Space charge tubes VS 12AU7 at LV

[demym]

Hi,

how does the space charge tubes (as 12U7, etc.) compare to regular 12AU7 running at 12V ? I mean, does space charge tubes act as they were bigger HV tubes, relating to regular 12AU7, given the 12V supply ? (and i'm talking about sound, distortion, etc)

Found some space charge on ebay at very low prices... i already made some experimentation with 12AU7 at 12-80V anode voltage; built some valvecaster; do you think that trying the space charge tubes would give me something different to those ?

Sorry if the question is not clearly posted, just wanted to know... would love to replicate an amp design (as we do with jfet) using tubes at LV... has anyone built a really high gain preamp (ala mesa, marshall, soldano, etc.) using space charge tubes ?

Thanks very much in advance

[Renegadrian]

Always nice to see people interested in tube projects...the more the better!
my humble experiments with tubes made me "create" a distortion pedal christened the TUBE STAR. That has a good amount of gain with an AT7. AU7 give a good overdrive, more than our beloved Valvy.

As some tubes are quite cheap I'd say try them and experiment, I believe a lot of good sounds are there waiting to be discovered. I myself just bought some other tubes to "create" something new.
PS I am curious to know the tubes source you mentioned on ebay, I'd like to get some too.

[demym]

Hey, hi italian fellow Adriano ! (i've been off the pedals for a good while, now i'm back)...  was reading one of your posts today: my favourite team is Inter :-) (you can improper now :-)))

Seriously,

i had some experimentation going on in this time.. i tried your tubestar, and several valve/double/casters... also the first two stages of a Legacy (all of these running 70V circa on the anodes, using a NE555 charge pump)..

But, as i don't still feel ready to go HV, and maybe i just like simplicity, i was asking if those space charge tubes react nearly as our LV projects at 50V and more B+, using only 12V (or maybe 24V)..

Today i looked on ebay after a while, and i found a lot of articles suddenly dropped down in prices: enclosures from China at 5€ (without shipping costs), 6111 submini tubes, and some space charge tubes... some month ago (when i last looked into it) the prices where not that small... SALDI ?? :-)


Do you have any good experience with those particular space charge tubes (other than, as we all know, your many mid voltage experiments on regular 12AU/X/T7) ?

Thanks very much, CIAO

(cmq meglio il vostro capitano che quello dei cuginastri dirimpettai quassù.... ! :-)  )

[demym]

For example there are some good prices on 12AJ6 (which should be a dual triode, right ?):

http://shop.ebay.it/i.html?_nkw=12aj6&_sacat=0&LH_AvailTo=101&_dmpt=Ricambi_automobili&_odkw=12v+tube&_osacat=0&_trksid=p3286.c0.m270.l1313

[Renegadrian]

http://www.junkbox.com/electronics/lowvoltagetubes.shtml
there's a list of SC tubes.
http://www.nj7p.org/Tube4.php?index=1
Always handy to see tubes' specs.

True about the prices, it seems they are lower on certain items...

I have no experience with SC tubes at today, I guess I have to get some and try myself! Sorry I cannot tell you something helpful...

as for football (or soccer as our US friends call it...) - THERE'S ONLY ONE CAPTAIN!!! 

[anchovie]

12AJ6 is a single triode + double diode.

[merlinb]

i was asking if those space charge tubes react nearly as our LV projects at 50V and more B+, using only 12V (or maybe 24V)..

The simple answer is no, strange things ALWAYS happen at low voltages. There is nothing inherently special about space-charge tubes, in fact, all signs point to the 12U7 being the same tube as the 12AU7, presumably selected at the factory for particularly low grid current. But even with the best will in the world, you will always be up against grid current (i.e., low input impedance), noise and unpredictable nonlinearity when using any kind of valve at low voltage. These things can to a large extent be overcome by using hybrid circuits, where opamps (rather than your weak guitar pickups) can do the hard work of buffering and driving the valves.

[PRR]

Yes, 12U7 may be a minor mod or sort on 12AU7.

FWIW: Duntemann is mixing true space-charge tubes with ordinary high-conductance (useful current at low voltage) tubes.

In an ordinary tube, there is a heated cathode, which barely wants to release electrons, a grid to repel electrons, and a plate to attract electrons. There's a crowd of low-energy indecisive electrons which discourage more electrons from leaving the cathode. Current is small, smaller at low plate voltage, even when grid isn't really repelling.

In a true Space Charge tube, there is a heated cathode, which barely wants to release electrons,  and then a POSITIVE grid electrode SUCKING elecrons off the cathode. 90% of these just flow to this grid and are wasted. However the 10% which miss the grid have high energy and really want to go somewhere.

See Sylvania 1956 12K5 datasheet:
http://www.mif.pg.gda.pl/homepages/frank/sheets/137/1/12K5.pdf

Grid 1 is Space Charge and held at +12V (not zero as in ordinary tubes), pulls 93mA out of the cathode, 85mA just flowing to +12V as "waste". Another 8mA misses G1. G2 is the Control Grid, is held at a small negative voltage, and throttles the flow to the Plate.

Although we waste 85mA to get 8mA useful flow, 8mA is many times greater than we'd get with a conventional connection (without Space Charge flow). And we already committed 450mA for heater, and in a car 12V power is cheap, so the "waste" is unimportant.

TungSol 1961 gives somewhat different values:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/1/12K5.pdf

12DL8 is apparently the same tetrode plus some diodes:
http://www.mif.pg.gda.pl/homepages/frank/sheets/106/1/12DL8.pdf

12DV8 is another Space Charge "power tetrode":
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12DV8.pdf
(12DV7 is not space-charge, passes a miserly 0.4mA.)

Is 12K5 12DV8 useful? IMHO, only if you need 8mA at 12V. Or resistance-loaded, maybe 6mA at 6V which leads to a 1K plate load. We rarely need such low impedances.