Hot heater voltage = extra noise?

[RockinRocket]

New Heyboer power transformer is putting out 3.7 on the heaters should be 3.2
This is excessive and will this increase the noise floor??

[sonicmojo]

Is that with all tubes loaded and what is the total voltage across both heater wires? Sometimes they are a bit imbalanced voltage-wise.

[Leo_Gnardo]

3.7V on each leg? 7.4VAC on filaments where we expect 6.3? Are all the tubes in place, filaments lit? If they are not, you have to expect an unloaded filament supply to read high. If they are, yes it's a bit worrisome. Cathodes will be running extra hot, more electrons will be available than there should be, higher current thru the tubes, yes I suppose you would have to expect a bit more noise.

[RockinRocket]

Yep tubes fully loaded.

More noise than usual. I talked to the manufacture and my order was on hold "for a defect" I replied "im in a crunch when am I going to get them". Looks like the heaters were the defect and sent me the bad power anyways.

[M Fowler]

Yes I've received bad Heyboer in the past too but they always took care of me. Really created a buzzy noise in circuit.

Mark

[RockinRocket]

Thanks for the good advise as always fellas

[R.G.]

You do know that you can drop the heater voltage in 0.5V increments by putting pairs of back to back power diodes in series, right? This sucks one diode drop, about 0.7 to 0.8 off the peak of the heater AC, that being about 0.5Vrms. Two pair roughly a volt rms. You'll have to tinker with balance. You could only use an existing center tap if you use balanced pairs in each side. But you could easily enough adopt the pair of 100 ohms resistors in series for a "center tap" reference on the heaters after the diodes.

[RockinRocket]

You do know that you can drop the heater voltage in 0.5V increments by putting pairs of back to back power diodes in series, right? This sucks one diode drop, about 0.7 to 0.8 off the peak of the heater AC, that being about 0.5Vrms. Two pair roughly a volt rms. You'll have to tinker with balance. You could only use an existing center tap if you use balanced pairs in each side. But you could easily enough adopt the pair of 100 ohms resistors in series for a "center tap" reference on the heaters after the diodes.

Hi RG thanks so much for the reminder. Iv never tried this.
I personally would prefer a transfomer that was to spec but I will try.
Can you point me to a schematic of this(even though its simple)?

[Stevem]

Are your plate voltages in line with what you would expect from this tranny?

[R.G.]

Hi RG thanks so much for the reminder. Iv never tried this.
I personally would prefer a transfomer that was to spec but I will try.
Can you point me to a schematic of this(even though its simple)?

Maybe a little ASCII text drawing will do.

Code: Select all

          --->|---
  _______|        |_____
(        |--|<---|           
(            
(   Htr                          to tubes      
(           
(          --->|---
( _______|          |_____
          |--|<---|           

If you use the winding center tap for grounding the heater voltage, you should probably un-ground the center tap and put two 100 ohm resistors in series across the voltage going out to the tubes, then ground the middle of the two resistors. This is commonly a better heater grounding arrangement than the CT anyway.

There are some caveats. You have to use diodes that can carry the current. This is simple enough up to about 6A, but it gets harder above that. the voltage doesn't much matter, as each diode sees only the opposing diode's forward voltage as a "reverse voltage".

You can also use a resistor to replace each diode pair. A wirewound resistor can be calculated to drop just the right amount of voltage at the actual heater currents. Again, you may want to balance the resistors, one in each side of the heater supply, and use a resistor centertap as above for referenceing the heater CT. The only disadvantage here is that you have to mess with getting the resistor value right, and that means estimating what the tubes draw at normal heater voltage, then calculating the resistor as being equal to the amount of voltage you want to drop divided by the current the tubes would pull at the proper voltage, then finding the resistors and maybe repeating this after testing.

Diode forward voltage is not constant, but is much more estimatable.

Edit: OK, so the ascill drawing didn't work all that well. If that doesn't get the idea across, I'll see about getting the drawing program out.

[pompeiisneaks]

Been playing with KiCad to get better at it, does this look like what you're talking about R.G.?

Edit: I didn't mean to specify that 1n4007 were the 'right' diodes, just common ones I knew

voltage drop.PNG

~Phil

[R.G.]

Yes, it does. Thanks for drawing that up.

If one is willing to deal with the asymmetry of the winding, you can use a diode bridge rectifier for two pairs in series by shorting the + and - pins. It gets you to higher current devices.

I did a quick review of currently available diodes to see if my current working set of ideas makes sense. You can get axial diodes up to about 6A each, at $0.50 to $1.00 each, depending on technology. The technology will probably matter, as these are high current diodes switching on and off at 2x line frequency, so the same set of conditions and possible complaints for them causing 120Hz blips of RF on turn off apply. Fortunately, most diodes seem to trending to fast recovery, and fast recovery soft turn off types are common. So it may be fine, or you might have to pick one of the varieties of FREDs, depending on your wire layout. Many of the bigger diodes are in two-lead TO-220 packages, which would be fine, but would require some thought about how to hatch wire them in. You can get relatively goliath current ratings in TO-220, up to 25A or so, but I don't see this as a common issue. Remember that each diode is conducting one half of the current going into the heaters, so each diode is heating up by the forward voltage times the current divided by two (that's not strictly correct, but it's the best simple rule of thumb). For 9A of heater current, each diode is running about 0.7V*4.5 = 3.15W. They'll get hot, and the TO-220s may well need a tab of aluminum heat sinking bolted on.

Note that the power in the diodes is almost exactly the same power that resistors doing the same job would do.

If you prefer to total up all your tubes' nominal currents, measure the voltage and current into the tubes under operation, you can calculate what would happen if you inserted a resistor or a diode. Really, the only things bad about resistors is that they slow down warmup time a little, and you have to measure, calculate, and adjust values. When messing with resistors for this job, first put in the standard value just bigger than the required one, the parallel it with higher value and lower-power resistors to tinker it in.