JTM 45 40-18054 dual secondary voltage switch

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John_tucker66
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JTM 45 40-18054 dual secondary voltage switch

Post by John_tucker66 » Fri Aug 28, 2020 1:40 am

I’m in the middle of a jtm45 build and I would like to add a 3pdt standby switch to switch between PT secondary voltages. The PT I have is the classic tone 40-18054 with high and low voltages. I’m having trouble get the wiring correct because the SPST switch on the layout I have goes to the mains filter cap and pin 8 of the rectifier tube and the high voltage secondaries go to pin 4 and 6 of the rectifier tube. How do I get all this wired to the 3PDT switch? Also, I’m aware that I need an added bias resistor for the low voltage setting and I’m pretty sure I understand that part. Getting everything wired correctly to the switch is challenging me though and it would be so appreciated if anyone could help! I’ve searched through the forum too and found similar threads but unfortunately none of the links are available. Thanks in advance!

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neikeel
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Re: JTM 45 40-18054 dual secondary voltage switch

Post by neikeel » Thu Sep 03, 2020 3:14 pm

Your dual voltage switch should ideally be three poles and double throw. The high voltage secondary pair will go to two bottom pins, the output to the rectifier will go to the middle pins and the low voltage pair to the top pair.

So on/off/on.

The bias feed will come off one of the central pin of the unused pin you will need to add a bias pot between the low voltage feed to the centre pin so that you can adjust the bias to be in the correct range when you swap the plate voltage over (for KT66s 70% is maxm safe so 70% of 25w is 17.5, shorthand is 17.5/plate voltage = bias current max in mA). You can use a fixed resistor
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Neil

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