[kentajalli]

I picked up an aging Edison 12 from ebay, and repairing it, modifying it, has been a little pastime of mine.
I did various mods. to it, as I read posts and articles here and there.
I have finaly settled on this:
pdf :  www.computer-surgery.net/ebay/new_edison.pdf





The circuit is nothing unusual.
My aim was to keep the signal path as clean as I could.
1- Reduce capacitors in signal path ( I am down to three)
2- Keep design to class A or almost! ( could not avoid the pentodes)
3- Get max. clean power out.
4- Keep stress down on output, to increase reliability and long life.
5- Keep noise to a minimum.

Good star-earthing is a must. What the circuit does not show (minor) is the fact that I used clean DC for heaters of input/driver tubes. EL84's use AC.
Twisted wire was used for heater supplies.
I had two leftover 10uF MKP's from an upgrade on crossover circuits of my Sonus Faber (i.e. quality MKP's) which I used to bypass the 230R on input tubes. The two 0.47uF's are Russian K40Y-9 paper-in-oil (about £2 each).
All 2.2uF bypass capacitors in powersupplies are quality polyP's.
Rectifier diodes in powersupplies have snubbing caps - two chokes (picked up from ebay for £15 for both ) are used with plenty of smoothing caps.
Bias is measured across 1R's on cathodes of EL84's , set at nominal 35mA, using individual multi-turn pots for each tube ( matched tubes are not needed), since I had no additional supply for bias, I used a tiny 0.6VA transformer and made the bias section on a tiny board.




Overall, the cost was very low, but it took a few days an hour here and there.
But it was all worth it, amp sounds great through my bookshelf (4 Ohm) Sonus Faber Concertino Domus.
Bandwidth is from about 10Hz to about +40KHz.
Amp clips at about 14W or so, but listening to it, you think it's a power-house, listening to 4th movement of Scheherzade recording on RCA label ( Chicago Symphony orchestra Freitz Reiner conducting ) which is as dynamic as you'll ever get (more than Led Zeppelin) , the amp will not run out of steam.
Noise is limited to a pure 50HZ (i.e. no buzzing) and you only hear it if you put your ear to about an inch of the bass drivers!

[JakeJ]

Cool beans!  Please post more pics and once she has settled in let us know what you think about the overall SQ.  The EL-84 is a mighty little tube from a sonic standpoint.  IIRC, you once had a pair of Manley Mahi Mahi's, how does this compare?  Have you experimented with bypass caps?  The Sonus Fabers are just 88 dB/W/m and you say they don't run out of steam?  That's pretty impressive.

Thanks for sharing.

[FullRangeMan]

I already listen the Concertino, it have great transparency, the xover is very small and smart, the cut is hi 3800Hz if I remember well;
Its a hard load for a small tube amp.

[kentajalli]

IIRC, you once had a pair of Manley Mahi Mahi's, how does this compare?  Have you experimented with bypass caps?  The Sonus Fabers are just 88 dB/W/m and you say they don't run out of steam?  That's pretty impressive.

Thanks for sharing.

I think you mistook me for someone else. What I do have is a pair of Hartung OTL monoblocks ( see http://www.audiocircle.com/index.php?topic=131237.msg1401271#msg1401271 ).
They are based on 6C33C's and they are OTL ! Unfortunately, the EL84's do not hold a candle to these, my reworked Hartungs can resolve sounds that is beyond my office amp. The Hartung's run on four 6C33C's , but I only use them with two (about 40W) which is enough to shake my house with my "Sonus Faber Grand Piano Home". They sound so fast and dynamic, it is scary!
Last night when I heard a hard crack on a Timpani, I could've sworn that it was either a thunder outside, or the one of the amps blew taking a speaker with it!
Thank goodness it was just a timpani. Eventhough the Concertino's supposedly have top quality HF drivers and the grand-pianos are somewhat older, but clarity of treble is miles apart.
When I said the EL84's sound very good and they don't run out of steam, it was all compared to what it was before (and compared to a similar Audio Innovation 200 poweramp I recently worked on).
In money terms, I would have expected to pay twice as much for the modified version compared to original.
The Electrolytic bypass caps are just muddled-sounding, eventhough I had bypassed them previously with PolyP caps. I could not experiment much with a replacement as you need large cap values, so my 10uF MKP's were the only thing available. I initially used 20uF thinking bass might suffer, but 10uF was large enough.
I did experiment with the smaller 0.47uF. I used standard white M-Caps , Wima red polyP, Vishay blue PolyP and expensive (£30 each) M-Cap supreme silver/oils - the finalists were K40y-9 and M-cap supreme - I flogged the supreme's on ebay recently! K40y-9's are the most under-rated caps in the world, they are inside my Hartung's too.
In my office ( 5m X 4m X 3.3m ) with bookshelf Concertino's - the amp has plenty of muscle and enough clean headroom for sudden dynamics.
I never claimed I get 100W ot of a pair EL84's ! 
Have you noticed how low the feedback is on this amp?
Yet it is clean as a whistle.

