• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Sub miniature pentodes - what shall I do with them?

A look at the data sheet suggests something battery powered. A "C" cell for the filament and 2X 006P 9 V. radio batteries in series for the B+ will do the trick.

That type yields only a few mW. of power. I'd think it would have been used in a chest mounted hearing aid. 22.5 V. carbon/zinc batteries were manufactured for exactly that application. Maybe you could use push/pull pairs to drive sensitive headphones.

BTW, read the data sheet carefully. Notice the mention of deflector plates. In fact, the 6418 is a tiny beam power tetrode, not a true, 3 grid, pentode.
 

Hearinspace

Member
Paid Member
2008-06-03 5:18 am
I can't say this is a "great suggestion" but looking at the triode curves there's a spot I might try for a low mu voltage amp. With a (good) constant current source on the plate (Vp=30V) set for 3.6uA you could get a linear 16Vp-p with 2Vp-p in. Good for something or other, . . . . How about direct coupled to something sandy that can add some current output drive. Tube sound in, sand grunt out? The voltages are roughly compatible.
 
Hey all, thanks for the suggestions. I quite like the voltage amp idea (although I'd need to look at how much drive that would be for a sand follower. I've some of Nelson pass's F4 PCBs here and that would indeed do the trick)

It's a bit beyond my knowledge levelto design at the moment, but is using these guys in some kind of mu-follower arrangement reasonable or sensible?

Having had a look at the oatley schematics it seems they typically boost the current drive by buffering after the tubes. I guess that'll be the potential concern for using these guys in a practical circuit...
 
Wow, cute little things. Seems triode-wired is a good way to use them, that data sheet shows curves that look like it'd be fairly linear like that.

The F4 has this resistor at the input, R2, pulling the input to ground with a 47k ohms per channel. But the front end of that amplifier has two JFETs which have extremely high input impedance. You should be able to capacitively couple a simple common-cathode gain stage to the F4 by increasing the value of R2 to ensure you aren't loading down the tube stage.
 
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oooh, maybe not so simple.

The F4 has a maximum output swing (unclipped) of +/-20V according to the specifications. Looking at the triode curves for that tube you can work out the amplification factor (mu) which I estimate at a lowly 7.6. With a mu-follower you'd have an input sensitivity of only +/-2.6V. This would require you to operate the tube with a grid-cathode voltage of at least -2.6V to avoid clipping at the input. And if you want to bias the tube where the curves are linear you'd be getting close to the maximum plate voltage of only 30V. If your speakers are sensitive enough you might not need full swing but that tube looks like you'd have to operate around -1V on the grid and accept a swing of +/- 7.5V on the plate which will barely reach 2W from the F4.... do I have my numbers right ??
 
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Shoog

Member
2002-08-15 10:16 pm
Eire
Personally I would say it isn't worth the effort of trying to get these to work in a HIFI application. From experience, and judging by the kits shown in the second post, these look like really microphonic little buggers.Coupled to that been the fact that they are a one shot wonder (ie not easy to change out) they look totally uninspiring.

Portable guitar preamp might be their best application.

Shoog
 
Shoog - noted!

That said I haven't a guitar. :)

I'm thinking I could parallel a couple, cascode them (with a Mosfet/bipolar), and thereby increase swing/sensitivity. If needs be I can then boost the drive with a jfet/mosfet and then I've a line stage.

Thoughts anyone?

PS I am aware that this is a somewhat nuts 'solution' to a few parts of limited utility!
 

needtubes

Member
2003-05-24 9:23 pm
I build a nice little headphone amp with these wired as triodes. They are microphonic for sure, but things are fine as long as you aren't constantly bumping into the amp. I have not run into any issues with constant audible oscillation, although I have not checked the amp out thoroughly due to lack of measurement equipment. Sounds nice to my ears, though. Still have two PCBs available from my amp build (which I would let go for a small price so as to recover some cost from spending so much at ExpressPCB for my single build...) if you would like to give it a go.

See here for circuit and here for pictures of the completed amp.
 
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Hearinspace

Member
Paid Member
2008-06-03 5:18 am
968's experience is a useful data point. You might experiment with oscillation suppression techniques.(stoppers, ferrite beads. . . .) I remember reading a post by Jeremy Epstein years ago in which he said he found that parasitic oscillation exacerbated microphonics. According to him, find and stop the oscillation and the trouble (with the albeit real mechanical microphonics) will go away. I don't have that particular experience but also don't have any reason to doubt his.
If you go ahead please keep posting as you go. It would be good to learn from your discoveries. Thanks.
 

968driver

Member
2008-05-17 5:37 pm
As I see it, the oscillation of these tubes is a pure mechanical vibration of the different electrodes in the glass tube.
As far as I remember, the most annoying thing was not manifested through the loudspeakers and the music, it was a weak audible whistle, ca. 8 kHz directly from the ringing of the tubes themselves.
After a while, this drives one mad and is unlistenable.
Apart from this, the sound of this line/RIAA phono preamplifier with dual tone controls was really good.
Some of the tubes started "singing" right away after power up, the "good" "silent" ones would start singing when excited by a knock on the table or even by the music playing at moderate room volume. No rubber rings or foam could suppress this behaviour.
I searched for a solution on the internet, but I found none.
 

needtubes

Member
2003-05-24 9:23 pm
Some of the tubes started "singing" right away after power up, the "good" "silent" ones would start singing when excited by a knock on the table or even by the music playing at moderate room volume. No rubber rings or foam could suppress this behaviour.

I hear the whistling you describe when I tap the tube or (less frequently) when I tap the amp, but in all cases it fades within a minute or less. My tubes appear to have no self-sustaining oscillation in the audio band.
 

Hearinspace

Member
Paid Member
2008-06-03 5:18 am
I don't know a lot about these things but I think the mechanical resonance needs to be driven by something or other. Jeremy's report was that oscillation is a likely cause and needs to be checked out. I would also question AC heating.
On the other hand, if it IS an old hearing aid tube as Eli mused, it might explain why we often used to hear those old folks trying to stop a squeal with the volume control. . . . . and all these years we thought it was feedback.:D