Artificial vs "Real" Centre Tap

[Ibazen]

OK, last question for a while ... promise! I'm grateful for all the help I've had so far.

The Mojo 5F2A I'm building calls for a virtual centre tap using two 100K resistors at the pilot light. I'm using a Hammond PT (291AX) that already has centre taps on the 650v and 6.3v secondaries. Which is better/safer, a virtual centre tap or a real centre tap? I was planning to ground the 650v secondary CT onto the first filter cap ground and the 6.3v CT on the PS mounting screw.

Is there a reason to prefer one system or the other? I'm guessing doing both is a bad idea.

Thanks!

 

[Wyatt]

If your PT has a heater CT, use the CT.

For a cathode biased amp like the 5F2a, ground the heater CT at the cathode if the 6V6.

 

[Ibazen]

Thanks, Wyatt.

Just to be sure I understand, you mean to ground the 6.3v CT to the preamp ground bus on the ground side of the 25uf cathode bypass, right? Should the 650v CT be grounded to the power supply ground bus, or should it be to a post on the PT? Thanks!

 

[Wyatt]

No, I mean ground the 6.3v CT to the cathode...connect it to Pin 8 of the 6V6GT. Research elevated heater center tap. You don't have to do this, but it's a convenient place to connect the CT and might result in less hum.

The high tension CT would be fine at the power supply ground bus. And I would ground the cathode bypass cap to the power supply Gotund bus, nit the preamp bus; all the power tube Grounds whould so to the same bus. This wasn't an issue for Leo's boys since everything Ground to the brass plate.

But plenty of vintage amps did just fine taking the mains Ground and CT's and soldering them all to the chassis In one spot. Sometimes we overthink these things because if what "might" happen.

 

[clintj]

If a PT has a center tap, I'll usually use it. It should balance the heater voltage more evenly, assuming the transformer winder did their job right. It's also easier to get a rock solid grounding with one wire and a ring terminal vs 2 resistors and 4 solder joints IME.

There's a couple of arguments for using an artificial center I'll add, in the interest of presenting both sides. An artificial center with a balance pot (humdinger) can be tweaked to quietest setting easily. I've also heard that the dual 100r resistors can act as a wannabe fuse in the event of a shorted tube.

Whichever you use, pick only one. Don't mix both centers.

 

[Ibazen]

OK, good advice. I'll research elevated heater CTs as well. Thanks!

 

[robrob]

The ultimate is a 100 to 250 ohm linear humdinger pot with it's wiper connected to the power tube cathode so it's elevated and tuned for minimum hum.

 

[Wyatt]

While a neat gadget, I've never actually had a situation where I needed a humdinger, especially not on a lo-gain amp like a Fender.

 

[Old Tele man]

Humdinger, et.al., are electronic "add-ons" to compensate for (a) the omission of a physical center-tap in the filament winding or (b) to electrically "balance" unbalanced currents in the filament winding caused by non-symmetrical windings (more on one side of tap than on the other...or...winding-to-wrapping (inner vs. outer) length differences).

 

[FenderLover]

... a virtual centre tap using two 100K resistors at the pilot light....
Thanks!

Just to be clear, you meant two 100 ohm resistors.

 

[Ibazen]

Just a follow-up: the amp is now done and sounds great! It's dead quiet -- only at max volume (not playing) there's a barely noticeable hiss and hum. The tone and master volume work great, except that when the tone control is completely rolled off, the audio completely disappears!

Fun fact: the preamp circuit and ground bus were connected with wire from a decommissioned Minuteman ICBM complex near Whiteman AFB, Missouri. Long story. Solid wire, but the insulation melts really easily.