Preamp tube on +/- 15V

Anyone have experience running a 12A*7 tube on +/- 15V? Seems easy enough to try it out. Float the tube between the rails and use a some blocking caps in/out. But would the noise performance at that voltage level be passable for pro audio line level?

Obviously this would be for a tube addition to general op amp circuits. Seems like a fun thing to play around with if noise can be low enough.

Here is a good paper on using the typical triodes at low voltages. Merlin's verdict also confirms my experience, the 12AU7 definitely works best.  Noise isn't the issue, linearity is in case you're not aiming at building a distortion effect. 30V of course gives much more to work with than the 12V he proposes. I've only ever built guitar pedals using this approach, if an effect is what you're after, here is a good starting point with a couple of circuits. Maybe Merlin will chime in as well  .

Seems easy enough to try it out.

Lost wisdom of the Ancients: you want B+ higher than Mu or the bias gets very fussy/unreliable. This is actually saying there is 1V uncertainty of grid potential. TungSol did some work to control that. But no need to hunt down obscure types. You do not *want* a lot of voltage gain in a low-volt circuit.

Adding to my first post, as Merlin mentions there is more variation between different tubes than between the two triode sections. Some tubes simply don't work properly at low voltages, you really have to try different ones, different brands.

In general, I'd much rather add a simple DC-DC converter than deal with the issues that the low voltages bring with them. A humble 34063 can easily do a couple milliamps at 150-200V. Or you could use a more modern switching IC like the LT8304 or LT8331 (circuit examples in the datasheets).

What are you aiming for? Clean/linear as possible with low voltage? An effect? Just experimenting? Are you married to using 12A*7 types? Or is that just what you have around? There are several routes you could go. I seem to recall John Broskie writing an article on low voltage hybrid circuits with opamps and common preamp tubes. I have that somewhere and will hunt it down. Blencowe has the writeup volker pointed to. Pete Millett has an interesting article on a headphone amp project he did using space-charge triodes/dual diode combo tubes that were used in 6 - 12 volt car radios, mostly in hybrid circuits just before all-transistor radios became practical. Rare tubes that are hard to find, so probably not something you just duplicate, but there may be some circuit elements that are helpful. Article here: http://www.pmillett.com/file_downloads/ax_hybrid.pdf

Not knowing your intent, if you're open to other low-voltage directions, there are a number of sub-miniature tubes that were made for battery operated amps and hearing aids. I hunted down some 5905 low voltage pentodes a while back to experiment with that run on about 24 volts. Haven't gotten around to playing with those yet. Then there's that VFD device Korg sells called the Nutube. Bought one of those from Pete M and still intend to see if I can make a workable Preamp with one of those plus a couple of 5905s.

And there are always step-up voltage approaches you can use with wall wart 12 volt power, like volker mentioned. Electro-Harmonix had a 12AY7 mic pre in a big pedal box that ran on 12 volts i think. It used a 125:12 volt toroid wired backwards internally with a voltage doubler to get 250 volts B+. Based on what PRR said, incorporating something along those lines might give better results.

Lots of ways to approach it. But I guess it all comes back to the original question: what are you wanting to do?

BT

Thanks for all the info! To be honest I'm not sure what the specific application would be yet. I started thinking about all the fun stuff you could do with a standard 12A*7 tube in solid state line amps, EQ's or compressors. I wasn't thinking mic preamp. More for saturation of line level stuff, soft clipping, that sort of stuff. I've made many hybrid circuits with dedicated power for op amps heaters and tube B+. But I thought it would be cool to just have a hefty +/- 15 to 18 volt supply and be able to get away with a 12AX7 somewhere in the circuit without voltage multiplying. You can get an amp out of a LM317 so even the heater is doable as long as your circuit isn't too crazy, Or you could use a LM350 with a big heat sink. 5watt 3 .1V  zener to the heater...

