Choosing Valve

[kmj]

Hi, I happend to stumble over an old add some weeks ago and due to this I'm about to pick up a pair of used outputtransformers. The transformers are from an "Andersson 60" which seems to be a slightly more exclusive version of the "edison 60". Their primary impedance is about 9,6kohms consisting of four windings at 600ohms each.

At first I thought that I'd look up a schematic and spend a year or so salvaging parts but why miss the opportunity of trying to design something yourself, how hard could it be? There are a lot of good books and articles on the subject so with some patience it should work.

Anyway, along with the opts I get some 15pc of EL34s. 4 new and 11 old which could be usable for testing . But just because I get them along with the opts there isn't any reason why I should use them other than that it's a cheaper solution..
But now to the point of this post, I want to give designing a go but to do that I need to pick an outputtube.

I thought of a pair of triodestrapped el34, kt66, kt88 or el84s in PP configuration but perhaps you guys have some other recommendation? And if not someone is bound to have some opinion on the above mentioned tubes. So if you discard the difference in price, which tube is the "best" choice? I have a pair of fostex168ez-sigma based Herukahorns with a sencitivity of 95dB.

So, what do you think?

[gingertube]

For info the Edison 60 was originally designed by Erik Andersson. So much the same amp as you suggest.

95dB/W/m is good speaker sensitivity and any 10 to 25 Watt Tube Amp will drive them adequately.

My advice from amps I've built is either Ultralinear connected EL84 (The 4 x primary taps allow you to use 50% Ultralinear easily) or Ultralinear or Triode strapped EL34.

For simplicity I would lean toward the Ultralinear EL84. This will give you 10W per channel. EL84s in Triode Mode just don't give enough power in my view (5W per channel)

The problem with Triode Mode EL34 is driving the large Miller capacitance of the EL34s - It is quite difficult to drive this 200 to 250pF of capacitance adequately and requires a high current diff stage driver or cathode followers, that is a 3 stage design. One exception is the Allen Wright PPC1A design which you can find on this forum by a simple search. It is a 2 stage design using high current diffamp/splitter and it gets around the usual "Not enough gain" problem of a 2 stage design by using cascodes for each half of the diffamp. You could use the same design with the EL34s in Ultralinear Mode. EL34 Triode Mode will give you 15 to 20W per channel and Ultralinear mode will give 30 to 35 W per channel.


Ultralinear mode EL84 are a breeze to drive and you can get away with a 2 stage amp and single dual device (ECC803S, 12AX7, 12AT7 etc.) for the front end.

Summary:
If simplicity (but still capable of stunning sound) is your bag then Ultralinear EL84 using the Morgan Jones "Bervois Valley" schematic is the go. As an alternative design, I will be posting a "Jack" (the giant killer) EL84 Ultralinear design in the next day or so.
If you want a bit more "wick" then Triode Mode or Ultralinear EL34 using the PPC1A circuit is the go.

Hope this is "food for thought". There are many other options and if you ask someone else they may well have different ideas to mine.

Cheers,
Ian

[kmj]

Oh, thanks for the fast reply. I have one of Morgan Jones books (among others) so i'll check it out.
I'm still open for other options ofcourse

I havn't even begun to think of a specific stage or part of it since i'm trying to avoid looking at specific designs. Since I've mostly built kits up to now I thought that I atleast could spend a month or two with datasheets and see what I can come up with. If that doesn't work someone else has already built an amp way better than what I can produce anyway and I havn't any problems with "borrowing" idéas

I know that the opt is made for kt66 and el34 but that's no reason to give up on other solutions.

[Fuling]

If those OPT´s are similar to the ones in Edison 60 they have an airgap and can be used in SE circuits. I have a pair that has been around in many different amps by now...
Since the primary has four identical sections you can get cathode feedback too, not a bad thing at all!

[kmj]

If those OPT´s are similar to the ones in Edison 60 they have an airgap and can be used in SE circuits. I have a pair that has been around in many different amps by now...
Since the primary has four identical sections you can get cathode feedback too, not a bad thing at all!

Oh, even more options, poor me

Edit:
Fuling, whats your opinion of the opts? Erik Andersson didn't want to give me more than some specs. Which is understandable

[Fuling]

Fuling, whats your opinion of the opts

They´re not bad at all, works good up to 10-15W in SE.
I´ve used mine with EL34, EL90, 811, PL519 and even PCC88

[kmj]

PL519, thats the valve that Rosenblit is so fond of...
Hm, 10-15W single-end, that would be something. It seems that choosing what to do wont be that easy after all.

So, keep suggestions coming

[kmj]

anyone else who want to have a go?

[Fuling]

PL519 SE with screen grid drive is one of my new favourites