6BX7 or 12B4A for small SE amp?

[G]

I have been saving money for the parts to build a SE 12B4A amplifier but have recently become aware of the 6BX7. Which tube would you use for a low power SE amp?

[zigzagflux]

I considered using the 6BX7 as a driver for my IT PP300B amp. Ballsy tube, very low Rp yet a decent mu; I would think it should work well. Plus, AES has a sale going on right now, 25% off !!

I steered away from it due to the response from others and Morgan Jones; difficult to find reasonable matched triodes in the same glass. Jones also didn't seem to be too keen on the distortion characteristics, IIRC. I'd have to look it up.

For your SE application, I suppose balance wouldn't be too much of a concern.

[Michael Koster]

G,

I have built a 6BX7 SE amp. The 6BX7 has some things going for it.

The spec allows up to 12 watts dissipation total and 10 watts for any one anode (leaving 2 watts for the other). This means that you can use one section for the driver and one section for the output. You can get over 2 watts into a 4K-5K primary with a spud amp like this. The gain is a little low but 2V will drive it. This is what I built and it sounds good, with a definite sweetness to the sound. The f2 on this tube/OP comes in at relatively low power levels.

I run the driver at 120V/15mA and the output at 280V a-k and 37mA. Self bias using 540R results in about 20V so B+ is a convenient 300V. Sounds OK down below 250V B+.

You can also parallel the 2 sides and use a lower impedance transformer, in the 2K-3K range which would get more power output but need a separate driver.

Neither of these approaches requires matched sections.

Cheers,

Michael

[Michael Koster]

Here's the basic idea. You could improve the input sensitivity by adding a cathode bypass cap to S1.

Michael

[Boris_The_Blade]

Originally posted by Michael Koster
You can also parallel the 2 sides and use a lower impedance transformer, in the 2K-3K range which would get more power output but need a separate driver.

I like this idea....seems like your getting more bang for the buck and using the BX7 more efficiently. If you feel like trying something different, you could do a little A2 with it with a follower (tube or FET). A little more headroom never hurt anybody.

[Michael Koster]

Yeah, watts out/watts in is not great in this design but it could be better in the parallel version. With A2 drive and all it could get into 2a3 space...

[plovati]

I'm currently using this ;

http://www.audiofaidate.it/forum/upl...uno_schema.JPG

Cheap project, output transformer very simple to build, pleasant sound. 1W power out over 8 ohm load, good volume from some fair efficient speaker. Distortion is not a problem.

The 6BX7 section's coupling is not so troublesome, over 4-5 NOS tubs one can find 2 with gain of the two triodes within 5%.

[andyjevans]

I've built PP amps with 12B4, 6BX7, 6S4 and 6EW7. Hard to say which had the best sound - they're all good. I guess the 12B4 was probably a nose ahead. But they're notoriously hard to match - worse than the 6BX7 for variance. Plus you get a lot of shagged out computer ones if you buy used.

Try the dissimilar triodes - 6EW7 and 6DN7 are very good.