Can real mctube power section support two tubes

[Ashurbanipal]

Just finished a real mctube. Anybody know if the power supply can run two tubes? That way I could have a two channel real mctube with totally different tubes and distortion settings without having to build another power section. I used radio shack 12.6v 450ma AC transformers.

[brett]

Hi
To be conservative, you could say that the real McTube draws about 150 mA for the heater, about 20mA for the B+ line, and about 40mA to magnetise the B+ transformer.  A total of a 210mA.  If you add another heater and B+, you'll be up around 380mA, so it should be ok. 

Another way to get two tones out of the McTube is to disconnect (with a switch) one of the bypass capacitors (the 100uF caps that bypass the cathode resistor).  Taking these out will reduce the gain down considerably.  At least in theory: I haven't tried it.
good luck

[Ashurbanipal]

Master R.G. stated once:

If your transformer is rated 450ma AC, then it can probably only support about 264ma of DC out of a full wave rectifier and capacitor filter. The transformer will see 450ma of current, but you only get the lower amount as DC.

So if the tube is using as much as you say, I guess my transformers support only one tube.

[brett]

Hi.
Master RG is truly wise.  The diodes are the main "power robbers".  4 x 0.7V x 400mA = 1.12VA, which reduces the 5.3VA available at the transformer to 4.3VA (and 360mA) after the rectifier.

HOWEVER, my McTube uses a pair of 300mA AC transformers, which don't even get slightly warm, and are well and truly underloaded and over-voltage.  Maybe they're more rugged than they're rated.

You can test your transformer's abilities easily enough.  Connect a 6W 12V car lamp to it and leave it on for an hour or so (but turn it off if it gets hot).  If it can push out 6W (0.5A) for an hour or two without getting hot, it should be able to power the two tubes.

However, as I noted above, there are other ways to modify the gain/tone of the McTube.  Changing between cascaded and parallel triodes would be another option.  www.18watt.com has schematics of a 12AX7 with its triodes in parallel.

cheers