Choke-loaded preamp

Jim de Kort

What's the DIY hobby all about? Fun right... well, I thought it was about time I built something that was just that: FUN.... No looking at what cores give the best detail for the tube in question. If the power tranny should be 3 degrees off magnetic north. None of that guru stuff!!!! Just plain fun and especially COOL looking.

Going through my stash of tubes I plucked out the 10Y and some 866A's. Wow, what a combo these would make... Bright glowing tungsten and blue mercury vapor. Wow!!!!

The coolest part of it all is that I can use my 26 preamp chassis for this amp without any major changes. All I need to to is change the current regulator resistor to get 1.25A and build a supply using the 866A's :)  This way I can use all the cool parts in my 26 preamp and just plug in the 10Y. The cathode resistor is the same (2K2), I use a higher voltage on the 10Y but it biases just like I wanted... 350V/16mA... The 10Y/801A will take chokes from 150H or up to give a -1dB of a 20Hz.

The B+ supply will have to be built from scratch. I will add the option to switch back to 26 as well, as a back up for my battery supply perhaps. The B+ tranny has two taps so I can hook them up for both 26 and 10Y (325-170-0-170-325V). The filament supply is done by my two battery towers which also house 12V batteries for the 26 filament. I can use these to get the 7.5V for the 10Y, no problem, only have to adjust the current regulator resistor to get the right current. The B+ I get for the 26 is around 200-205V and for the 10Y about 390V. This is including bias voltage of course! (12V for 26 and 32V for 10Y).

Below is the schematic, I will put up pics as soon as it is finished...



The 866A diodes may be a bit of overkill, an 83 will also give a nice effect and is easier to implement as it does not require such heavy filament windings. if you want the double mercury glow, try the 816 instead. This is a smaller version of the 866A. You could say in terms of specifications it is half of the 866A.


The 816