Well, here it is. After the GPAF 2006 where this amp debuted and used with the Maxxhorns I brought it home and installed it in my livingroom as "my amp". It did its work driving a pair of extreme vintage Stromberg Carlson 12" full-range speakers in an EV cabinet alignment. It's been serving faithful hi-fidelity service there for the last almost 4 months.

I hardly know where to start with this one. It's entirely a unique design by yours truely, abraxasaudio. I wanted an amp with more power than anything I could get in a reasonably priced triode. I was also rather bored with the fat warm triode sound. The amp needed a relative low output impedance to accomodate a wide variety of potential speakers, but most notably my 12" S-C's mentioned above. I also didn't want another "me too" amp; but those that know me know that's just the way I am. I also prefer a wide bandwidth clean sound, even if it is from a tube amp. More distortion? No thanks! We all get plenty of that from our speakers anyway. I'd just as soon hear what the speakers are really doing rather than listen to some combination that only works when together.

This amplifier represents the top of my skills to date. Carefully designed, it went through many tests and auditions prior to becoming flesh and blood in the chassis you see here. The layout is impecable, with every connection taken to the highest level of my skills it was three days - 36 hours - in final construction not counting the chassis layout and prep.

The results? Amazing. The pentode as an SE output device is unfairly taken by the general audio class. With proper design and care, the pentode works wonders. This amplifier produces 10W/ch. of full range music and avoids the over-fat sound of so many triodes. I've established before that the pentode probably produces the cleanest midrange of any triode and the greatest detail. Imaging is razor sharp and the depth of field is very good. Vocals are in your face in front of you and locked into position. On my 12"-ers (depending on the material) the image is wider than the speaker placement. Magic happens at the edges, with no hint of blurring to the middle. And with up to 10W/ch and reasonable speakers, this baby can rock as well.

Yes, mom ... I put the grid stopper right in the grid cap on those 24's.

Construction: I used my .080 anodized aluminum in my usual bend. The interior supports are aged pine (update: I use only oak interior these days). The exterior trim is white oak stained with Pecan and then clear-coated 3 times. The pecan matches and blends nicely with the deep red theme of the tranny covers. As usual, I only use stainless steel fasteners except where impossible. The amp is very heavy, and there is a minor amount of lateral flex in this chassis if you try hard, but should be plenty strong.

The power supply uses vintage Stancor and Hammond power tranny and choke. The output iron hidden under the hood is vintage Hammond. The heavy duty supply provides 2.5V for the 24a tubes, the high voltage, and the 5V for the rectifier. The heaters for the output tubes are provided by a second new stock power tranny under the chassis. this relieves the main tranny of some power disappation as the 6.3V windings are not used at all.

Rectification is by 5U4 or 5AR4. Either tube will work fine, and the circuit is set up to run properly using either. With the 5AR4 10W/ch. is available, while with the 5U4 it's about 8.5W/ch. I found I preferred the 5U4 in general, but the 5AR4 provided punchier bass.

Switch Selectable Load
Using high-quality silver-contact heavy-duty switch.
Accomodates 4, 8 & 16 ohms at full power each.
The WOW-WOW knob is from an old Hammond Organ.
This can be changed - LOL

As is my practice, I use a CLC (choke) based main filter with B+ taken after the choke. Further B+ filtering provides for the input tube power. It's the screen power supply that is special about this amp. Often over looked by many is proper management of screen power supplies. Copying the errors of "vintage production amplifiers" where cost was a factor and major feedback was used to correct errors, is part of what leads many to think of the pentode as non-hi-fi. In this amp, the screens are provided a fully regulated low impedance source, thus stabilizing their operation and ensuring a deep enough supply for their variance under load. This technique reduces the degenerative feedback caused by audio on the screen, and is what I think is key to bringing out the fine midrange the pentode is capable of. There is additional by-pass directly at the screen as well. Further, no matter how hard the B+ drops during loud passages, the screen voltages remain rock-steady.

The input amp/driver stage uses the venerable 24a tube. This is not a rare tube (but getting there), but is, like so many tetrodes, overlooked in the hi-fi guru area. I got my first taste of using a tetrode while developing my Loftin-White amplifer. I loved it there and I love it here as well. I also wanted a linear response from both the driver and ouput tubes. The amp is designed from the beginning to be as linear as possible even though I know I was going to add feedback to get the output impedance down. As a design this turned out to be a success. I had plenty of gain to play with so I could fine tune the input operating condition AND the feedback to work in harmony. The result is an amp using only just enough feedback to keep the output impedance as low as possible throughout the audio spectrum, while not needing much feedback for error correction. I have a theory that says any amplifier that is "error correcting" is busy doing something, and that something will have an effect on the audio signal, producing "grain" or other side effects. So I avoid making an amp busy correcting (or busy biasing etc.) itself as much as possible.

The 24a Tube

As I mentioned, this amp was used after closing on Saturday night, while a bunch of us sat in Maxxhorn's room listening to this amp on Bob's (and team) fine speakers. Bob and the group were impressed with the clarity and performance of this amp on the wide range of selections we listened to. There was about 6 of us all total, and we all had our favorite selections to hear. If you've never heard these speakers, you owe it to yourself to visit their web page and find out where your can hear them.

UPDATE 5/2007: Another really great speaker system, but more reasonable in cost are the Audiokinesis Jazz Modules.
I had the priviledge of hearing these at the Lone Star show 5/2007, and Duke is a wonderful guy intensly interested in sound quality.
These deserve serious consideration for ANY quality home hi-fi.

I'll let the pictures say the rest.

Amp at GPAF May, 2006.