Heater on Real McTube

[smashinator]

I'm looking at the schematic for the real mctube II, (http://www.dogstar.dantimax.dk/tubestuf/mctube.htm) and I can't figure out how to wire pins 4, 5, and 9.  

It looks to me like the + wire goes to pin 4.  Pin 4 is then connected to pin 9.  Then the - wire goes to pin 5.  Pin 5 is also connected to pin 9.  Pin 9 goes to ground?

Am I right?  If so, can someone explain WHY it's wired that way?  

If I'm not right, how is it supposed to be wired, and why does the diagram look like that?

Thanks!

[Paul Marossy]

I built the McTube not too long ago. It is using a DC heater supply. The schematic shows the heater filament the way most schematics indicate the filaments.
The + side connects to Pin 4, and Pins 5 & 9 connect to ground. This is how I built mine and it works fine. I would also like to know why Pin 9 connects to ground. Usually on a 12V heater supply, Pin 9 is just left unconnected...

[The Tone God]

The + side connects to Pin 4, and Pins 5 & 9 connect to ground. This is how I built mine and it works fine.

Doesn't that short out the second heater ? It should be noted that in those schematics the bottom rail of the fillimant supply is not true ground. I don't think you wire pin 9 at all. In both supplies you are using 12v so pin 9 is left alone. You could tie the bottom rail (pin 5) to ground and things would be okay but connecting pin 9 would be bad since its the center tap of the heater.

It may be a typo. To be honest I don't know why he wired SW2B that way either. Kind of a useless connection.

Andrew

[Ardric]

I think I see a couple reasons why Fred's way makes sense.

It's important to reference the heaters to ground to prevent hum.  In tube amps they often use the center tap of the 6.3VAC heater winding on the power transformer.  When the center tap isn't available, it's usually a pair of 100 ohm resistors to ground... one for each leg.

Tying V1's pin 5 directly to ground could add more hum, not reduce it.  We just need a resistance between the heaters and ground for a reference voltage.  Rather than add another resistor, Fred's using the heater resistance itself to do that job... like a transformer center tap.

The strange wiring of SW2B makes it easy to solder the 470 ohm resistor directly to the switch.  It'll be supported by two lugs then.

I haven't built the effect myself, so this is all just guessing.  Take it with a huge grain of salt.

[markusw]

I´ve wired the tube in an one channel Alembic preamp clone exactly this way and it works. Though it runs on DC there is still a small AC voltage present that might cause hum. If I got it right, connecting pin 9 to gnd should give you an additional humbucking effect. Like Andrew mentioned, pin 5 is not connected to ground! Tying the filaments this way should give you about 6V on each filament. Leaving pin 9 not connected obviously gives you 12V on both.

Markus

[smashinator]

It looks to me like the + wire goes to pin 4.  Pin 4 is then connected to pin 9.  Then the - wire goes to pin 5.  Pin 5 is also connected to pin 9.  Pin 9 goes to ground?

Hey, I quoted myself!

So, essentially, if I wire it up this way it will work and I'll have 6v on each heater?

[Paul Marossy]

Oops, my mistake.  :oops: Sorry for the confusion - posting while trying to work again...

Pin 5 doesn't connect to ground in this case, as has been pointed out. I was thinking more along the lines of the Shaka Tube, which is an AC heater scheme.

Pin 9 is the heater center tap, which is referenced to ground. BTW, my McTube is very quiet.

[smashinator]

So do pins 4 & 5 get connected to pin 9?  As in, do I hook extra little wires from 4 & 5 to pin 9?  Or am I just reading the schem incorrectly?

[Paul Marossy]

The + off of the rectifier connects to Pin 4.
The - off of the rectifier connects to Pin 5.
Pin 9 connects to ground.

[smashinator]

Ok, I'm glad to have that straight!  Thanks!

[Paul Marossy]

No problem. Sorry for the confusion. I need to pay closer attention to what I'm doing while I'm at work and posting on the forum.  

[EdJ]

Hey,now i am confused!
Are there going wires from pin4 to pin 9 and from pin5 to pin 9?
Thanks in advance,Ed

[puretube]

those little "wires" you see in the schem, are the symbols for the
tube`s filaments (heaters), which in fact are wound "wires", like in an incandescent [terrible word] bulb.
Do not short-circuit them with extra connexions!
:roll:

[EdJ]

Thank you!!
Ed

[puretube]

bump  :wink: (for "tabs")

[wui223]

Is it sound nice as the tube amp? can someone post a sound sample?
I am going to build one also but the transformer quite confusing.

[Paul Marossy]

Is it sound nice as the tube amp?

It's just a tube overdrive preamp. It might make your solid state amp sound a little more tubish, but it won't transform it into something that it's not. There were soundclips at the dogstar site at one time.

[MartyB]

http://www.freewebs.com/martyb/realmctube.htm