problems with The Real McTube

[lightningfingers]

So I have a 230V-12V @ 250mA transformer for the 2nd one, but I can't find one rated at 500mA :?  where can I get one in the UK (can I get one in the UK? :shock: )

[puretube]

wire 2 in parallell, or find one >500mA
(that would be the "1st" trafo - the one that gets the mains-voltage, right?)

[lightningfingers]

super cool :mrgreen:

[markusw]

Hi,

look at http://rswww.com/ for the product# 201-7066. This one gives you 830mA at 2x 6V = 12V or #201-6990 which gives you 500mA at 2x 6V = 12V. I recently built the Alembic tube preamp (single channel only). I used the 830 mA version for getting 230V down to 12V. However, when transforming the 12 V back to B+ I only got about 176V AC (~217V DC). So I went with a 230V/6V trafo wired reversed which gave me finally about 370 V DC or 250 V DC plate voltage. For any reason the back transformation was not that efficient (any comments are highly appreciated). I also used the power supply part of the Real McTube for the heaters. Be sure to measure the heater voltage once the thing is assembled. I had to increase R2 (12 ohm 2w) to, if I remember right, 20 ohm since with the 12 ohm R the heater voltage was too high (around 14 V).

Good luck,

Markus

[puretube]

this goodie has quite some development behind itself,
and is good for 1 to 4 12AX7s in standard configuration;
works equally on 12V~ 50/60Hz wallwarts, is almost hum-free:
http://diystompboxes.com/sboxforum/viewtopic.php?t=22846&sid=262a3aa42973cdff1b39234745a6692d

(scroll down to 5th post)

[lightningfingers]

#201-6990 which gives you 500mA at 2x 6V = 12V.

So I would use this for the the first transformer, and the 250mA one from Maplin for the second?

[markusw]

yes, by wiring the two secondary windings in series you get 12V with 500mA. transforming the 12V back with your Maplin trafo you should get ~230V AC (-loss) which should be pretty enough for the McTube. Alternatively, you could just use a 12 V AC wall wart transformer and use your Maplin trafo to get B+. This way you can use a smaller enclosure. By inreasing R1 and/or inserting another R in front of C1 you can easily adjust B+ to 140V. Or you could use a 9V AC wall wart which should give you theoretically ~170V AC (-loss), still more than enough for the McTube.

[lightningfingers]

230VAC :shock:  won't that blow the tube? and if 230VAC IS useable can I not just wire two maplin ones like the schematic (it does say IDENTICAL transformers)

Excuse my ignorance as this my first tube project :oops:

[markusw]

I am by no means an expert, but to my knowledge you can go up to 300V DC plate voltage with a 12AX7 tube, which should be in the range of 300 V AC. So this should not be any problem. But to be on the safe side you should use a "first" transfomer that gives you at least 500 mA at 12V (in the schematics it says two at least 500 mA transformers). Since you already got the 250 mA Maplin which provides enough current for B+ you just need one 500 mA trafo to get the 230V DC down to 12V (one 12AX7 will draw about 150 mA at 12,6 VDC with the heaters). However, the design of the McTube is for 140V DC or about 104 VDC plate voltage. So if you start with 230V AC (thats the voltage in the UK I suppose), you will get 12V AC for the heaters. After transforming the 12V AC back with the 2nd trafo you will end up with something like 180V (maybe more) AC for B+. So you will have to increase R1 or add another R in front of the C1 cap to lower B+ down to 140V DC. Thats the reason why I suggested to use a 9V AC wall wart. But forget about this. It was a mistake. I forgot that you just have 9VAC for the heaters which is too low. So my advice: either get a second trafo with 12V and at least 500mA or use a 12VAC wall wart instead if you want to keep the enclosure smaller.  The two 12AX7 stage will each draw about 0,1 mA at B+ = 140V. Buy a couple of resistors in the range of 10k-200k and replace R1 or add it in front of C1 until you measure a B+ of 140V DC (actually it should be about 200k). Maybe someone else with more experience than me could comment on this.

Take care (especially since you are working with 230 V which can kill you  :!: ).

Markus

[lightningfingers]

OK thanks for all the help :mrgreen:

[Paul Marossy]

Some vintage Fender schematics indicate over 300V on the plates of the preamp tubes - 320V or more.
I don't know if new manufacture tubes can handle that or not...
FWIW.

[lightningfingers]

about wiring two 250mA ones in parallel, how exactlu do you wire 2 transformers in parallel?

[smoguzbenjamin]

Just like you wire two resistors in parallel. You take the outputs and put + to + and - to -

[puretube]

wire the hivolt windings parallell;
wire the lovolt windings parallell;
use it as if it were one transformer.

specified for 12AX7 is absolute anode to cathode = 300Vmax;

B+ may well be 400V (remember, there is a big resistor - usually - between the B+ and the plate...)

[Paul Marossy]

I should have been more specific... I meant to say B+ voltage. Sorry for the amibiguity.  :oops: