Floating the heaters
I am working on a direct coupled preamp design and was thinking of using a 175v on the plate of the first stage.
I was thinking of using an Ef86 for the second stage which has a 100v cathode to heater max. Can I simply float the heater that far and if so, what voltage would you recomend?
Do I need to make sure that the heater of the first stage is not floated?
Thank you for your insights! Have a great day!
What is the other tube that you are using? If it also has a cathode to heater max of 100V you could float the heater on about 85V, than both heaters will be within their specs (although pretty stressed).
I believe that in this case it's desirable to use separate heater windings. That will reduce stress, other reason is that if the heater is floated below the cathode there is some kind of diode action between both. If you float your heater some 30V above the cathode voltage this is eliminated. So if you can reference heater for first tube at around 30V, and for the second at around 200V.
I am newbee, but if I am wrong others will correct me
Floating the heater is just fine. Do bypass it with a capacitor (say 2.2 to 4.7uF) to ground for noise considerations. If possible, lift the heater +30V or so above the cathode potential (keep the noise bypass cap too). This is easily done with a divider/bleeder from the supply, as the lift draws no current.
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