[Freo-1]

 When I obtained a pair of ATC SCM-19 speakers, the need for more power became readily apparent.  At the time, I had a pair of 55 watt mono amps sporting a pair of 1625 output tubes.  So, why not stick with what works (which is the path that I wound up following).  This is a picture of the 55 watt amp:
 

 My buddy (who actually made the amps) and I decided to employ the same basic topology, but increase the power from 55 watts to 110 watts to get the ATC's to really perform at their best.  So, the basic design was modified to include a MOS-FET to provide more power/stability to the voltage rails at the first tube stage (12SX7, or 12SN7).  The MOS-FET design works very well, and provides added depth and ambience to the presentation.  The output tube stage was increased to four to get the added power.  Fixed bias for each tube was employed, so matched tubes are not required.  Having said that, the quality of the 1625 tube is such that the gm from a given lot measures pretty close to begin with, so it’s not much of an issue.  Here’s the best part:  replacement tubes are plentiful, and can be had for 4 to 5 dollars a tube!  Pretty good deal there, if you ask me. 
 
The transformers were custom made by Heyboer, who makes very high quality power and output transformers.  The chokes are NOS, and are rated quite a bit higher than required for this application.  Mundorf Silver/Gold coupling caps were used throughout, as were 1 watt PRP resistors, and Cardas speaker posts, along with Vampire RCA jacks.  A separate screen grid power supply is required for the 1625 tube to sound its best. 
 
The results exceeded expatiation.  The ATC speakers have never sounded better.  The low level detail from this amp/speaker setup is something to behold.  With live recordings, one hears details not previously revealed in such a natural sounding manner.  One can hear such details as glasses clinking, very low level background conversations, fingers running up and down the fret board, etc.  Vocals seem more realistic and natural as opposed to a hi-fi reproduction.  No solid state amp I’ve ever heard can quite capture that feeling. 
 
The amp is still breaking in, so I expect the sound to improve as the caps break in.  I’ll report back is a week or so, but this amp sounds so good, may finally be able to get off the equipment merry go round and just enjoy music for some time to come.
 

[TrungT]

Very nice.   

[Triode Pete]

Sweet! 

Enjoy!

[Freo-1]

Thanks, guys!  I plan to use these extensively. 

[rodge827]

Very nice, enjoy! 

[Ericus Rex]

I had the pleasure of briefly hearing Freo's 55 watt versions.  If he's this excited about how the new ones sound compared to the 55s then he's got a mighty fine pair of amps!  I can't wait to hear them!

[poseidonsvoice]

Suhweet!

Can you show pics of the underside and describe the topology input to output?

Enjoy!

Anand.

[Captainhemo]

Cool stuff,  enjoy   

-jay

[Freo-1]

I may be able to show some underside pictures in a week or so during Christmas break.   The topology is pretty much what was described above:  The input 12SX7/12SN7 has a MOS-FET wired to the first stage to provide more voltage/current drive to the incoming signal.  The next 12SX7/12SN7 acts as a phase splitter to the output tubes. 
 
 The output is wired for pentode operation, with separate screen power supplies and regulation.  The 1625 can handle up to 700 volts on the plate thanks to the top cap, but the screens can only handle 300 volts safely.  I'm sure the screens can be pushed a bit beyond that, but why tempt fate?  This tube should be run in pentode vice ultralinear to get the best overall sound. 
 
 All the output tubes are fixed bias, with individual bias pots.  Each tube is biased at 40 ma.  All the tubes are DC Filaments, and there are separate voltage rails throughout.  The input tubes have a separate choke, and the main choke is a NOS unit, with plenty to spare.  The tube sockets are NOS from the 40's.
 
 All told, this amp is simply one of the best sounding I've ever owned.  It just seems to do everything right, and is a near perfect match for the ATC SCM 19/C1 combo.  With this amp, the ATC's can get life like dynamics very few speakers can achieve.  My buddy  (who made the amps) thinks they are the best sounding amps he has made to date, and he has made some world class units.  He was also very impressed with the ATC's, saying they sound better than most speakers heard at the audio shows. 
 
 I'll report back after they break in some, but so far, so good.
 

[Freo-1]

Duplicate post... (Sorry)

[RDavidson]

Those are AWESOME!

[Freo-1]

Thought I would post an update:

After using these for a number of months, was able to get a handle on the amps strengths and limitations.  Noticed that when pushed hard, it could get a little congested.  Also, low volume level was not quite as pristine as I would have liked.

So, after conducting some research, had the amp modified to use 6BL7 driver and phase splitter tube vice the 12SN7.  I had very good results using the 6BL7 with a pair of 6883B mono amps.  The power transformer has plenty of reserve for the extra current draw.  The mod involved changing out the filament resistors, and wiring the filament in series.

The results from this change was that the amps performance was improved across the board.  The noise floor dropped to the point that the music seems to come from a black background.  No hiss whatsoever.  Midrange was very much improved.  The 6BL7's seem to drive the 1625 tubes better than the 12SN7 by a fair margin.  The music can now definitely play louder without congestion.  Treble also sounds cleaner.  Low level music playback is a lot better as well, much better clarity.  Before, the volume needed to be closer to moderate to get the music to sound alive.  After changing the driver tubes, the amps now sounds better at all volumes.  At louder volumes, the minor congestion has disappeared. A major improvement across the board. 

Going forward, will be looking at using TV tubes for audio.  They are readily available, well made, and had to be low noise to work in TV sets.