B+

[lightningfingers]

I'm building a real mctube II for a friend (as a gift, i'm not selling it), and i'm using different transformers for this one as the ones i used in my first one arent availible any more...i have a 12V 500mA one for the heaters, in parallel with an isolation transformer for B+, and I can't get the bloody thing below 210VDC. I have a 10K 2W resistor on the positive side (and one on the negative side for that matter) I have yet to put caps in there but I don't think they affect the Voltage (do they?). Any suggestions as to what I should do, I have 9 days to get it finished and I haven't even started the enclosure......

[toneman]

what's the question?
2 hi of voltage?
filament question?

[Peter Snowberg]

Do you have the whole circuit built and the tube installed? If you don't have a current flow, the resistors won't drop any voltage.

I really like higher voltages for tubes so I don't think I would worry about things. Can you point me to a schematic?

You could always add a zener diode to drop the voltage more. Just make sure you use one that can handle the current it will be seeing.

[toneman]

can U google?

http://www.geocities.com/stompboxworktop/mctube.html

http://www.diyguitaramp.com/tech.html

http://www.diyguitarist.com/DIYStompboxes/RealMcTube.htm

http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

all in 2 minutes.....
took longer to type this reply......
afn
t

[Regan]

I wouldn't bother worrying about it unless your components aren't rated for that high a voltage. unless I'm mistaken, you would have over 300v  coming out of the isolation transformer(assuming you use 240v at the wall) thats a lot of voltage to drop.
Regan

[Peter Snowberg]

I don't like to make assumptions about what schematic was used.

As long as C1 and C2 can take the additional voltage, things look fine. Build away!

[lightningfingers]

I think the components will handle the higher voltage, its the tube i'm worried about, I don't want to break it, it will be kind of hard to get another one.
How do I drop the voltage with a zener diode (where do I put it etc)?

Thanks

[Peter Snowberg]

The (at least some) 12AX7 spec sheets say 300V max on the anode. That's not the B+ voltage, but the voltage on the anode. The B+ would be quite a bit higher.

I've used the "drifted" Fender values of 120K for the anode and 1.8K on the cathode with a B+ of 420V and had no problems. The tone was great too!

[lightningfingers]

Ohh right, they're ok with higher voltages :oops:  

Thanks for the replies :mrgreen: