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EL84 & 12AX7 Amp possibilities

Polytix

Member
2010-09-09 12:51 am
Well it's time to dive into the world of tubes and too my luck a friend found a nice plastic bag full of goodies at the dump. The contents consist of 4x new EL84's and 5 x new 12AX7's, not bad for a free score I thought, so anyway I have been foraging through the oodles of info on the interwebs and to be quite honest I will end up tackling something a bit over my head, so was wondering if some of the seasoned pro's out there could post up some good schematics, I'm an electronics tech by trade so can work alot of problems out but I want a build to learn the art of tubes alongside the theory that I have been reading. So in a nut shell

Simple build preferably somewhat integrated so I can plug line level inputs straight in.
Would prefer PP but SE would be fine.
Volume controll and if possible Tone controll aswell.
Stereo Hifi.

I also scored an old Columbus console which I was going to raid for a few bits and bobs, mainly the rectifier tube (adds that bling factor), and possibly the main transformer, most likely I will order all new iron anyway.

Any help would be grand otherwise I will most likely create a monster of a beast that will end up more like a boat anchor.
 
Hello,

A quick description of a circuit which works well :

1/ 1x12AX7 in SRPP circuit driver, 1,2K biasing resistors, 470K grid resistor
2/ 1µF/400V coupling cap
3/ 1xEL84 in ultra-linear operation, cathode bias resistor + 220µ/25V, 220K grid resistor.
4/ plate supply : 250-300VDC required.
5/ output Xfo : 5K + 3 or 2,5K tap / 4/8/16 10-15W
6/ the cathode resistor of the EL84 to be calculated to not exceed 12W plate + 2W screen dissipation.

You would be able to expect 3-4Wrms, 3-4%THD without FB loop, with an even harmonic spectrum.

A+!
 
Well it's time to dive into the world of tubes and too my luck a friend found a nice plastic bag full of goodies at the dump. The contents consist of 4x new EL84's and 5 x new 12AX7's, not bad for a free score I thought, so anyway I have been foraging through the oodles of info on the interwebs and to be quite honest I will end up tackling something a bit over my head, so was wondering if some of the seasoned pro's out there could post up some good schematics, I'm an electronics tech by trade so can work alot of problems out but I want a build to learn the art of tubes alongside the theory that I have been reading. So in a nut shell

Simple build preferably somewhat integrated so I can plug line level inputs straight in.
Would prefer PP but SE would be fine.
Volume controll and if possible Tone controll aswell.
Stereo Hifi.

I also scored an old Columbus console which I was going to raid for a few bits and bobs, mainly the rectifier tube (adds that bling factor), and possibly the main transformer, most likely I will order all new iron anyway.

Any help would be grand otherwise I will most likely create a monster of a beast that will end up more like a boat anchor.

Nice score! Whereabouts are you? If in NZ, we could correspond as I am in the process of building something similar. Kiwi radio consoles generally will not have large enough rectifiers or PTs for stereo builds unless you double up on everything.

Gary
 

Polytix

Member
2010-09-09 12:51 am
Nice score! Whereabouts are you? If in NZ, we could correspond as I am in the process of building something similar. Kiwi radio consoles generally will not have large enough rectifiers or PTs for stereo builds unless you double up on everything.

Gary
I'm down in Dunedin, yeah my mate was dropping off a horde of crap at the recycling center and just saw the bag sitting there on the corner of a table, recycling center was closed so it was a yoink kinda manouvere, lucky really as most people (including the recycling center Staff) would have just thrown the bag straight into the landfill, so he kinda saved some nice tubes.

And cheers for all the info guys, will probably start with the baby huey and go from there, I do like the RLD and I also downloaded the BOM and Schem for the Fisher. Cheers for the direction, I think in the last week I must have downloaded and pondered over at least 50 different schematics.