[kentajalli]

Concertino review:
http://www.absolutesounds.com/pdf/main/press/SF%20Concertino%20HFN%201105%204web.pdf

[JakeJ]

You are right.  I confused you with another member on the Mahi Mahi amps.  That's what happens when you try to post way too late in a very long day.  Apologies.

Thanks for the run down on your cap experiments, very interesting.

[kentajalli]

Pictures:




1- Ready made 6V powersupply, for large 12V fan, at 6V it spins very slowly, thus very quiet!
2- Smoothing Caps.
3- Bias board, with multiturn pots
4- rectifiers with snubbing caps.
5- Small 8H smoothing choke, the large one to the left is the main 2.5H choke.
6- The black one is the only cathode bypass cap, next to K40Y-9 signal cap.
7- small bridge rectifier and smoothing cap for DC heater supply to input tubes only, EL84's run on AC.







There are many components missing, due to the fact that middle tube is taken out of circuit together with its components,
and the amp no longer needs autobias resistors and caps anymore




The front is now a black glass-effect perspex with an added orange LED in the centre, fed from heater supply.

P.S.
I had neglected to take into account the screen current which is about 5mA - I now bias the tubes at a reading of 40mA across the 1R resistors, this gives me a plate current of 35mA which should keep the tubes not too hot, but stay in class A most of the time. balance is about 0.5mA or less, with no drift!

[JoshK]

Very cool, I'm impressed.  I have always loved the EL84 amp.  It can be quite impressive when details are cared for.

Just as a thought, you know about the DC Link caps?  They come from the power switching supply industry as do most innovative tech these days.  They could in moderately high uf at 600V and they aren't too large...oh and they aren't Electrolytics.  Lots of guys on the diy tube audio forums are using them now in the smoothing supplies.   Wima, Vishay and some others make them.  Mouser carries them.

I think the next logical conclusion might be to regulate the screen supply.  Don't know how much of a difference it would make but that seems to be the general consensus.

[kentajalli]

Thanx for your post.
I had considered the large plastic caps for powersupply smoothing, But somehow I chose not to.
They do not have the same ripple current as electrolytics. what they do have, is very good high frequency response (i.e. switch mode rejection), which is no good to me.
Bypassing electrolytics with a few microfarads of plastic capacitance (as I have done), should give you the best of both worlds.
As for the screen supplies, though I have not regulated them, but I have used an 8H choke and 220uF cap to further clean and stiffen the supply for them ( see diagram).
So what current the plates pull they do not have an effect on screen supplies (nor the input section supply).
There is a limit to what can be done to a little amp.
I always believe the art is, to upgrade and improve at realistic costs - both financially and in labour time.
so far I have spent peanuts on this amp (relatively speaking) and made it sound as good as any $1500 amp or more.
It'll never be a Ferrari, I  turned a Corolla into a Lexus! no more. 

[kentajalli]

I found a picture of an unmolested Edison12 for comparison:

[kentajalli]

UPDATE

After a while of listening to the plastic bypass capacitor (the large black ones in the picture), I have decided I don't like them. I have replaced them with a pair of Nichicon Fine Gold electrolytic (100uf 63V), so far I like them better.
Will update in a week or two.