© Don Sachs, 2015
Vintage Tube Audio  Restoration and Repair

Custom KT88 Tube Amplifier $3199

This may well be the best amplifer I have ever heard!!

UPDATE:  OK, I am only building one version of this amp from now on.  This one has fully regulated separate high voltage supplies for each channel using a Maida style regulator setup.  It has literally about 20 micro volts of ripple - basically unmeasureable.  Still uses a large Mundorf Tube cap film capacitor as the first filter cap and then goes into regulated supplies for each channel.  This is essentially a dual mono amp in one chassis.  The background is completely black and you literally cannot tell the amp is on when it idles away.   Stereo separation is just insanely good!  Also, I have added a fully automatic biasing feature.  The tubes idle for a minute and heat and then over the next minute they slowly ramp up to full bias.  The supply monitors the bias on each tube and corrects it constantly to keep it right where it should be set.   The combination of the fully regulated independent high voltage supplies for each channel and the fully automatic and constantly adjusting bias makes the amp at least another 15-20% better than the all film cap previous version.   I was trying to build this version for $2995, but it costs about $200 more in parts and takes at least 4-5 hours longer to build.  I have to price it at $3199 with a premium tube set included.  I have now built 5 of them and they are fantastic.   I just did a citation II full rebuild for a customer and played it for an evening.  Lovely amp of course.  Then I put the fully regulated version of my KT88 amp back in.  NOT EVEN CLOSE.  The citation, while lovely, sounded almost flat and uninvolving compared to this KT88 amp with the fully regulated high voltage supplies for each channel and the adaptive bias system.  The fully regulated version outputs 65 watts/ch from about 27 Hz to well over 35 KHz.  It is only about -1.5 dB at 20 Hz, which means it still outputs about 54 watts at 20 Hz, both channels driven.  There is no music below 30 Hz and the bass response of this version is just stunning.  These transformers are REALLY GOOD.  Those are real measurements with both channels driven simultaneously, not extrapolated from transformer specifications or some other nonsense.  Needless to say it has amazing bass!  It is flat to well below 30 Hz and can still deliver over 80% power at 20 Hz. Yet it renders every subtle detail in the music.  It has a warm, rich, powerful and detailed sound. Backround:  I was looking for a way to build a nice 60 watt/channel amp that would give most of the performance of a Citation II amp for considerably less money.  I built a prototype amp using Roy Mottram’s octal driver board and a transformer set for Bob Latino’s ST120 amps that Roy had in stock.  That amp featured a very large power supply with dual chokes and the project was a success.  The amp sounded wonderful and delivered 64 watts/ch.  The only flaw was that it started to roll off at about 40 Hz and was almost – 3dB at 30 Hz.  The sound was excellent though and you would never realize the bass response was rolling off without putting it on the bench and measuring the frequency response. I wanted to repeat the project and build an amplifier that was flat to below 30 Hz.  So I did a little research and sourced some excellent 80 watt potted toroid transformers that had much better low frequency response.  The resulting amp is just stunning!  This amp may be the best one I have ever had in my living room.  I like it every bit as much as my completely rebuilt and tweaked Citation II.  This amp sounds a little warmer, but has all the detail and power.   These transformers are REALLY GOOD.  Those are real measurements with both channels driven simultaneously, not extrapolated from transformer specifications or some other nonsense.  Needless to say it has amazing bass!  It is flat to below 30 Hz and can still deliver over 80% power at 20 Hz. Yet it renders every subtle detail in the music.  It has a warm, rich, powerful and detailed sound. I had Roy Mottram make a special version of his octal driver boards with some modifications.  My special version is twice as thick so it will not flex at all when you change tubes.  It now uses Belton sockets instead of the Chinese ceramic ones.  It has separate power supply inputs for each channel now, facility to add AC balance pots for each channel, and the CCS is now the larger one we use in the Citation II rebuilds.  So the 6SN7 driver tubes are now run a little hotter where they are right in the middle of their linear range.  They are still cruising along at 40-45% of their maximum dissipation rating though.  