AWA

                                              BenniesBlog -  Single Ended tube amplifiers .

Hi, after more than forty years in professional broadcasting and a bent for tube Audio Design I thought I would start this blog to display current projects and past builds of mainly Single Ended tube Amplifiers .

Output transformers used in the past Hashimoto ,Tamura , Electra Print , Finemet , Amorphous , Grain Orientated Steel both EI and C cores M6, M4 and M3 often called HiB and Super HiB 

After many builds including Single Ended 45 , 2a3 , 300B, PX4 , EL84 , EL509 and now launching into KT66 and KT88/6550  both Ultra Linear and Triode strapped .

My latest build was inspired by the now obsolete AirTight M101 Kitset Single Ended KT88 .

I saw a number of pictures on the net including under the Chassis and pretty much reverse engineered the circuit , quite a simple affair of 12AX7 plates strapped togeather and a KT88 biased with a 600ohm cathode resistor starting with the premise of a B+ rail of 400v DC .

It looked as though the 12AX7 input tube with plates / grids and cathodes strapped togeather was biased at about 1.3mA , I lifted mine to 2mA .

I first tried the Amplifier with a SE KT66 triode Plate 373V bias -43 V 63mA 470R Cathode Resistor and Ultral Linear at 70mA Plate 360v bias -27V 380 ohm Cathode R .

The KT66 was first designed back in 1937 by MOV in the UK for Audio Used from its inception to work both in Kinkless Tetrode and Strapped in Triode from day one .

With  the latest Muse Coils custom design for me a primary of 4.6K / 3.3K /2.25K / - 4 /  9 and 16 Ohms with %40 Ultra Linear tap for the 4.6K load 70mA max at 4.6K and 80mA at 3.3K 

Model MinotauR  OTSE - 060NHY-046 is the first Alexander had made of this model and very useful for experimenting with many tube types .

The Triode KT66 after the run in period was very pleasing indeed as MOV designed this tube in triode to emulate the highly regarded PX25 directly heated triode the British answer to the 300B .

The sound had excellent transparency , full and extended bass and high frequencies , the midrange had all the qualities of a 2A3 but nearly twice the power at 5.8 watts .

The overall impression was excellent with Muse Coils transformer , Single HiB cut C core with double coils similar design to Lundahl and the best Japanese Nogoshi and the Tamura F2007 .

In Ultra Linear at 7.2 watts the KT66 exhibited more bass slam and high frequency dominance , the midrange was neutral but to match the 2A3 I have to remove the Mundorf Silver and Oil capacitor and replace with a Toshin DUCT Paper in Oil Vita Q capacitor from the Hifi Collective UK .

Next after this will be the GE KT88 / 6550 although labelled GE KT88 it look s certainly to be a 6550 which is a highly regarded NOS tube .

The 12AX7 I decided to use was a NOS Japanese 12AD7 the very low noise closely matched version the the 12AX7 with very close matched plates suitable for strapping togeather .

All my wiring is point to point copper foil at present and resistors I have chosen are mainly Amtram AMRG  and AMRT carbon resistors and capacitors from Vishay Sprague ATOM and a full bridge rectifier using HEXFREDs bypassed with 0.01 uf 1200v ceramic capacitors .

My test gear is an old AWA Oscillator G231 just waiting in a AWA Distortion & Noise Meter.

I also picked up a Peter Millet audio analyser using a PC sound card .

Skope is a n older Techtroix 

NOW are few pictures of the chassis and woodwork to be fitted and Muse Coil Transformer Plots 15Hz - 52Khz -1dB 1K driving impedance