Sometimes I simply have to go on, make new stuff, try out new things. This time the victim was the Kapittel I SV811-10 amp. I liked it really, the sound was very nice, but had the idea it could be better, especially with the new sources I have now. Of course again the amp has to deliver low power, but perfect power, this is the only aim I go for. This means as less as possible paper/oils, much coils, as less components as possible, good iron, and of course no feedback.

The AV302B-SL as power tube was very easy choosen. The good name this tube has together with the 300B hype and the fact that I was distributor for AVVT for the Benelux at that time add up to a logical choice. I could use a more heavy type made by AVVT, but the 8 watts I can get out of the AV302B-SL will be more than enough to drive my 104 dB Lowthers in the range above 150 Hz, 2nd order crossed. I choose to bias of this tube to be 400 Volts/100 mA (-90V), simply because of the fact I still have the 2*500V/1A toroidal transformer, coil coupled this transformer will give about this voltage with more than enough power. I only had to remove the first capacitor out of the old power supply of the Kapittel I.

The driver was a more complex choice, I wanted it to be a directly heated triode, and preferable only wanted to use one tube. This means I was looking for a DHT with quite some power, lineairity and high amplification, to keep the amp at least a bit sensitive. Possible candidates were the KC1 (too less current to drive the slew current of the AV302B-SL), the 220P, 45 or 10Y (too low amplification) and the S6 made by electro mecano. The last seemed to be very suitable for this application, having an amplification of 25, 15 watts, 5k plate impedance and DHT. With the 330 Volt transformers I could get 350 volts, and bias the S6 at 17 mA (-5V). The S6 is loaded with a 40H signal coil. This gives an easy very lineair 100 volts output with enough slew current with only 4 volt input sensitivity. To prevent the S6 of amplifying also the low frequencies and the high output belonging to them, I choose to add an extra filter capacitor at the start of the amp. The amp filters now at 150 Hz, 2nd order, this also keeps out extra low frequencies out of the Lowthers, since they are not backloaded they are very sensitive for high cone excursions.

The iron I got from a new discovered brand in the Netherlands. The owner makes very good stuff. I choose to coil load the S6 instead of an interstage load, because making a good 10k interstage with good bandwidth is a very hard thing (if not impossible) to do. I have enough bandwidth at the low side with 40H, simply because I use the amp from 150 Hz. I wanted to have something real special so I used the van den Hul MCS-200 high purity matched crystal silver cable on it. Especially in the low amplitudes of the prestage I think this is a very big improvement. The output transformer uses normal copper cable, the amplitudes are much higher there and thus less cable sensitive. I used double-C permalloy cores, because permalloy is so very beautifull sounding and double-C simply has some very good technical advantages. The bandwidth of the signal coil and the output transformer is 40Hz-80kHz!

All wires internal are silver (including the powersupply), resistors Allan-Bradley and Kiwame, capacitors in the signal are CDE silver-mica and Jensen paper/oil, powersupply capacitors GE paper/oils (8*20 uF/370Vac) in the prestage and SCR MKP's (4*100uF/630V) in the power stage. There are no solidstate and very few electrolytics in the complete amp except for the heather supplies. Negative bias voltage supply, pre-stage supply and powersupply are all separate and tube rectified.

And well, the most important, the sound.... Only two words, real love, from the start... Even when the still had to break in, voices were so very impressive you simply are drawn into the music. Also this amps really knows how to seperate the different instruments and has tremendous depth and detail. Listening to especially vinyl has become trippin'. Enough said now, have to listen again, it is quite addictive....

The schematics are presented here.

Photo Gallery:
Powertube heather supply (2*6.3 V/4 A, 10 mV ripple).
High voltage supply (2*600 V/200 mA).

Technical data:

Supplies          : 425 V @ 100 mA, 350 V @ 20 mA, -125 .. -65 V bias (tube stabilised).
Tubes             : S6 Electro Mecano, AV302B-SL AVVT, 5R4WGTB Chatham, 5Y4GT RCA, 
                    OA2 Philips, 5W4GT National Union.

Iron              : 40H Signal coil custom made Agro double-C permalloy with van den Hul MCS-200
                    silverwire. 4k-5 Ohm Output transformer custom made Agro double-C permalloy.
Measurements      : Monoblocks 335 x 455 x 230 mm, power supply 440 x 220 x 30, heather supply 200 x 250 x 100.
Weight            : About 20 kg each side, power supply about 20 kilos, heather supply about 10 kilos.