Switch in B+ ST120

Hi, i was wondering what rating should a switch in the b+ should have? for the ST120 amp. I see most of them are rated for 250vac and ST120 has a b+ of ~500vdc. With ss has about 520vdc.

What i was thinking, is to add the option to switch between solid state and tube rectifier, without adding additional resistance to power supply (like wz68). In order to keep the voltage to the driver board between 370v-400v, i need to change the drop resistor from power supply. I was thinking to add a ON/ON switch, so i can change between resistors. I won't act the switch while the amp is on.

I think you're overcomplicating matters.... Many of us have used both SS and 5AR4 without resistor changes.

WOW! As the effect of a different resistor is only on the B+, and not the load on the transformer (but for the 5V filament between the tube, present, and the solid-state, not present), that fine an adjustment is not really necessary. Nor, in fact, would it be audible.

Now for the hard part: DC will arc far more than AC. And you are dealing with some significant voltages here, more than an arc-welder as an example. Nor do I think you want to invest in a mercury-wetted relay to reduce arcing, or a massive DC-rated switch. I suggest a relay-plug and a jumper block that would require you to choose your option before applying power. Again, you are considering switching DC, which if your intention is to do so on the fly - will be both expensive and dangerous. Not to even consider what the 'blink' effect may be on the amp itself.

That is what i'm afraid of. But would this matter? As i don't want to switch live DC. If i want to use ss, i'll turn of the amp, wait a few minutes, switch from tube to ss, and power up.

ureche wrote:That is what i'm afraid of. But would this matter? As i don't want to switch live DC. If i want to use ss, i'll turn of the amp, wait a few minutes, switch from tube to ss, and power up.

My advice continues to be DO NOT use a switch. Anything that under any conditions could be operated with the unit running. The exception *might* be something under the chassis that would require dismounting the unit to operate. Murphy will have his way, otherwise. And that is why I suggested a socket and jumper-block device such that you could set the block the way you wanted at the beginning of each session, but would be unlikely to operate it while running.

Also, be aware that NONE of the VTA or Dynaco tube amps of any vintage enjoy being short-cycled.

Or:

https://www.tubedepot.com/products/8-pin-octal-tube-base-only-deep?taxon_id=102    Plus

https://www.tubedepot.com/products/8-pin-pc-mount-socket?taxon_id=102#productImageModal    

And insert the correct resistor at the correct pin - the rest being straight-through - and simply plug one or the other in before starting. No switches, no muss, no fuss.

If you must use a switched resistor.....

Wire the higher value one permanently in circuit and when you wish to have a lower value then switch in a parallel resistor to reduce the value to the desired setting. That way you are not breaking a high DC voltage and are only dealing with the differential voltage across the fixed dropper resistor.

https://photos.app.goo.gl/pw6NhCif4QsKAmBH6

Thanks for the ideas. I think i'm going to forget about the switch.

I've read the forum all day, and i think it would be better to use a bucking transformer to lower the voltage from 230vac to about 223-224vac. I'll use the transformer when i want to use ss. I've played a little with psud, and manage to find a resistor that will accommodate both ss and 5v3a tube.

Another advantage would be, getting the heaters to the right voltage 6.3v. My pt has 2 x 6A wingdings and if i've done the math right, with the load of 3 x 12au7 and 4 x kt88 i'll get about 6.47v. Unloaded voltage is about 6.76v. Unfortunately when using the tube, i'll stay with 6.47v.

Wondering if using wz68 (with has the 38ohm resistor) vs ws1 (no resistor) would have any audible impact? Low to medium listening volume.