Best way of reducing heater voltage?

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Hi,

I've just finished breadboarding Mikael Abdellah's KT88 amp, and everything seems to be working just fine (on the 1st try, woo-hoo!) except for my heater voltage, which is a little bit high. I've measured between 6.5 and 6.6V, depending on line voltage.

What is the best way of lowering it? Just a big fat power resistor, or are there any other, less-brute-force approaches?

thanks,
Rolf

[ErikdeBest]

I think the 'big resistor' approach is good! If you are using DC try to break the resistor in smaller ones, and decouple each section: provides for a nicely filtered DC voltage.

If you are using a toroidal you can try to take one turn of the filament secondary.

[Sherman]

Congratulations! I have built this amp (I built mine as monoblocks) and believe you will love it.

The power resistor method is definitely the easiest. With the KT88s drawing 1.6A and the 6N1P pulling 0.6A just make sure it is big enough!

As Eric said you could convert the heaters to DC and use a couple of smaller resistors and some caps to filter the current. However with this design DC heaters aren't at all necessary to have a hum-free amp.

[danFrank]

Tube heaters can take a 10% variance in either direction' so 6.6v is OK. Why bother? And don't tell me about tube life!
If the heater winding is CT to ground then get TWO equal valued resistors (figure out the voltage drop needed for the value) and put one on EACH of the 6.3v windings. This will be best for reducing hum. If one side of the winding is grounded then only one resistor is needed.
Daniel

[Merlinb]

Originally posted by danFrank

If the heater winding is CT to ground then get TWO equal valued resistors (figure out the voltage drop needed for the value) and put one on EACH of the 6.3v windings. Daniel

Or use one resistor and place it between ground and the centre tap.

[rdf]

Another option if you want a little RF filtering is a pair of hash chokes terminated with a good film cap. Many chokes intended for this current have a less than 0.5 ohm internal resistance. Pick a pair that gives the proper drop. Make sure to oversize to avoid buzz.

[tubelab.com]

Or use one resistor and place it between ground and the centre tap.

There should be no current flowing in the center tap, so a resistor here wouldn't do anything. The center tap is used to create a balanced transmission system (similar to the electric power in the US) so that hum radiation from the filament leads will be equal and out of phase, cancelling out.

This is why you would use two equal resistors to keep things balanced.

[jane]

What about reducing the heater wire size?
You don't need many extra milliohms to drop 0.2-0.3V.

[agent.5]

Add a common mode choke in the primary.

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Thanks for the replies, everyone!

I'm using AC heaters for now, since a 3.8A DC arrangement would be a little more complicated. The winding is center-tapped/grounded through 2x 220R.

I'll try a resistor on each side of the winding, I just hope I have the right parts around here.

Originally posted by agent.5
Add a common mode choke in the primary.

In the primary? Would something from a switched PSU work? I have a couple in the bin, measuring 0.3 ohms each side.
It won't matter if the HT is lowered, it's already 430V (vs. 400V in the schematic).


thanks,
Rolf