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I lashed one of these up on a breadboard. Need something to
do while I await the outcome of my job search. :)
This is part of a bigger project to consume a bunch of spare
parts left over from a few years ago. The end target goal is to
use the 1628SE as a plate load rather than OPT and use a pair of
Ni pinstripe MQ parafeed transformers. Also, I will be choke loading
the CF driver to improve its linearity.
This initial prototype sounds really really good... What more, I am
using AC heating of the 811A. With a humbucking potentiometer spanning the filaments, I can null the hum down inaudible on 98dB eff speakers.
This is a very easy circuit to build out too. Classic 6SN7 gain
stage. The only weirdness is the Class A2 driver stage. :) I'm
looking forward to picking away at measuring performance and distortion at the 811A grid during October before wrapping the
project up. But, whats cool, is that this is a very easy to do
project and more people should try the 811A. Key being the fact
that you can get away with AC filaments and the buggers are $20/ea
Sound wise it is very direct and very powerful. The amp itself
should do 9W but I need to adjust R3 and R5 to go full power.
I expect significant improvements going parafeed. I know the Hammonds
blow out their HF response at 13kHz or so. FYI, the 811A at 100mA
isnt even sweating yet. But it does throw heat. This is most definitely a winter amp. :)
That's got to be the lowest parts count 3-stage amp I've ever seen, pretty damn cool.
Have you played around the 6SN7's operating points and tried unbypassed cathodes? Very neat breadboard btw =^). Thanks.
My altec 1570s use pair of 811as in pp and im doing a full tutay mod.I did try dc filaments on the 811s but it dropped the filament down to 5.2 so I scrapped that idea.Now he changes the pair 6w6s to a pair of el34s for the cf and those are driven from the 6sn7 phase splitter.The cathodes of the el34s are choke loaded and coupled as the 6w6s were but even tho the el34s have no gain and are wired as triodes the sound seems to influence the 811s in a way i dont like..Im thinking of using a pair of 6bx7s or possibly transformer coupling the 6sn7 to the 811 outputs.I have 4.1v on the grid of each 811a and a pair give me about 170watts triode power.
Nice Jim but what are you going to do when your wife asks for the leaf you built that amp on back for her kitchen table? hehe. Very nice tho its looks good.
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