Any love for the Mazda 6/30L2?

[Mick Bailey]

I have a half-dozen NOS boxed 6/30L2 dual triodes. They appear to be quite unpopular, but has anyone found a good circuit to use them in?

[Justin Thomas]

Seems they might be closest to a 12AU7, which may explain their unpopularity... seems they may be more of a "function" tube than an amplification tube, and why bother with a strange type, when an AU7 can be used without rewiring heater supply? No reason not to try them as PI tubes, or as tiny PP outputs... Maybe a Super-Twin style reverb...

Justin

[J M Fahey]

"Unpopular"?
ALL tubes are obsolete and unpopular, except for a very very very small niche of users

You can very well use these in any project, including a guitar preamp, they will just not "drop in" a Fender Twin 12AX7 socket and work the same ... specially considering filament wiring is different

Try to get its datasheets, or those of the supposed equivalents (ECC804?) and post them there.

That said, Musicians often plug horribly mismatched 12AU7 in 12AX7 sockets to "tame amps" and such, and he World hasn´t split in two (yet).

I always buy stashes of obscure tubes for peanuts , design and use them successfully.

Such as 1000 ECC189 (similar "TV" tube) for $250 (so 25 cents each), and made great distortion pedals, tube buffers , hybrid amp drivers and such.

IF you do not get the datasheet, do it the practical way:
1) get filaments and +150V (many TV tubes have max rated 160V plate voltage)
2) add a 47k plate resistor
3) try different cathode resistors: 820r/1k/1k5/3k2/3k3/4k7/6k8/8k2/10k to find which gives you about 90/100V on plate
In fact I have built such a test box, with said switchable cathode resistors, plus switchable 22k/47k/100k/220k plate resistors, to cover all bases.
Bypass said cathode with a large cap, say 25uF, just to put it out of the picture
4) inject audio at the grid, measure voltage out, divideb them to get audio gain .... enjoy.
So far I have never found a "useless" tube.
Usually gain won´t be as high as with 12AX7 (around 50/60X) but a more modest 10X to 40X .... still pretty usable.

I have a friend in Brazil who uses nothing but TV tubes, and gets impressive results.

EDIT: http://www.r-type.org/pdfs/6-30l2.pdf

[Justin Thomas]

One day, I would like to build a super gain amp using triodes with u of like, 6... Just daisy-chain about 20 together. Damn the efficiency, damn the noise... let's just have some fun again!

Also note, the 6V6 is listed as a (I think) "vertical deflection amplifier," which has nothing to do with audio, I think... that tells me I can use a VDA tube backwards...

Justin

[Leo_Gnardo]

Also note, the 6V6 is listed as a (I think) "vertical deflection amplifier," which has nothing to do with audio, I think... that tells me I can use a VDA tube backwards...

There are some hi fi home builders that like to repurpose these "sweep" tubes, AND in pro amps no less than McIntosh used a raft of them (8 x 6LQ6) in their classic MC3500/MI-350 mono power amp. A dozen of these powered the original Woodstock festival. You may be onto something...

What could be done with those kool old Mazda's, who knows? Yes look at what Juan's friend has done with his odds n ends. Crikeys I got boxes full of all kinds of orphan tubes. If I had tons of time on my hands and a market for my creations, could turn out some oddball amps.

[Jazz P Bass]

What's up with pin #9?
It appears to be a screen between the two sections.

[Enzo]

When I emptied my shop I pitched boxes of odd tubes. I always thought something interesting might be done with a pentagrid converter tube, maybe a trem interface or something.

[Mick Bailey]

There are still plenty of NOS TV tubes around and they make good hi-fi amp projects. Sometimes the heater voltage is higher at 15v but they're a fraction of the cost of 6.3v types. Compare, say, a NOS Mullard PL84 against an EL84. Interestingly, B&O did a little stereo SE hi-fi amp called the Grand Prix that used the PL84.

I got 30 NOS boxed Mullard PL84 for £5 about 4 years ago. Also a few NOS PCL86 thrown in, otherwise they would have been thrown out.

EDIT: The Mazdas have the price on the carton. At today's price (according to the Bank of England inflation calculator) they come to £19.22, or $25. That's pretty hefty considering tubes were a commodity item back then.

[Chuck H]

A half dozen odd tubes probably doesn't merit a special build just to purpose them. I have a small tub of 12au7's pulled from an old Conn organ. Most are good-ish. I've considered building an all au7 amp. Three preamp triodes, a concertina PI and two power tubes for about three watts. Or something like that. But I probably have enough au7's to keep such an amp operating for the remainder of my life. Even if I don't, more wouldn't be hard to find or at least there wouldn't be any socket modification to change to different 12**7 types.

Maybe a project at half that count. Like two triodes for the preamp and a single 6/30l2 in self split for the power amp. It wouldn't be a gain monster but it might sound good anyway and you have enough bottles to tube it three times.

[Mick Bailey]

I have a little self-split 12AX7+12AU7 amp that uses a simple SMPS to give about 270v from a 12v laptop supply. That thing plugged into a decent cab is insane. More gain than enough and some reasonable clean tones. Loud enough to annoy my wife while she's watching TV, too.

[J M Fahey]

Hey, we deserve something about it
Some sound sample, YT video, pictures, schematic, whatever

[Chuck H]

You could mod that amp to use the 6/30l2 for as long as you still have them I suppose. Not sure how the filaments are wired in your amp, But if it's do able without changing a transformer it would probably be a good solution.

[Mick Bailey]

Here are some pics of the mini amp. It kind of evolved so ended up being rough to try out some ideas and was never intended for public display. If I was re-doing this I'd skip the fan & heatsink and bolt the switching MOSFET to the case. I used an IRF740 but more recently I've discovered auto ignition IGBT devices that look like they may be more efficient. The output transformer is poorly positioned (I added the rails afterwards and they're too-thick kitchen cupboard handles. The OT needs a perspex cover over the terminals. 270v is 270v whichever way it's generated.

I intended to re-visit the tone stack to reduce the insertion loss even more. The SMPS DIY coil is a piece cut from an old radio ferrite antenna, though I bought a few commercial inductors to try out but never got round to it. The current/voltage is a function of MOSFET 'on' resistance, coil inductance and resistance. Higher inductance means more voltage, but higher resistance reduces current. It's a tradeoff. The PSU is very low noise though, down to the high switching frequency.

No schematic, but I'll try to draw something up if there's any interest.

[J M Fahey]

No schematic, but I'll try to draw something up if there's any interest.

Do you need to ask that?
Just do it
Killer little amp

And add a sound sample whenever possible

[MarkusBass]

I plan to build similar amp. I'd like to see the power supply schematic. What are the power supply requirements for such an amp?

Mark