GLASS AUDIO

VT-1SE

This is my first Vaccum Tube Single Ended Amplifier based on famous 300B Triodes.

Maximum output power is 2x 9W/ 8 Ohms Load. That is a limit which can be produced by single Tube of this kind. The main idea of Single Ended design is to simplify circuit and eliminate any components which can distort or degrade the sound. The Single Ended triode tube amps were considered by many audiophiles, to be the most natural sounding amplifiers and become almost a cult. Main active component of this kind of Amp is Power Vacuum Tube and output Transformer. In this case I chose proven from 1938, to be best sounding Tubes , Triodes 300B .

Construction

This Amplifier is my first Tube prototype, so that was no reason to make it from expensive and exotic parts. I wanted to reduce to minimum costs without of losing performance. For Output Transformers and Main Power Transformer I used very good and very inexpressive set, made for Single Ended Amp by "Eastern Audio Co.", acquired on Ebay. These Transformers perform really very well and I'm not planning to replace them with any recommended Toroidal Transformers (Plitron) any time soon.

Driver Stage is also very important. The 300B tube requires very high grid bias voltage. It is about -105V. For that reason many projects using transformer driver for 300B tube in Driver Stage. I decided to use a White Cathode Follower getting high performance with high peak voltage output and superior power supply ripple rejection. The low noise tube 12AX7A is used for first stage a 6189W tube for driver stage.

Final Stage the tube 300B works with 400V plate voltage and 5V DC for heater, driving 3K Ohms primary tap of output transformer. Secondary taps of that transformer are hooked up to the switch for 8 Ohms and 4 Ohms speakers terminals.

Power Supply is most innovative part of this Amp. Typical Tube Amplifier has High Voltage power supply for Tube cathode and Low Voltage power supply for Tube Heater. In most designs heaters power is supply from the same power transformer using special high current 5V - 6.3 V tap. It is forever open discussion, what kind of power should be used for tubes heater AC or DC. I think DC voltage for tubes heater is better. The most Vacuum Tube Amplifiers have a problem with ripple noise from AC power line. To eliminate that to absolute minimum, a few big, heavy and expensive chokes must be used for all power supplies. That would increase weight and size of that amplifier. Some "modern" constructions of tube amps are built old fashion way, with lot of chokes and ancient capacitors to supply power for tubes. Sometimes that kind of Amp reminds wooden crate full of junk or some artifact recovered from "Titanic's" garbage room. Hey, its 21 century already. When we using old technology like vacuum tubes, it doesn't mean we have to use everything else from that era. In my opinion any amplifier should be integrated into the rest of your audio system.

My approach to this problem was inspired by publication in magazine "Glass Audio" Vol.12 Nr.6 2000 by Satoru Kobayashi about using Switching Power Supplies for vacuum tubes heaters. I used Switching Power Supplies for heaters for Driver Stage ( 12 VDC) and Final Stage ( 5 VDC). Also Grid Bias Voltage(-100 VDC) for Triodes 300B is created by Switching Power Supplies. Prices for Switchig Power Supplies went down so much and almost every kind is avaiable on line in Surplus Store like "All Electronics". Only high voltage for plate (400VDC) coming from Power Transformer.