considering octal fatness build with 6sj7 pentode preamp

[xtian]

I my pursuit of low-wattage studio builds, I'm gravitating toward the Octal Fatness schematic (similar to Gibson GA-5) with 6SJ7 pentode followed by 12ax7 in the pre, and octal tube in the power.

Important consideration: I've read that it's difficult to find a good 6SJ7 tubes--you might have to sort thru quite a few until you find a good one. I don't have any on hand. Can I count on getting lucky if I order one?

Mark, if you were to build a low-wattage, single ended octal amp, which would it be? As you know, this would be my second build, and I'd want to start with a tested design.

What is required when you swap, say, a 6V6 for a KT66? Looks like some schematics have a 1K pot and some have a switch for changing the resistance of cathode to ground.

[Phil_S]

Others will caution against it, but see if you can buy some inexpensive used 6SJ7 on eBay. Good ones can be had and you might get a few bad ones, too. So what? I heard the new production is just as hit and miss.

Look at the Gibson GA-20. Great circuit. 6SJ7>6SL7> 2x 6V6. You won't be disappointed. Just build one channel. Like this: http://home.comcast.net/~psymonds/GA20.htm

[Cliff Schecht]

I've got a whole bag full of good ones.. Having a tube tester on hand helps a lot but every single one of them tested at full or near full transconductance and the brands/styles vary all over the map. I think I even have one or two glass bottle versions, although I could be mistaken there.

[tonewood]

Are these available to Xtain?

Also, I don't think the tube tester can check for the noise that is often complained of. Anyone know?

thanks

[xtian]

Now I'm considering sticking with 12ax7 type preamp, and a single octal design, single-ended. Anyone recommend a good design?

[Phil_S]

If you want to get off the beaten path a bit, use one triode at either end of this tone stack: http://www.freewebs.com/balijukka/roybeanstack.htm and your choice of octal power tube. This is a very versatile tone stack. Twiddle the knobs enough and you'll find just about anything you want. This is based on personal experience.

[xtian]

This is a very versatile tone stack.

Nice tip, Phil. Thanks!

I don't understand the green elements in his drawing. On the right, "TPv1" must be the output signal. But on the left, is "Vs1" a volume pot?

[Structo]

I hadn't seen that stack before nor that website, thanks Phil.

[Structo]

This is a very versatile tone stack.

Nice tip, Phil. Thanks!

I don't understand the green elements in his drawing. On the right, "TPv1" must be the output signal. But on the left, is "Vs1" a volume pot?

I think that is a Spice simulation.
Vs1 would be the signal source.

[Phil_S]

Oh! I'm very glad people seem to like this. You never know what will fly where.

Yes, Vs1 is the voltage source. That would be the plate or cathode follower of the first triode in a 12AX7 or whatever you decide to use. Tp1 is just a voltage test point, for taking a reading. You run the side on the right into the grid of the next triode. It is made for a back end load of about 330K ohms. This is how I did it: http://home.comcast.net/~psymonds/Maxie-tone.htm the 500K pot and the 1M grid leak are parallel, so that's 333K.

I don't have the technical knowhow to modify this circuit for another load. I've had exchanges with the inventor over at AX84.com and he seems to really have his arms around how things work. Like I said, if you want something different, it's worth building this. I don't think you'll be disappointed. It's been built and it works well. A really nice feature is that it is a lossless tone stack.

So, you can just hack off the concertina PI and the power section. Keep everything before C10. Change C10 to a larger value? Replace the power section with your basic SE cathode biased octal tube. I think that would be a great amp. Its on my list of things to do. I've got an old coke bottle 6L6-G waiting to go. So many projects. So little time.

[Andy Le Blanc]

that looks similar to a wein bridge, single freq, band cut boost.

don't give up on the pentode pre., you can dress an octal socket, a 9-pin,
and 7-pin together to run the same generic circuit and run anything you want.
One at a time.... Any tube type you run you should test run a good handful of
each and listen for what suits your ear. Pentodes have a lot gain,
so you can set up a gain structure following with all the "other" tube types,
you don't have to suffer 12ax7's mashing the tone of your instrument.

I've seen some old SE student amps that where a pentode stage with vol. and tone,
is driving the power tube, and that's it

[brewdude]

I have the Judge Roy Bean stack in my heavily modified Valve Jr. It is very versatile for a two knob tone stack. It seems to be very interactive though, and somewhat less intuitive to me. Small adjustments can shape the tone quite a bit. I wired it up point to point on the pots. It is definitely worth trying.

[Andy Le Blanc]

Hold up, that's the same old thing you see in a old premier, big muff, with an old one knob
tone stuck to one leg. It tilts on a center freq. determined by RC.
differing values (non-symmetrical) will give you a boost or notch around the center freq..

[Cliff Schecht]

I've seen some old SE student amps that where a pentode stage with vol. and tone,
is driving the power tube, and that's it

My Silvertone 1331 is exactly this, a 6SJ7 driving a 6V6. Not very loud but has a decent range of tones and some bluesy rattles (think Hound Dog Taylor ).

[Phil_S]

I have the Judge Roy Bean stack in my heavily modified Valve Jr. It is very versatile for a two knob tone stack. It seems to be very interactive though, and somewhat less intuitive to me. Small adjustments can shape the tone quite a bit. I wired it up point to point on the pots. It is definitely worth trying.

Yeah, I built it for someone else, so I didn't have much time with it. I also found it to be not very intuitive. I rather liked the interactive nature of the two pots. Every turn gave a pleasant surprise.