Quickie phonostage Idea, will it work?

[Paul Joppa]

Metal octal tubes had been made for a long time by 1959-maybe very early forties?

[Doc B.]

I think the potential issue is that nuvistors were not created with the idea of low current draw for use with batteries. I haven't checked specs, but I think they may draw more current than "battery specific" tubes.

[Noskipallwd]

Here's a link to a data sheet if anyone is interested, http://frank.yueksel.org/other/RCA/RCA_Nuvistor_Tubes.pdf , nope, doesn't look like they are suitable for this purpose. Oh, almost forgot, Paul there is a portable headphone amp kit based on the Raytheon tubes available, I remember reading some reviews, seems the tube is a little too microphonic for a portable device but the opinions on the sound of the tube were favorable. All heresay of course, never heard it myself.

Cheers,
Shawn

[Dr. Toobz]

Those little "Christmas tree light" tubes are unbelievably microphonic. I have a bunch in my junkbox and have played around with them on a breadboard to learn about how tubes work, try various operating points, etc. The filament only draws something like 10mA and is the thickness of a hair, and if even so much as wind goes by the tube, it rings like a bell. The ringing lasts for at least 30 seconds. I was going to use these in a Quickie to make a headphone amp for my guitar (using a few of the little tubes for gain and tone duties, then taking the output to the pair of 3S4 tubes wired in push-pull), but never found a good way to damp them. In between the ringing, they do sound good, however. I've only wired them as triodes.

[Paul Joppa]

Bummer about the microphonics - that's a killer for a phono preamp! I'll keep looking though, there may be a better candidate. If they made hearing aids there must be a mic preamp tube. There is the 1U5 in regular 7-pin form which was intended for mic preamps, but it's drawing 50mA heater and we'd need two per channel, minimum.

That 6418 is a beam power tube, capable of a full 0.002 watts output !   :^)  I just read the data sheet.

[Dr. Toobz]

Is there anything in the battery tube (7-pin) world that even comes close to a normal hi-mu triode like a 12AT7 or 12AX7 when used in pentode configuration? The problem I've found with the battery tubes is that they all seem to have rather low amplification factors. This appears to preclude their use in things like phono or guitar amps without using a bunch of stages....

[Paul Joppa]

Yes, that's the problem. Tiny cathodes need little heater power but can't muster very many electrons. So you need either very high impedance loads, or you settle for low gain.

Example: 1LH4, mu is 65 but rp is 240K. That's nearly 100 times higher than the 6922 used in Seduction!

[cpaul]

Steve Bench has done a bunch of work with the pencil tubes (including a phono stage using 6088 tubes) using battery power.  Check this out:  http://www.jacmusic.com/techcorner/SBENCH-PAGES/sbench/6088pre2.html

He's no slouch, either.  Check his no R no C amps.  Fun!

[Doc B.]

Steve is a very smart and very nice guy. I "licensed" a hum cancellation circuit from him years ago, for our long since retired Parabee amp (I think I traded a kit to him). It worked for me, but for some reason it didn't work too well in the field. So I would simply suggest that one might do their own due diligence when trying his very intriguing ideas. They are often tantalizingly off the wall. Could very well be that we had some other issues in our designs that didn't allow his circuit to work. I think that he is one of the most cutting edge guys out there.