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Jan 28 2015

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1, 2, … THREE Component Amplifier


The SINGLE STAGE tube power amplifier.
With only ***THREE*** components in the signal path.

A very interesting development indeed.

I returned to the concept of the “High Voltage” version of Ultra-Linear, but connected to the Suppressor Grids (S3). The amplification of the single stage tube pair simply skyrocketed. So did the cathode currents. Fortunate that I had those semi-constant current sources on the cathodes, as it would have been a possible run away situation. The Suppressor Grids need an obligatory, anti-melt-down resistor, otherwise the S3 simply glows red, and shortly after disintegrates.

The films (on facebook):





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Please do not ask me what type of “mode” these, formally speaking, “pentodes” are actually driven in. I suppose that it is something similar to a tetrode, but not quite.
With the input signal upon S1,
With a fixed voltage upon S2, … (errrr …. fixed in relation to GROUND, and not to Cathode!),
With a high voltage 43% Ultra-Linear signal fed back from the transformer to the suppressor grid S3, via the anti-grid-melting resistors, …
… to be honest with you, I do not have the slightest idea as to what I should “call” this mode of operation. I think that it is the “Ziggy Thing” {{ (C) 2015 by hiend-audio.com }}

It would seem that as a “tetrode”, this configuration should have an area with a messed up anode characteristics. Especially with the “negative resistance” thing, at low anode voltages and high currents.  Frankly speaking – I do not see or hear any problems. Maybe the signal swing simply does not reach those areas, or maybe the stabilizing effect of the non-standard auto-bias. Anyways, it seems to work.

What I wanted to signal to you is that the amplification of this output stage topology / configuration has risen to such an extent, that a single stage amplifier has become possible.

Essentially, an amplifier consisting of a CD player and an OUTPUT STAGE.
Forget the preamplifier.
Forget the drivers.
A simple 1:4 step-up interstage on the input and off you go.

The significance of this topology is the COUNT, the NUMBER, of components within the signal path. Currently, I see the following:

  1. A step-up interstage, symmetrical transformer
  2. The output tube (counted as “one”, although there are two working anti-phase in each respective push-pull leg)
  3. The Output Transformer

After that – it is just the speaker.   Just THREE components.

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A very tempting scenario indeed, as the sound is crystal clear.
No feedback mechanism implemented.
I do not see any need for such, at this present moment.
The sound – crystal clear.
Well, obviously, not as crystal clear as in my OTL amplifier. The clarity of output-transformerless solutions is unbeatable, but I believe that within this present, transformer based topology – this is as far as you can go. Or close.
The transformer – poor thing. Getting a bit warm, aren’t we ?  :)
With over 200mA of cathode current running through the tubes, the transformer is warm.
The tubes – are glowing red. Make the calculation for yourself.
(0,2A X 1500V + 140W filament).

The bass – simply to die for. The amount of SLAM simply took me off guard. Amazing.
A special Chest-Protector-Vest would be highly advised, as a Lung protection means, to protect the lungs and torso from the bass slam.
All this, mind you, with a missing preamplifier, with missing driver stage.
Just the freaking output stage.
In terms of the sound pressure level achieved from this amplifier-lacking output stage, I would reckon that having a stereo pair of such output stage monoblocks in a room of up to 60m2 would be sufficient to provide a listening volume that would drive the neighbours nuts.

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The setup is running off some 1500V DC, give or take a few.
Screen grid is at a daring 730 V DC, . .. BUT … BUT … BUT ….
The cathode is elevated by 150V of my crazy auto-bias, so that the 600V limit is not exceeded, albeit close.

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P.S. Please pardon the controversial “music content” that is being demonstrated. I simply needed to find some lively piece of music with some heavy bass content and slam. I found the Backstreet Boys CD in some old drawer, and I think that It suits it’s purpose, although not my favorite cup of tea … To offset such a faux pas, some other films / pieces, with different kinds of music are also included.

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