The reason I asked about noise is I'm messing with a varimu, 6BC8 100V B+. typical UA175 deal. I wanted not to have an input attenuator. Just a straight line in. In order to not overload the tube at 100V I had to stay under +15dbu input signal with a 4:1 step down transformer. So with such a step down at signals around +4dbu the background tube hiss is audible. Yes I could give the tube more gain by adjusting plate and cathode resistors but more gain = less headroom.

So at a certain point I felt I had to compromise between headroom and noise with 100V B+. Which in turn had me thinking about even lower voltage on a completely different project.

Thanks again for all the links and info... I'll go ahead and "try it out" ;D

Some of those little switching ICs are really nice. Could be very handy for stand a lone tube mic pres. Pity they all seem to be in SMT packages.

Cheers

Ian

"More for saturation of line level stuff, soft clipping, that sort of stuff."

N-JFets are the better low voltage tubes!

Vox Valvetronix [Korg designed] and several other Korg applications of the same circuit are doing this exact thing running +-12V rails on a 12AX7 generating mild distortion. It kind of works. It does saturate like expected - tube like characteristics and all. With guitar and rhodes sounds like a good stomp box overdrive.

Marketing is of course claiming silly things like power amp-like distortion and sag etc. impossible things.

Further - UI designers slapped on a LED under the tube because tubes are supposed to glow apparently.

rock soderstrom said:

"More for saturation of line level stuff, soft clipping, that sort of stuff."

N-JFets are the better low voltage tubes!

Click to expand...

I'm sure! but I'm addicted to the glass bottle...Hey that reminds me, the solid state tube we were talking about in the brewery:
https://groupdiy.com/index.php?topic=67023.0

Wonder if that would actually function at low voltages like a real tube. They definitely have the LED going on!

I have a passive attenuator feeding a 6201 gain stage....I like it....ok Love it.......Quiet as a mouse.....

It's only 1 stage however so it's inverted on the way out.....

Got it from Ebay from a guy who modded an older Yamaha monitor controller/selector....

It uses an HP printer wall wart.. it's 16/32vdc/////

ECC86, EF97 and EF98 (6GM8, 6ES6 and 6ET6) are probably the only widely-available tubes that were specifically designed to work at transistor voltages (doing the VHF work in early transistor car radios).

The important question is if you like what tubes do at low voltages.

Jakob E.

Good to know scott, do you mean it can take a range of input voltages when you say "16/32///vdc"? If thats so it probably has a DC/DC converter in there. Or do you know the plate just runs on the 32vdc?

gyraf said:

ECC86, EF97 and EF98 (6GM8, 6ES6 and 6ET6) are probably the only widely-available tubes that were specifically designed to work at transistor voltages

Click to expand...

Thanks Jakob!

gyraf said:

The important question is if you like what tubes do at low voltages.

Click to expand...

Only one way to find out...

bluebird said:

Good to know scott, do you mean it can take a range of input voltages when you say "16/32///vdc"? If thats so it probably has a DC/DC converter in there. Or do you know the plate just runs on the 32vdc?

Thanks Jakob!

Only one way to find out...

Click to expand...

I'm not sure what voltages it's using. The wall wart is an ac to dc regulated converter supplying 16vdc and 32vdc and 0. All three  are wired into the tube and I'm not familiar enough with circuits to know how he's supplying the load.... It looks like the 16vdc is going to the heater and the 32vdc is split after a 270ohm resistor then goes to  a couple of 5.6kohm resistors before each plate.........I don't know how to draw a schematic but could tell you what goes where in this thing if you wanted.. and poke around for voltages if you want..... there's some .22uf caps at the plates too........etc.... quite a few things in there....

I really like the sound , it's range of usefulness is nice and the inverted /unbalanced output doesn't bother me because I have some transformers after it in a compressor I use that invert it back.....

Let us know where you go with this, Bluebird. This low voltage thermionic stuff is always interesting to me.

Thanks,

BT