[kentajalli]

OK
Previously in a different post I had asked if anybody could help with an Audio Innovation 200 poweramp circuit or PCB diagram.
Since no-one stepped up ( have you guys been busy? it's only Christmas!) 
I photographed the PCB and manually drew the circuit (board).
But before you have a look, I must say, I do have a modified Edison one poweramp, that uses the same tubes, but in a different configuration. It sounds much better than this amp. Having eliminated the possibility of it being the quality of the output transformer, I set about modifying the the board to change the circuit to make it same as my modified Edison.
Can any of you guys check my work out before I start the modification please! Ignore irrelevant discrepancies.

The right side of the mainboard is the modified bit ( in green ), so that you may compare it to original audio innovation 200 factory layout (left side). the second page in the pdf, is the daughter-board which is the input buffer, that I shall do without once I modify the amp.
Thanx in advance.
Happy Christmas to you all.
Ken

Modified Edison one circuit is:
http://www.computer-surgery.net/ebay/edison%2012.jpg
Audio Innovations PCB is:
http://www.computer-surgery.net/ebay/Audio-Innovations%20200%20PCB%20modified.pdf

[ElliottStudio]

I can't see any mistakes. I would, however, put a choke where the big 47R resistor is between the first and second stage of filtering on the PSU.

[kentajalli]

thank you!
I already had, and a second low power 8H choke for the grids, and the pre section takes it power from there.
It works very well, I have finished!
However, today on my original Edison 12, I went one step further, and did away with "Auto-Bias" -
On a small board, with a tiny mains transformer, a few resistors and caps, and four multi-turn pots, I changed the circuit into fixed bias mode (adjustable for each tube).
The amp was previously running on 65mA (300V) previously (dis 19W !! per tube) and it sounded fine for about a year with a set of Chinese tubes.
Now I have it running at 30mA (dis 9W per tube) and it sounds superb!
at very low listening levels it keeps its composure, as it does at high levels, no more compressed dynamics.
It is a different amp, more muscle.
Is it the fixed bias?
is it the fact that less current is running through the transformers and the tubes (i.e. less strain) ?
is it the lack of capacitors on the auto-bias resistors ?
All of the above!
I don't know.

[ElliottStudio]

Great work! I prefer fixed bias. It's not so much that the bias level has changed (though I would run them a bit more into class A or more current). It's that more of the B+ supply is available to the plates due to the fact that you don't have a 15 - 20 volt loss across the cathode resistors anymore. Add to that, you get to eliminate the dreaded cathode bypass cap. All of this will improve the performance of the amp, as you have found!

[kentajalli]

I wonder if anybody has any suggestions. (specially You Elliot!)
So far on my Edison 12 amp ( see circuit in previous posts) - I have eliminated many signal capacitors, there remains only three.
Two are between the driver section and the EL84's ( 0.47uF) which after many try-outs, I settled for the Russian K40y-9's .
I tried Auricap, various MKP's from M-cap ( even M-cap supreme silver/oil ones at £30 each) - but the K40y's are my favorite (@ £2 each).
It just occurred to me, that there's a last signal cap, namely the 100uF bypass on cathode of the first tube!
I am thinking of replacing this, with 2X 10uF MKP caps I have leftover from upgrading my Sonus Fabers.
Does anyone expect any change in audio quality?
does an electrolytic capacitor sound any different to the MKP's (in my case) ? 
I wonder . . .

[ElliottStudio]

Yes, the quality of cathode bypass makes a big difference but 10uf MKP's are pretty big! If you can fit them, give it a go!

[kentajalli]

Yes, the quality of cathode bypass makes a big difference but 10uf MKP's are pretty big! If you can fit them, give it a go!

Yes there is plenty of room.
It is 9am, I came to my office early (that's where the amp is) and just did that, didn't take that long.
I am listening as I type.
The difference is not light and day, but everything across the board is a little clearer. Lower bass (didn't expect this) is more tuneful, upper mid is also clearly better defined.
Over all, with such little effort and cost involved, it is probably one of the more useful upgrades I ever did.
For the life in me, why didn't I think of it before ? !
Thanx Elliot

[ElliottStudio]

Did you bypass the existing 100uf cap with the two 10uf? or replace the 100uf with them? You may be rolling off your lows. That cathode bypass cap's value usually is chosen to have an f3 of around 10Hz or less.  You may want to add a high-quality 47 to 68uf electrolytic back in there. I use the solid polymer types and bypass them with 1uf MKP.

By the way, congrats on the good sound upgrade!

[kentajalli]

last year I had bypassed the 100uF (incidentally nichicon gold) with 2.2uF wima poly p's.
this time I replaced them with 2 x 10uF MKP's first, thinking (as you did) that my bass is going to suffer.
To my surprise it did not, so 'for the heck of it' I went down to just one 10uF -
high and behold, bass was still there.
So I tested it the next day with a sig-gen and scope.
Bass rolls of at about 12Hz, probably -3dB point at less than 10Hz. No problemo!
starts to clip at about 13V peak ( about 14W into 6 ohm ?)

On a different note, although the output trannies are rated for 8 ohm - it manages to drive my Concertino's (rated 4 ohm) flawlessly.
My less efficient Concerto's are rated 6 ohm, and the amp is good enough to shake my office of 5 x 4 x 3.3 meters, without any noticeable stress.
Why some people need 100's of watts, is beyond me.

[ElliottStudio]

Great! Looks like you don't need as much capacitance as I thought.
As for amp power, it all depends on speaker sensitivity, how large a space and how loud you want it. For most homes 50W is more than adequate with modestly high-sensitivity speakers. But if you have a huge room, low to mid sensitivity speakers and listen to Mahler, you need power! 

Also, the -3dB point you chose works in concert with the cutoff of the output trannies. This creates a filter that removes power robbing ULF from getting to the transformers and helps to keep them from saturation and that my friend is why your bass seems tighter.

I use small value coupling caps from input to driver to do the same thing.

[kentajalli]

Just a last word on this topic.
As good as I have managed to make this amplifier
- only three signal capacitors of high quality
- upgraded powersupply smoothing
- fixed bias for individual tubes
- quality internal wiring
- DC heater supply for input and driver tubes
and it sounds great through my office speakers, but it can not hold a candle to my home amplifier, which is a modified Hartung mono-block OTL amp, based on 6c33c's.
http://hartung-amplification.com/page3.html
The amps are rated at 50W  with four 6c33c's ( they clip at about 80W)
but I use them with just two ( clip at  50W+ )
No comparison!