Please help me make a schematic

[eitanwaks]

I have recently acquired four KT 88 and twelve 8sn7 tubes. I have never built an amplifier before and I'm not too strong with electronics. I am on the other hand very good technically.

I would love to build a tube amplifier with these components. I noticed that there is a publication of glass audio that uses of these tubes to build an integrated amplifier. Supposing I can get the schematic how good will it be?

If I decide, as I prefer, to draw the schematic up by myself from scratch where should I begin? If there are good schematics available (meaning, my schematic won't be any better) where can I find one?

Thank you,
Eitan Waks

PS I would like an integrated amplifier.

[jeff mai]

You should start here:

On Line Tube Learning for newbies....

[lineup]

Originally posted by eitanwaks
I have recently acquired four KT 88 and twelve 8sn7 tubes. I have never built an amplifier before and I'm not too strong with electronics. I am on the other hand very good technically.

I would love to build a tube amplifier with these components.

your tubes is a good start and nice reason to get more knowledge
and good practical samples of how to use them

the little I know, KT88 is very much used
so, a search for KT88 in this forum as well as on the web
will probably get you a great number of hits

[Rocky]

Take Jeff's advice, read the recommended readings before getting electrocuted, remember tube circuits very often involve lethal voltages (yeah yeah, lethal currents) and even if you don't get shocked building something, your dog can still get shocked when sniffing on the chassi if you wired something the wrong way, or even worse, someone other than your dog.

The KT88 is a beam pentode, a very powerful tube mostly used wired as triode. The fact that you have four of them lets you choose between single ended and push-pull, and I think I am speaking sense when advicing that push-pull (i.e a differential pair per channel) is your best shot, to get some power and still stay away from superhigh voltages.

Now, the 8SN7 is actually 6SN7 with 8.4V/450mA filaments, a widely used medium mu dual triode (two triodes per bulb). Apart from the filaments, they are the very same tube spec wise, and I can assure you you will find existing designs based on those two tubes. Do a search for 6SN7 and KT88, and ignore the ones with B+ voltages that exceeds your maximum comfort level...

[Tom Bavis]

The Williamson circuit is the first one that comes to mind, of course. A Google search on "Williamson Amplifier" will find many variations. A very nice 20W should be available in triode, much more in Ultralinear.

The 8SN7s can be operated on 12.6VAC by connecting them in series and wiring a diode across each filament, in opposite polarity. They will get about 8.3V RMS by my estimate, close enough to their 8.4V rating. If 6SN7s are to be used later, remove the diodes.

Still... the transformers are the most expensive part of this amp. I think it makes the most sense to pick transformers for the power level desired, then pick the rest of the circuit, rather than just starting with the tubes at hand.

[eitanwaks]

jeff mai,
thank you for the resources. I'm sure I will have many hours of enjoyment covering old sites inside this thread. In addition, I own a copy of Morgan's book.

lineup,
I have been a search on this site for KT88 and for 8sn7. When searching alone I find several hits of interest however, I haven't found a thread that entails details about a schematic using both of these. I get the same results when I search with the Google. This is a reason that I am asking here. Thank you however for your suggestion. I have found many sites and schematics for the KT 88.

Rocky,
your advice for going for a push pull design strengthens my initial decision. Although this is the first valve based amplifier that I am going to build (or hear for that matter) I think that a push pull design is warm to instead of a single ended design although the latter is supposedly a holier grail (from what I've read of till now).

Suppose I find a schematic that fits my needs. How do I change the schematic so that it is good for 8sn7 instead of 6sn7?

Tom Bavis,
I do not plan to use 6sn7's in the future. I didn't quite understand the second paragraph of your reply, could you please elaborate?
You mention that I should choose the Transformers before I continue. Could you please give me a set of guidelines/suggestions that I should follow when choosing the Transformers?

[Rocky]

Originally posted by eitanwaks
I have been a search on this site for KT88 and for 8sn7. When searching alone I find several hits of interest however, I haven't found a thread that entails details about a schematic using both of these.

That is, my friend, because the 8SN7 is not very widely used. The 6SN7 on the other hand, is THE most widely used tube perhaps. Search for 6SN7 and KT88, and leave 8SN7 out of your search. The implementation will be the exact same for the 8SN7 as for the 6SN7 apart from the filaments wich should have 8.4V for the 8SN7 instead of 6.3V for the 6SN7.

Originally posted by eitanwaks
I think that a push pull design is warm to instead of a single ended design although the latter is supposedly a holier grail (from what I've read of till now).

Depends on whom you ask, many people rates differential way over SE, some swear pentodes are the way to go. At your first project, build something that will work for you, and you will learn the direction where your grail may be. Or you will be hunting someone elses grail.

Originally posted by eitanwaks
Suppose I find a schematic that fits my needs. How do I change the schematic so that it is good for 8sn7 instead of 6sn7?

The transformer windings need to be slightly higher voltage, that's all. If the schematic includes a DC regulator, it too should be adjusted so it'll regulate a higher voltage.

Originally posted by eitanwaks
...I do not plan to use 6sn7's in the future...

As you have probably understood, there are lots and lots of 6SN7 variants around and one of the reason so many people uses it is, apart from being a good sounding tube, because it is available in hundreds of variants so one can easily adjust the character of the sound by trying different brands. The trick Tom suggests will let you drop in 6SN7 in the future if you want to. I think that is a pretty darn good advice, as you will have a much wider choice of tubes available to make the sound as you would like it.

edit: typos..