Post
by ivan H » Fri Dec 14, 2018 6:51 pm
I haven't done this "dual voltage" thing but have used one of these 3PDT switches to switch between "tube rectifier/32uf filter" & "SS rectifier/64uf filter", which uses the same switching principals. To do what you want is, with a little thought, really quite simple so I'll hold you hand through it.
Ok, the first thing you want to do is to determine the resistance value needed for correct bias when switching from high to low HT voltage. So, using the lower voltage secondary taps on the PT, power up & correctly bias the output tubes. Now take note of the negative bias voltage (write it down). You can take the voltage reading from the junction of the 220k bias splitter/grid reference resistors. The next step will be to swap to the high voltage secondary taps on the PT. Again, power up & correctly bias the output tubes. You can again note the negative bias voltage but it's no really needed so long as you don't disturb the bias trimpot. Ok, REMOVE POWER TUBES & break the AC feed to the bias circuit & insert (in series) into the feed a Linear pot of say 100k, wired as a variable resistance. The AC bias feed MUST still be connected to the high voltage secondary tap on the PT. With the pot set to zero ohms & the bias trimpot set where it was correctly biasing the output tubes with high voltage supply, connect you voltmeter to the junction of the 220k bias splitter resistors & power up. Now slowly increase the resistance of the pot that you inserted into the AC bias feed, until you read the same negative voltage required for correct bias with low voltage HT supply. Power down, drain filters & without disturbing rotation of the pot, remove from circuit & read it's resistance. That's the resistor value you'll need to wire onto the 3PDT switch to effect proper bias in low voltage HT mode.
Ok, wiring the switch. So, you'll be using 2 "poles" of the switch for HT (AC) switching & the other "pole" for the bias feed resistor switching. Of the poles for HT (AC) switching, the middle lugs connect to your rectifier. The high voltage PT secondary taps connect to one side of these 2 poles, the low voltage PT secondary taps connect to the other side of these 2 poles. This, you can switch either secondary taps to the rectifier. Easy.
Now you have one pole still free. You MUST get this right. Ok, of this "bias supply" pole, connect the lug that is adjacent to the High voltage secondary connections on the switch to the lug beside it (one side of "high" voltage AC supply). You'll be putting your bias feed resistor on this bias supply pole so you can use one lead of the resistor to make the connection. Now to the MIDDLE lug of this bias supply pole connect the other lead of your resistor & also the white wire that supplies AC voltage to your bias circuit. The other lug of this pole is unused.
So, you should now have that bias supply dropping resistor (mounted on switch) being fed from one side of the PT's "high" voltage taps. When the switch is switched to select the high voltage PT taps, the resistor is "shunted" by the switch & so the bias circuit is feed the same high AC voltage. When you switch to select the PT's "low" voltage taps, the bias circuit is supplied AC voltage via the dropping resistor. Before putting power tubes in, make sure that you are getting the correct required - bias voltage for both high & low HT supplies. Hope you can understand this & hope it helps. Sorry I can't put a diagram up (digitally challenged). Cheers
