There's no DIY forum here?! So I'll post this here: Space charge vacuum tube phono preamps, anyone?

mhardy6647

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I have this perverse interest in space charge tubes.
These were vacuum tubes designed for automobile radio use near the very end of the age, when good (it's all relative!) AF power transistors existed, but not much for high frequency use. The use of low voltage tubes permitted the elimination of vibrator HV power supplies (get all y'all's minds outta the gutter...).

Scan_Pic0034 by Mark Hardy, on Flickr

They were tubes that only required 12-ish volts on the plates, so they could run off a low-voltage storage battery/in an auto.
These were (are) a weird, wussy lot of vacuum tubes.
I should mention that it's also completely possible to operate some normal tubes at very low plate voltages -- folks do this with things like 12AU7s all of the time sometimes occasionally well, it's happened. ;)

I was a (very) early adopter of Pete Millett's original hybrid low voltage vacuum tube/solid state headphone amp/preamp design, because it looked like fun (it was/is).

I've built a couple of them, and I even have parts to build one more. :)

realhifiglows by Mark Hardy, on Flickr

013a by Mark Hardy, on Flickr

But, here's the thing. I've been thinking about space charge tubes again -- trying to think of an interesting niche for a fun, weird, easy and safe ;) DIY project for a lazy under-achiever (that'd be me). After poring over datasheets, I got to thinking "I wonder if there are any space tube triodes with enough oompf [amplification, that is] to concoct a phono stage with 'em?"

I really haven't seen much evidence of any DIY designs. I did find this "commercial" (well, kinda) item, which would be OK even though it's - not surprisingly - a hybrid:


I know there's some bright folks here -- anyone feel like dippin' in the shallow end of the pool, vacuum-tube-metaphorically speaking :confused: :smoke and speculating, or sharing, ideas or designs for some sort of simple/practical* vacuum tube or hybrid space charge phonostage?

_______________
* Notice I didn't say it had to be good. :)
 
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paulbottlehead

Active Member
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I know there's some bright folks here -- anyone feel like dippin' in the shallow end of the pool, vacuum-tube-metaphorically speaking :confused: :smoke and speculating, or sharing, ideas or designs for some sort of simple/practical* vacuum tube or hybrid space charge phonostage?
The challenge with the phono stage will be both noise and gain. As you observed, that "tube" phono stage you found has some op amps hanging off the end to deal with the issue. This is something we have toyed around with and when you start with the basic needs of a phono stage, then fit battery tubes to those requirements, it gets complicated in a hurry and the tube count goes way up, as does the probability of noisy operation.
 
The challenge with the phono stage will be both noise and gain. As you observed, that "tube" phono stage you found has some op amps hanging off the end to deal with the issue. This is something we have toyed around with and when you start with the basic needs of a phono stage, then fit battery tubes to those requirements, it gets complicated in a hurry and the tube count goes way up, as does the probability of noisy operation.
I would assume similar issues would apply to headphone amps? Not so much the gain, but the issue of noise?
 
By the way, @mhardy6647 - I appreciate your thoughts on utilizing misfit tubes- there’s so many oddball tubes left behind in the bins, that finding some element of New England thriftiness, ingenuity and resourcefulness in the possible practicality of them is appreciated.

I’m interested in those headphone amps you built- what were your end results? Satisfactory? Shinola, or….
 

paulbottlehead

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I would assume similar issues would apply to headphone amps? Not so much the gain, but the issue of noise?
Noise requirements for a headphone amp aren't as bad as for a phono preamp, but the HP amp will need to run a fair amount of current and use some output transformers. You could make a 6GM8 do this, but it's not a particularly linear tube and it may not compare favorably to something running higher voltages.
 

Thermionics

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It's not space-charge, but I've had fantastic luck with the John Broskie Aikido LV running 6BM8 tubes at 24VDC. Quiet as the grave, extremely musical, and excellent gain. I'm using it to drive my office system (when I'm actually physically in the office) consisting of Pass ACA amps driving Linaeum LFX monitors + Dayton Audio Titan 2 10" sub:

1635782189511.jpeg

(that's a Hypnotoad phono stage above the LV board, BTW)

-D
 
In keeping with the spice charged space charge tube discussion, The Audio Round Table has this schematic titled "Ecstacy Space Charge" which is an amp using a 12AU7 driving a pair of space charge 12K5 tubes for a (very) low power stereo amp. The power supply would be a 12 volt wall wart.

The 12K5 used to be a very cheap tube. I could fund my retirement for a number of years selling my stash of these tubes at the prices the Bangy Bang folks are asking on eBay.

 

mhardy6647

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By the way, @mhardy6647 - I appreciate your thoughts on utilizing misfit tubes- there’s so many oddball tubes left behind in the bins, that finding some element of New England thriftiness, ingenuity and resourcefulness in the possible practicality of them is appreciated.

I’m interested in those headphone amps you built- what were your end results? Satisfactory? Shinola, or….
Sounds good to me. I know that's not a very audiophilic response, but that's what I got. ;)
I used one as an active preamp for a year or two BITD and thought it sounded nice. It did add a little hum, though (and oddly enough) -- I've never bothered to suss out why, though.
 

mhardy6647

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That is post of the year material, right there. Now, we just need a post of the year contest! :)
do I hear you volunteering to be a moderator? :confused: :rolleyes:

Back on topic (and I don't even really know what that means in the context of a thread that I actually started!): A DIY forum would be nice.
To moderate... well... I am, unfortunately, pretty immoderate. Plus, fall is teachin' time; I am pretty distracted until December... but maybe after the first of the year, we could revisit this -- unless, of course, someone else is bracing at the bit!
 

paulbottlehead

Active Member
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In keeping with the spice charged space charge tube discussion, The Audio Round Table has this schematic titled "Ecstacy Space Charge" which is an amp using a 12AU7 driving a pair of space charge 12K5 tubes for a (very) low power stereo amp. The power supply would be a 12 volt wall wart.
A 12AU7 on such voltages will draw some grid current, so you'd want a 100 ohm pot at the input and a very, very stiff source to drive it (like a headphone amp or a small power amp).
 
Adding to the list of challenges, space charge tubes can create a lot of heat and consume considerably more power than their higher voltage counterparts.

I’ve used the 12dw8 several times with fantastic results and one of the simplest ways to tame some of their undesirable characteristics is to lower the heater voltage thus lowering Mu, noise, power consumption and heat. In most instances, this leaves a considerable amount of usable gain making an all tube space charge design somewhat possible.

If you're looking for something simple and practical a hybrid is going to make the most sense. In fact, space charge is best suited for simple low gain designs and you can make some really great sounding stuff with considerably less effort and parts.IMG_0925.jpgIMG_0928.jpg
 
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