heater issue w/new build

[daz]

Fired up the new build today with a hammond 272JX PT and there are issues. Among them is the heater v whick is at 7.1 VAC. I was told not to ground the CT of the heaters so i didn't. How do i rectify this and get it down to 6.3v?

[Bruce / Mission Amps]

Fired up the new build today with a hammond 272JX PT and there are issues. Among them is the heater v whick is at 7.1 VAC. I was told not to ground the CT of the heaters so i didn't. How do i rectify this and get it down to 6.3v?

Put all the tubes in it.
I don't know why you were told not to ground the 6.3v filament windings. Are you sure they weren't talking about the 5v rectifier center tap... which of course can not be grounded if you are using a tube rectifier.

[daz]

No, they told me not to ground the 5v center, but they also said i didn't need to ground the heater CT. And this IS with tubes in. Anyways, i grounded it and there was some hum that went bye bye, but of course the VAC is still 7.1. i was told a .1ohm R in series with each heater leg would get the V down to about the needed 6.3 V.

I also had some serious issues where the amp sounded broken. Turned out to be a brand new yert defective bass pot ! Sounds great now ! (just needs the obligatory endless tweaking )

[cbarrow7625]

Daz, I think what Bruce meant is that the PT you are using may have been designed for more tubes than you have in that amp. If you are not drawing the correct current that tap was designed for, then you will get the wrong voltage on the heaters.

What is the PT? What was it desinged for? What is the current rating of the heater tap? What tube compliment are you running?

If the .1 ohm fixes it, that is fine. Go with it. It may take something larger though (even up to 1-2 Ohms, again depending on the current draw and the design specs).

Oh yeah, and ground the heater center tap. It is there to help remove noise (as you found out).

Chris

[daz]

What is the PT? What was it desinged for? What is the current rating of the heater tap? What tube compliment are you running?

Hammond 272JX 236VA, sec. 300-0-300, DC ma 250, Fil.#1(rct) 5.0v @ 4a ct, Fil.#2(htr) 6.3v @ 8.0a ct

Amp has 3x12ax7, 2 EL34, 1 5AR4.

Someone also pointed me to a page that showed using pairs of diodes paralleled together but pointed in opposite directions to each leg of the heater to drop it. Which would be the better option?

[tubeswell]

8A in the heater circuit? Plenty of current rating there. But I thought that current is drawn by the heater filaments, not 'pushed' by the winding, so how does having an 8A rated heater winding make the voltage go higher? Doesn't voltage depend on the load? (I must be missing a few brain cells)

[Bruce / Mission Amps]

8A in the heater circuit? Plenty of current rating there. But I thought that current is drawn by the heater filaments, not 'pushed' by the winding, so how does having an 8A rated heater winding make the voltage go higher? Doesn't voltage depend on the load? (I must be missing a few brain cells)

I^2*R losses.
The small DC resistance in the primary and secondaries of the PT is what is causing the voltage drop.
No or low load means less voltage drop.
Free running with no load, the PT is higher then the actual rating.... which is usually based on the full load capability, which in your case is 8 amps.
Plus what I've mentioned before... Hammond PTs seem to be wound with 115vac-117vac primary voltages in mind.
Here in my shop today with all the lights on and my bench fired up, the line voltage at my 4 way outlet is 124.3vac

[Don Moose]

Hammond PTs seem to be wound with 115vac-117vac primary voltages in mind.

They'll also give you every bit of 6.3VAC at 8A, so if you're pulling less, the voltage will creep up.