Here you will find random info on an "evolving" P-P EL-34/6CA7 amplifier.
Heres a pic of the "breadboard" mess..
Pretty ugly huh? The "proto" chassis is underway..(I'm thinking of making the Final chassis out of Glass!!(Any tips? Contact me!)
Behind the proto amp you can see the 6V6 amp(left),and the "topless" ST-70(right).
The idea for this amp was to make something about 50W,using common,inexpensive tubes,that sounded _good!_
My Dad wanted a somewhat powerful amp to use.We started by getting a pair of RCA output transformers.These transformers were used in the RCA MI-12188A PPP-807 amplifiers (More info here: http://www.montagar.com/~patj/mi12188a.htm )
After opening them up,and replacing the cracked,brittle wire leads,I sorta crudely calculated the primary Impedance (Z) to be about 3.5K ohms,which was inline for PPP 807's.These Transformers should be good to about 100W,which is twice our goal,and so far seems to have been a bit of an advantage!The amplifier has lots of extension,and plenty of low end.(yay!) I think the "oversized" iron helped.I was slightly concerned,in that the transformers were designed for "PA" use,and not "hi-fi"..But it all worked out nicely in the end!
The power Transformer was salvaged from an old dead Tektronix tube oscilloscope,"Type 564 Storage oscilloscope"
(Here's a scan of the O-scope's original PS. Does anyone have any idea of the current ratings of the various windings?)
It has plenty of heater windings!,However I'm not sure of the current capabilities of the HV windings.I suspect they may be a tad-bit small (wire dia.) for the peak current levels,but so far in testing,and listening it's been okay up to "Rock'n-the-house" power levels,and fairly light -Vg1 (bias voltage) and heavy Ia (plate current).. Ie: heavy idle dissipation,and more class A power!.)
The tubes are biased at about 50ma (Ik) per tube.The (currently 33ohm) cathode resistors provide a nice point to calculate bias current. (The future chassis will have a test point,next to the adjustment for each tube.) I may also include a simple LED bargraph circuit to give a visual monitor of the bias condition. (Patrick Turner has a neat design he uses at http://www.turneraudio.com.au/ )
And perhaps some kinda of overload protection,incase plate current (Ia) goes over the "safe zone" (maybe just a fuse in the cathode circuit..A borderline undersized (wattage wise) resistor would be similar. (lots of people use this method! common in guitar amps.)
I do have other larger power trannies from other Tek scopes (Tranny's marked as #120-120,and ,Please contact me if you have info on either of these transformers!) Incase this one dies on me. Hopefully they will be similar/suitable.
I chose EL34's because they are fairly common,and inexpensive.Other choices might be KT90-88-66's, 6550's 6L6's (Dads original choice.) 6550's would probably make the most power,but may strain our poor power transformer,so I picked EL34/6CA7's. They are fairly well suited to a 3.5K load,and produce about 50-55W with a 420-ish B+ voltage..PERFECT!
( http://www.duncanamps.com TDSL has info for differnt tubes,and different setups!)
Duncan' says that THD will be about 5%,which is a little high.I've used a little cathode(auto) bias,along with adjustable fixed biasing to help with (crossover) distortion a little.I have also included a rudimentary (soon to be refined!) feedback loop to help aswell.
So far in Listening tests the distortion hasn't even been a concern,theres plenty of power/headroom for average,to "rockin" listening levels.
Suprisingly,As of yet I have not done any "technical" tests with the 'scope,or anything.I just started with a simple circuit,tweaked and fiddled it a bit on paper,built it up,and fiddled it by ear,and so far,I have to admit,I'm impressed! Once I get it into the "proto" chassis (more manageable!) I'll start testing and tweaking (finalizing) the circuit more.But really,thers no telling what might get changed around next!
Speaking of which,You may recognize the input circuit from the 6V6 amp! Yep,I copied it.It's simple and it seems to work well in both amplifiers,But it's probably not the best choice for real "Hi-Fi". I plan to make the input circuitry on a PC board,Much like the ST-70 (even the same dimensions!) this way,if one wants to experiment with the circuit,it's much easier,just swap the board!
Making it the same size of the ST-70 seemed obvious.If you wanted you could use the original ST-70 circuit,or one of the many "upgrades" (SDS labs,Triode Electronics,etc,etc,etc) or make your own!.. Killer!
This sort of fits into the theme ("Prototype"/Erector set,experimental type of thing.),I also wanted to be able to use different types of tubes,So of course I choose the venerable Octal layout of the whole EL34/6L6/6V6/6550/KTxx/etc. family of power tubes,of course!What else?! I even made the bias adjustable over a wide voltage to accomidate the different tube's requirments.. There may be provisions made for switching to different B+ voltages,etc. for the different tubes...We'll see..
The Power supply schematic(s)
Page # 1 (hand drawn)
The Amplifier schematic(s)
page # 1 (hand drawn)
page # 2 (CAD drawing)