No.56
Hi-Fi Audio
SV300B

Introduction

It has been some time since the micropowered single-ended triode (SET) movement has hit the U.S. At first it was dismissed as Japanese silliness, yet the SET amplifier now appears to be here for good. I recall hearing my first 300B amplifierfrom that point on, I was hooked. A search for a good quality 300B amplifier floored me with the steep retail prices. Being a Biomedical Electrical Engineer by trade, I knew I could build an amplifier just as good as the commercial unitsand for a lot less money. Many years passedthen I discovered the Svetlana Tube Zone on the Internet. This sparked my interest again. The results are described in the following text.

Circuit Description

The circuit of my amplifier was pieced together from various schematic sources including, but not limited to, Glass Audio, Vacuum Tube Valley and the Svetlana Technical Bulletins on the Internet. The driver circuit is a Mu Follower, as seen in the Svetlana Technical Bulletin No. 27 by Cyrus Brenneman. I did make a few changes to it, however. It ended up using a floating DC supply for the SV83 and 6N1P's heaters. The plate voltage was lowered to 400 volts. I did this to increase the life of the tubes. Even after this mod, the circuit will swing over 200 V p-pmore than enough, as the SV300B only needs about 200 V p-p input to drive it to full power.

One thing worth noting about this driver circuit: it is very linear. This allows the amplifier to "display" the natural harmonics of the SV300B. The SV300B is one of the few tubes that I have found to require a good deal of overdriving before the distortion becomes unpleasant. Also, when this amplifier clips, it happens so gracefully I can hardly detect it. Keep in mind that the driver circuit only has a gain of about 30, so a good pre-amp is a must if you plan on using a CD or tape player as your source. I measured the input sensitivity to be about 6 V p-p for full power.

The output section is an SV300B in pure Class A1 with "fixed" (adjustable negative grid voltage supply) bias. I had tried both self bias and "fixed" bias, and the latter gives a more solid sound to the music while allowing complete control of the tube's operating point. The plate voltage is 420 volts and the grid voltage is -82V, giving the SV300B a plate current of about 90-95 MA. This easily gets me the super-clean 7.5 watts that my +100dB horn speakers need. This is on the conservative side, as far as the SV300B is concerned. It can be run a little harder to get more power, if needed.

I chose to use AC on the SV300B heaters. AC just plain sounds better than DC to me. All the rectifying and filtering seems to affect the sound, and hum has not been a problem with this amplifier. If you wire things as shown in the schematic, and you keep the transformers away from the signal path, you should not have a hum problem. All you have to do is adjust the 100 ohm pot until the hum is essentially gone. The remaining hum is not audible from my speakers.

Construction

The top plate is 1/8 inch thick aluminum and is 12 _ X 13 _ inches. The back plate is 2 _ X 10 _ inches. I coated the plates with a special primer that is made for aluminum and then painted them with Hammerite spray. The transformers are also painted with Hammerite. The bottom plate is just 20 gauge powder-coated sheet steel. I cut a large square hole in the top plate to mount the power transformer in a horizontal position. This puts the power and output transformers' magnetic fields at a 90 degree angle, thus preventing inductive noise coupling. The base is made from white pine and is stained a medium walnut with a semi-gloss clear coat.

Parts selection

As with any SET amplifier, parts selection plays a major role in the amplifier's sonic signature. It is very important to use good quality parts. I have labeled the make and ratings of the components in the schematics for your convenience. All of the iron is from Hammond and the resistors are Multicomp metal films, plus a few Vishay wirewounds. Most of the capacitors are ASC polypropylenes, with a few Solen Fast Caps. The power supply uses one large Chemi-Con electrolytic, while two 220uF Elna Cerafines are used for 6N1P cathode bypasses. I used 40 volt 8 amp Schottky diodes for any low voltage rectification duties. Note: If you plan on using the Hammond output transformers make sure you get the new, improved version. The old 1627SE weighed 6 pounds, while the new 1627SE weighs a whopping 11 pounds! The new version is excellent, with lots of bass and a very open and detailed sound. I measured the -3dB bandwidth and found it running from 25Hz up to 38KHz at 5 watts.

Measurements

Although I feel that measurements mean a lot less than a good listening session, I have included some of the measurements for the spec-loving constructor. Input sensitivity is 6.13 V p-p for the onset of clipping. Output voltage across an 8 ohm resistor at the onset of clipping is 25.76 V p-p or 10.368 Wrms. Power bandwidth is from 22Hz to 39KHz at 1 Wrms. At 10 Wrms it is 27Hz to 28KHz. I don't have a distortion analyzer so I don't know how much distortion the amplifier has, but it really isn't noticeable until around the 8 Wrms range.

Listening Impressions

So how does it sound? Well, it's downright beautiful. The SV300B has a wonderful midrange that is to die for. I have been listening to this amplifier for about 4 months and have enjoyed every minute. I have tried two other brands of 300Bs with mixed results. The KR Enterprise VV300Bs have more bottom end, but their top end had a bright glare. The Audio Note nickel-anode 300Bs have about the same bottom end as the SV300B, but really lacked in the high end. They were very soft, appearing to lose some high frequency detail. Then there is the SV300Bit has good bass, and high-frequency information has a nice evenness to it. The midrange is alive with detail and smoothness. I have never owned an amplifier that could sing like this one. The SV300B is the way to go, in my opinion.

  Final Thoughts

After looking everything over, I am completely satisfied with the results. Svetlana makes excellent tubes, and I have not experienced a failure with them. Whenever I have called Svetlana's engineering staff for help, I have been treated with respect, and Eric Barbour was a big help during this project.

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.

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