Newly built with less than 20 hours of playing time.
I have just completed building this compact (12x7x7 inches) single ended pentode EL84 power amp using the David Gillespie schematic. It is packed full of high quality parts to achieve superb sound. The amp chassis and transformers are reborn from a previous life in a Zenith console stereo from the 1960s. If you are uninitiated in the single ended sound, I promise you will be blown away. Along with the total circuit rebuild, the unit is freshly painted with hammertone light blue and black, which is a textured paint and shows as a dimpled appearance. Not a quick and dirty rebuild. User friendly features such as; on/off switch that is fused, IEC power socket for after market power cable that is grounded with x1/y2 safety caps, and new gold plated input jacks and speaker output jacks. Please read on for more information and don't hesitate to email me if you require more. Have a great day, and thank you for checking out my ad.
This amp follows the schematic that was developed by David Gillespie in Audio Karma under the title "Magnavox Flea Power: Getting More Out Of The 8600 Series". This amp was initially a Zenith from a large console stereo which was the norm for audio back in the 60s and 70s. My build is a further development of the Maggie amp with an even better power supply than what is described in the article. One thing about single ended amps is they are only as good as their power supply. Much discussion in the tube DIY world and esp for SE use is about the power supply. With typically a minimalist circuit and just 1 gain stage, SE amp's deficiencies are exposed by their power supply. Most amp builders agree for a SE amp a LCLC , choke/capacitor/choke/capacitor, type PS is the best. Next best is a cLCLC, type, which uses a small value capacitor, typically less than 1 uf, The small value capacitor in the first cap position helps to fine tune the B+ of the power stage and also helps to increase the speed of the amp. In the pic of the underside of the amp, the big white cap is that small "c" in the cLCLC . Its value is .5uf x 600v. the next L or inductor/choke is a vintage Stancor made in Chicago, next C is 40uf film cap, next L is a very special and expensive oil filled military type choke with a glass seal probably made for the air force. The next C is a 80uf Nichicon. The PS in the AK article is just a 20uf cap to 10H inductor to 40uf cap, good but not good enough for me. The PS for this Z amp has the ability to deliver more current at a faster speed than the original PS plus there is no hum or noise to intrude on your listening experience. The PS also uses over sized 1 and 2 watt resistors in critical areas. What this works out is an anode voltage at the plates of the el84 power tubes of 245v. which is almost spot on to the design target of 250 vdc with the 5y3 rectifier tube. All other voltages are also in step with the design schematic. The other mod that I implemented with this amp over the original design is the use of a CCS or constant current source to set the el84 bias. This uses a SS solid state device (LM317T) to keep the bias voltage at a fixed value. The sonic result of this mod is a firmer sounding and stronger bass over the simple bias resistor and bypass cap in the original design. Other components that I implement in all my builds are careful component and wiring layout as seen in the photo. I also use large 18 gauge wire or larger for the PS and a 12 gauge ground bus wire which is only grounded at the power transformer. This along with the film capacitors allows the amp to deliver much more current instantaneously when music demands are high. For signal handling I like to use paper in oil capacitors for the coupling as they have the best sonics at a reasonable price. The ones installed are military specified and are ceramic ends with a brass body. Capacitors of similar construction made today cost over 50.00 each! Well, I could go on but the final aspect to all the fuss about the construction of the amp is the sound quality. In my home system which consists of vintage JBL L112 speakers, I think they were made in the 80s, a simple Cary AE3 preamp, and an OPPO dvd/cd player for digital music source. I am not sure of your experiences with single ended types of amps but to me this amp sounds excellent and produces a very strong 5watts of power and even though it only has about 10 hours of use on it. It has an excellent frequency response as shown by the graphs in the AK article and as confirmed with my listening experience. It has great bass that is dynamic and plentiful. In fact, comparing the single ended el84 to a push pull el84 I can definitely say that the single ended amp has more substantial bass. The other areas of frequency response is also excellent. The midrange is pure and expressive and especially, in the area of dynamic shadings and micro detail. The ability to hear and see in your mind's eye a solo violinist's playing in front of an orchestra is a wonderful experience with this amp.
Finally, in closing, the amp is sold as is as shown and the buyer accepts full responsibility for its use. The amp uses vacuum tubes which can burn when touched. There is no bottom to the amp and as such children and pets should not be allowed to play near the amp.
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