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Help with elevating a heater supply

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I made a Broskie Unballancer that I'm running with DC heaters on 6SN7 tubes.

I'd like to elevate the heater supply as I think I'm exceeding the 100V heater-cathode voltage tolerance.

Broskie recommends a voltage divider from the B+ with an aim of 1/4 of the B+ for the heater reference voltage.

I've tried to read about the ways to do this but have seen conflicting ideas and as I'm a noob and not using Broskie's PCB I am a little weary about elevating the heater supply without some confirmation that I wont do damage. So I've sketched out what I think I need to do and would be really grateful if someone could confirm my plans.

I've shown a resistor divider from the B+ using 144k and 47K resistances to get the 1:4 ratio. What I'm not sure about is if I connect this directly to the ground output of the heater supply or do I need some balancing resistors?

The pdf attached shows the power supply I'm using (the red pen is amendments I've made as a result of kind advice on this forum). The circuit in green I have already added and the circuit in blue is what I propose to add to rebias the heaters; the yellow is my interpretation of the alternative for connection to balancing resistors.

If anyone can comment on connections and component values shown that would be great as I've read conflicting opinions on values for balancing resistors (i.e. the power supply sticky thread shows 10k+ balancing resistors and elsewhere I've seen the need for a much larger value for the capacitor on the voltage divider).

Thanks.
 

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  • Heater biasing.pdf
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The resistor values sound fine.
Put the resistors across B+ then put the joint of the two resistors to one of the heater rails.

I found this works very well and I got away with using an AC heater supply.
Noise was so small it was irrelevant.

Thanks for your reply. So no need for the balancing resistors?

Is there any advantage in adding them in addition?
 
You might find it necessary to increase the value of the decoupling capacitor. I use a 47uF electrolytic.

Thanks Ian,

This was one of my concerns as the Broskie manual shows a 0.01uF but elsewhere I have read 47uF, 10uF or "large value" so I am not sure which I should use.

Does this capacitor help maintain a constant reference voltage or is it filtering noise or other?

Is there a way I can tell if a larger value cap is needed, or is it generally better as a matter of course? A 47uF cap is physically quite large if it needs to be rated above B+ no?

It's all running nice and quiet as it stands but will get some larger caps if it will improve things...
 
Thanks Ian,

This was one of my concerns as the Broskie manual shows a 0.01uF but elsewhere I have read 47uF, 10uF or "large value" so I am not sure which I should use.

Does this capacitor help maintain a constant reference voltage or is it filtering noise or other?

Is there a way I can tell if a larger value cap is needed, or is it generally better as a matter of course? A 47uF cap is physically quite large if it needs to be rated above B+ no?

It's all running nice and quiet as it stands but will get some larger caps if it will improve things...

It is important that the heaters are at 0V as far as signals are concerned. To ensure this over the entire audio frequency band you really need to use a reasonable size electrolytic, possibly in parallel with a small film cap.

Cheers

Ian
 
For your 240V B+ and 144K/47K voltage divider you're looking at 1.3 mA of current across the resistors for a power dissipation of 0.30 Watts, FWIW. Sounds reasonable (maybe go with 1/2 Watt or 1 Watt resistors?).

I have an iPhone app called Perfboard and wanted to test it by sanity checking the values.
 
That would depend on the impedance of the voltage divider wouldn't it?

Hi Mr.dB - I'll put in my two pennies. The decoupling cap on the bias supply should be low impedance to keep the bias voltage from wandering and possibly modulating the tube. A good low impedance (i.e. - high value) capacitor is usually used. The 0.047uF cap is pretty small, I will cosign ruffrecord's advice on using at least a 47uF cap to keep the heaters nice and solid. You can still use the 0.047uF cap if it's already there, but it really needs to be a lot bigger.

On another topic of heater elevation, I believe the EL34 filament on your pass tube (V2) needs to be elevated as well. I don't see on the schematic where it is elevated - in fact, it seems to share the heater supply with V1.

V1 (the error amplifier tube) cathode is at around 100V (depending on what the plate of V2 is at, I assumed around 340V). V2 cathode is sitting up at near 290VDC - well above the 100V heater-cathode rating on the EL94 datasheet I have.

The 12AX7 (12AT7) usually has a 90-100V max heater to cathode rating as well.

It might be necessary to split the 6.3V heater supply and bias them separately to stay within the tube spec limits, as the difference between the cathode voltages seems to make it impossible to stay within 100V of the cathode both tubes with a single bias voltage.

~ Sam
 
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