Substituting dual triodes

[Carlp]

I'm VERY new to working with amplifiers, tubes in particular, but I'm keen to try a simple SET design by Fred Nachbaur (http://www.dogstar.dantimax.dk/tubestuf/miniblok.htm). I think I understand the basics and will build it as he designed it. But I'm curious about substituting other dual triodes for the 13EM7/15EA7 he specs (his goal was to design a very cheap SET - $100 or less). Can you give a beginner some thoughts about what to consider when substituting tubes?

[SY]

A beginner shouldn't substitute tubes!

But this could be a good platform for you to get past being a beginner. There are a lot of more-or-less similar dual triodes (and pentode triodes) that can work in this topology, but to get anywhere, you'll have to have the basic tools (generator, scope, power supplies) and crack the books to learn the arts of biasing and determining loadlines and operating points.

That said, and the usual high-voltage warnings, one similar tube I've used in simple circuits is the 6GF7. It seems to work very nicely.

[tubelab.com]

I spent some time playing with a circuit similar to his. I tried many of the vertical output dual triodes. Almost any of them can be made to work, but you will need to play with all of the resistor values to get it to function. This requires some basic knowledge of how tubes work and how to read the plate curves. The 13EM7 is about as cheap as any of them and works as good as any of them. If you haven't tweaked up a tube amp before, it would be best to build the design as intended for your first attempt. At least you know that you are starting with a known good design.

[Tom Bavis]

6EA7 / 6EM7 is the same except for heater voltage, so it should work without any other changes. 13GF7 / 6GF7 is same with a different base - sockets will be hard to find.

[Miles Prower]

Originally posted by Tom Bavis
6EA7 / 6EM7 is the same except for heater voltage, so it should work without any other changes. 13GF7 / 6GF7 is same with a different base - sockets will be hard to find.

Not anymore: Antique Electronics has 9 pin compactron sockets.

[refference]

Hi Miles Prower ,

Sorry to say it , BUT :

6GF7 / 13GF7 - Have 9 pins base , called NOVAR or MAGNOVAL

Compactron is another kind of base with 12 pins .

Regards ,

Carlos

[Miles Prower]

^^^^
That's what they call 'em over there at Antique Electronics: Nine pin compactron sockets.

[tubelab.com]

GE copyrighted the term "Compactron" in the late 1960's. They used it on both 9 and 12 pin tubes. To avoid infringing that copyright RCA called the 9 pin compactrons Novar types, and the 12 pin tubes Duodecar's. Other companies made up their own names. Today they get called by whatever name you can remember (if you are old enough). I worked in a TV shop that repaired mainly Philco and GE TV's, therefore they are all "compactrons".

[Carlp]

Thanks for the replies. I know there's a lot to consider, and I know I don't even know what I need to know at this point. My question came from the idea that I'd build as designed (I'm starting it now), and then try substituting tubes as a practice to force me to educate myself. I work best by having a tangible goal for which to learn, but I can't shell out a ton o' money to try my skills. Swapping tubes seemed like it would get me into some basic tube and circuit theory. BTW, I have a pretty good idea of the dangers with the voltages present, capacitors, etc., so I do intend to be cautious.

Thanks again, and if anyone would like to point out some texts for me, I'd be grateful.

[SY]

Texts? <broken record>"Valve Amplifiers" by Morgan Jones</broken record> is the very best place to start.