The board is built with premium Takman carbon film and metal film resistors in all the key spots as I do in the preamps.  There are also Kiwame resistors in a few key spots.  Coupling caps are very good film and foil bypassed with small Russian Teflon caps.  There is a separate 6 volt DC regulated filament supply for the input tube.  There is an LED panel meter so you can monitor the tubes, but it is fully auto biasing.  There is nothing to adjust!  So this amp features: Fantastic power supply featuring independent fully regulated high voltage supplies for each channel and a bit toroidal power transformer that is 25-30% larger than required.  Think ridiculous amount of headroom! The power and output transformers are epoxy potted toroids from TOROIDY.PL in Poland Triode switches right on the top deck so you can change between normal 65 watts/ch and triode mode (about 35 watts/ch) Fully auto biasing for each tube.  Just put any KT88 or 6550 tube in and the amp will bias it.  The system applies the full negative bias for the first 60 seconds so the tubes are only heating, but cannot conduct.  Then it brings them up to full operating point over the next 45 seconds.  A very soft start for the tubes so they should last a long time. Driver board is Roy Mottram’s Tubes4Hifi 6SN7-based octal one, built with premium parts and fantastic coupling caps.  Better CCS and AC balance pots to precisely balance the toroid output transformers. Custom wooden case with anodized aluminum top plate.  Output speaker terminals are American made Superior Electric. Individual plug in cathode fuses on each tube socket.  If you ever have a tube red plate it will simply blow the little fuse.  Simply remove the bottom of the amp, plug in a new fuse, replace the bottom and put a new tube in.  The amp will bias it and you are ready to play again. Why does it cost $3199? Because the transformer set alone costs about $650 delivered, and the amp is built with premium parts.  These parts and almost three days of labor result in a stunningly good amplifier!  This amp is so good it is scary, and driven by my custom line stage it is heaven.  At 65 watts/ch with fantastic output transformers and an oversized power transformer it will drive most speakers with ease.  The amp is built in a solid cherry case with anodized aluminum top and rear plates.  The top plate can be either black as in the photos, or standard natural aluminum color as in my preamps.  I comes with a brand new quad of Shuguang Golden Voice KT88 output tubes and three CV181z tubes up front.  As with the preamps, they are built to order.  I require 70% downpayment and can usually ship the completed amplifier within 10 weeks. A listening test: I just finished completely rebuilding a classic Marantz 8B tube amp for a customer.  All new everything and top shelf parts.  I have run it for a few hours and it is giving about 46 watts/ch at clipping with both channels driven.  That means it is running better than factory new.  It has K40 oil caps on the output tubes and very good quality resistors.  It idles away perfectly and is dead quiet.  I put it in the living room system and listened for an hour.  Very clean and clear.  Really a great classic tube amp and deserving of all the hype.  I just swapped this KT88 amp back in the system.  Big difference.  All the detail of the 8B, but much richer and fuller sounding.  Sure, my system, my ears, but all I am saying is this KT88 amp is easily the sonic equal of the Citation II and the Marantz 8B.  I am not saying which amp is better, just that this amp is easily in that league and to me it sounds better than the 8B.  This KT88 amp ships out Friday.  I am going to order another set of those transformers tomorrow because I have to build one for myself.  I will miss it! Warranty: This amp should be totally reliable.  So I will guarantee it for one full year.  If anything goes wrong (exept for a tube) I will pay shipping both ways, fix it, and return it for free.  If a tube that I supply fails in 90 days I will replace it.  Now if you have a party and someone spills beer on it then I will fix it for you as cheaply as possible, but the cost is on you.  Realistically I would expect this unit to be trouble-free for many years.  If you get a problem more than a year down the line and you pay the shipping I will fix it for only the cost of parts, forever.
Front view.  Epoxy potted toroid transformers in stainless steel cases. Latest version with LED panel meter on deck. Ignore the blue poly on the transformers.  It is just for shipping. Rear, featuring Superior Electric American Made binding posts.
New version with independent fully regulated high voltage supplies for each channel, and fully auto biasing!