronic tubes

The cascade configuration worked well, but now I broke up one of the EL84. I thought to hit my head against the walls, and almost cried as the getter vanished. I got stunned. But I relaxed on time, and now I'm looking for a solution. While still alive, there is always a solution.
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Old 7th May 2012, 8:14 pm   #104
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Default Re: My first experience with electronic tubes

Hi Miguel,

When the signal passes through zero volts at the speaker and at the input, the valve is biassed just as in its no-signal state, and the anode is passing 40mA.

40mA may make only a little heat, in the resistance of the copper wire of the primary, but more importantly the current creates a magnetic field in the transformer core. The collapse of this field as the valve current falls below 40mA is what powers the positive half of the cycle, when the anode swings far above the HT voltage.

Transformers for single-ended stages like this are more like chokes with an added secondary. Design effort has to go into making enough inductance, and for it to not saturate when the valve is at peak current... approaching 80mA

An ideal transformer would have no resistance from anode to HT, so the average anode voltage must equal the HT, and the anode must swing equal amounts below and above the HT voltage.

Have you heard of 'load lines'?

David
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Old 7th May 2012, 8:20 pm   #105
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Default Re: My first experience with electronic tubes

Not sure of what you mean with "load lines", David. An explanation will be welcome.
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Old 7th May 2012, 9:16 pm   #106
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Default Re: My first experience with electronic tubes

OK, they're easy enough and make output stages a lot more understandable.

Take a graph of characteristic curves for you EL84 (for example) from one of he better data sheets.

Anode voltage is scaled horizontally, increasing to the right. Anode current is scaled upwards, more current is higher.

On this graph there is a nested family of curves, all starting at 0 volts and 0 mA, climbing as they go rightwards and then levelling-off at different amounts of current.

Let's forget transformers for a moment, and do a simple anode resistor load amplifier.

At maximum current (80mA = twice the quiescent current) we want the anode voltage down around 0v (in this simple example) with the valve at its quiescent current (idling) then we want it to have half the HT voltage on its anode. At zero current there is no voltage dropped across the load resistor, and so the anode must be at the HT voltage.

If we say the HT voltage is 300v, then the resistor must drop 150v at 40mA which means it must be 150/0.04 Ohms = 3750 Ohms.

when it's carrying 80mA the 3750 Ohm resistor drops 300v... exactly as required.

We can plot the anode voltage and currents which are forced by this load resistor on our characteristic curves. It intersects with the anode voltage axis at 300v and it intersects with the anode current axis at 80mA. It is a straight line, and you can now read off where each different curve (representing a different grid voltage) crosses the load line. You can see how the output circuit works.

Now with the transformer output stage, the inductance to the HT says that the average anode voltage is equal to the HT voltage. The anode swings to twice HT voltage at zero current, and to 0v at max (80mA current)

The load line is created by the impedance of your loudspeaker, increased by the square of the turns ratio of your transformer. You now have twice HT swing over 80mA swing

600v/80mA = 7500 ohms load. If you use 8 ohm loudspeakers then you need a turns ratio of Square root(7500/8) = 30.6 to one.

Of course, doing this for real, you'd not take the anode down to zero volts, but pick a limit where the valve is still reasonably linear.

Both my typing fingers now hurt! so it must be time to stop.

David
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Old 7th May 2012, 9:23 pm   #107
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Default Re: My first experience with electronic tubes

Thanks David. I understood now. We did that for biasing transistors in the college, not for transformer output but for simple class A pre-amplifiers. I have to admit that I've not done that for the valve amplifier, but I understand your point.
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Old 8th May 2012, 12:15 am   #108
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Default Re: My first experience with electronic tubes

It's how the turns ratio of output transformers are calculated.

Of course, the impedance of real loudspeakers wanders all over the place and spoils all your nice calculations, but you have to try.

With transistor amplifiers, negative feedback is used to control distortion and to allow direct coupling of stages without creating DC problems, the feedback reduces the output impedance until the circuit becomes virtually an ideal voltage generator, and so quite tolerant of load impedances.

Simple circuits with valves have little or no feedback and load Z becomes much more important because it sets the load line directly.

David

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Old 31st May 2012, 4:20 pm   #109
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Default Re: My first experience with electronic tubes

As there is the possibility to use the ECC83 as preamplifier (sent by my friend Andy) I want to clear a little doubt I have regarding this valve.

The ECC83 have the filament in two sections. In order to connect it in 6,3V:

Should I shortcircuit pins 4 and 5, and connect the 6,3V supply across pins 4-5 and 9?

Thanks.
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Old 31st May 2012, 5:04 pm   #110
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Default Re: My first experience with electronic tubes

Yes.

http://www.r-type.org/pdfs/ecc83.pdf
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Old 31st May 2012, 6:22 pm   #111
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Default Re: My first experience with electronic tubes

Quote:
Originally Posted by Radio Wrangler View Post
Transformers for single-ended stages like this are more like chokes with an added secondary. Design effort has to go into making enough inductance, and for it to not saturate when the valve is at peak current... approaching 80mA
That's not quite right, Radio Wrangler. But if you designed according to this, you'd not have a bad transformer, just one rather bigger than necessary!

The transformer is indeed like a choke with a secondary, as you say. It needs to be away from saturation at the DC anode current (say 40mA). Now, when the valve is working at full output, the anode current goes from 0 to 80mA. But the secondary is then passing a cancelling current, as well. If the primary inductance is high, then the field in the core is practically constant throughout the cycle. At really low frequencies, the finite inductance of the thing gives rise to a very low output, and yes you could argue that the core's field would go from zero to twice the steady-state value as the cancelling current is peanuts. But under those conditions, there will be almost no output from the secondary anyway so who cares if it is distorted by saturation.

As a reasonable engineering approach, you might drive the valve at full output down to a frequency such that the output drops by 3db due to the finite transformer inductance. Under those conditions, with 40mA steady anode current, it turns out that the core gets the equivalent of 28mA of swing. Thus if you design such that it does not saturate with 68mA, you'll be fine - and at higher frequencies, everything will be even more OK. Good quality design would put not drive the valve anywhere near the transformer 3db frequency anyway.
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Old 31st May 2012, 8:09 pm   #112
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Default Re: My first experience with electronic tubes

I'd just allowed for the load to have some highish impedance peaks, and had taken a fairly conservative viewpoint. It only has to reach a few times nominal and you're most of the way to no secondary flux component.

David.
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Old 2nd Jul 2012, 3:19 pm   #113
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Default Re: My first experience with electronic tubes

Finally the amp is working. Thank to all of the friends who help me to reach this goal. Specially to Andy (aka. Dr Wooble) for his huge help.

You can see the magic eye playing here:
http://www.ziddu.com/download/198199...05013.AVI.html
http://www.ziddu.com/download/198199...05018.AVI.html

and some pictures
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