The reason that I was concerned is the 7788 datasheet specifies 6.6V max.
That sounds finicky to me. However, you still be OK. If you measured 6.8 VAC, with the filament winding unloaded, the voltage will come down, under load.
Sounds like the P/N junction of a diode drop will clear this up just fine getting you 6.1V..........get a healthy current rating & PIV rating..lots of designers fine tune voltages this way.
PS, At 6,8V.....just about any PIV rating is gonna do.
_________________________________________________________Rick......
PS, At 6,8V.....just about any PIV rating is gonna do.
_________________________________________________________Rick......
An alternative of course is just a plain old resistor. It has the advantage of inrush current limiting to a very expensive valve. Might matter, might not, but can't hurt.
That's my preference also. You can more easily fine-tune the voltage if you need, and as Chris says, the resistor will limit the inrush current when the filament is cold. (Since a cold filament has much lower resistance, the resistor will drop a much greater share of the voltage until the valve has warmed up.)
Uhhh guys, that diode is going to half-wave rectify the AC... Is that really what you want?
Bill
Back-to-back (cathode to anode) in parallel, with two diodes.
Chris
I have frequently used both diodes and resistors; even for out put tubes, 811A's 4 amp heaters. Don't find they get very hot certainly not with a driver tube. You can easily work out the size of resistor to use with Ohms law. In Perth west australia the ac voltage is all over the place; supposed to be 240 volts but often rises to 255 volts or more.
Phil
Phil
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