Minimum dropping resistors?

[joesatch]

Getting 286vdc at the reservoir cap. I really don't want to bring this down. What dropping resistors would be appropriate between B+2,B+3,B+4,B+5 ?  100 ohm 2 watt ?

[tubeswell]

Getting 286vdc at the reservoir cap. I really don't want to bring this down.

HT Choke

[joesatch]

Getting 286vdc at the reservoir cap. I really don't want to bring this down.

HT Choke

Ok between the reservoir cap and next filter cap. What about between B+2,B+3,B+4  what is the smallest resistor i can put there?

[thetragichero]

this may be helpful: https://www.ampbooks.com/mobile/amplifier-calculators/RC-ripple-filter/calculator/

i think maybe we're going at this from the wrong way.... what voltages do you NEED in the power amp and preamp?
that's generally how i go about power supply 'design' figure out my operating points and the current draw of each node and then i just need to solve for R

[joesatch]

this may be helpful: https://www.ampbooks.com/mobile/amplifier-calculators/RC-ripple-filter/calculator/

i think maybe we're going at this from the wrong way.... what voltages do you NEED in the power amp and preamp?
that's generally how i go about power supply 'design' figure out my operating points and the current draw of each node and then i just need to solve for R

There is no power amp. This is a slo preamp build. I will investigate the link you provided. thanks

[thetragichero]

in that case your voltage is already way too low, but you don't need a lot of current so instead of the bridge use a voltage doubler

based on the schematic i'm looking at (https://schematicheaven.net/newamps/soldano_slo100.pdf) you're needing 378v, 359v, 350v. have a play around in duncan power supply designer

[joesatch]

in that case your voltage is already way too low, but you don't need a lot of current so instead of the bridge use a voltage doubler

based on the schematic i'm looking at (https://schematicheaven.net/newamps/soldano_slo100.pdf) you're needing 378v, 359v, 350v. have a play around in duncan power supply designer

according to the Robinette slo micro (which is what i'm building minus the power section) he uses this PT with a FWB running into a choke 10H 125ma and it outputs those voltages. I dont have the choke yet so i will get one then measure voltages

[Williamblake]

If you do not connect the B+ nodes in a row but in a star you have the least voltage drop with the most resistance between the nodes.

[thetragichero]

according to the Robinette slo micro (which is what i'm building minus the power section) he uses this PT with a FWB running into a choke 10H 125ma and it outputs those voltages. I dont have the choke yet so i will get one then measure voltages

slo micro I'm seeing uses a 275-0-275 power transformer with a full wave rectifier



with the power transformer you have (210v) the only way you're going to get proper voltages is with a voltage doubler

[joesatch]

Not that one.  This one, How is he getting the voltages up?  Just by use of the choke?

[thetragichero]

i wouldn't build that if i were trying to build a slo preamp as the voltages have been reduced quite a bit for the ef80 power section to work properly. every other slo on that page have pretty much dead on preamp voltages. i would highly suggest using a voltage doubler instead of the bridge to get the proper voltages. not saying what you're planning won't work but it won't work as well as running the preamp at the right voltages. right now you haven't got too much to undo to do it right

[joesatch]

i wouldn't build that if i were trying to build a slo preamp as the voltages have been reduced quite a bit for the ef80 power section to work properly. every other slo on that page have pretty much dead on preamp voltages. i would highly suggest using a voltage doubler instead of the bridge to get the proper voltages. not saying what you're planning won't work but it won't work as well as running the preamp at the right voltages. right now you haven't got too much to undo to do it right

So something like this?

[thetragichero]

close. something like this. i would use 300 or 350v capacitors here as playing around in duncan's power supply designer showed me like 560vdc peak on that first node. what's good about this is you can use bigger resistors (i think 4k7 got pretty close to the 378v you want on your biggest b+ node) and have a very well-filtered power supply

also make sure you're using 1n4007 diodes

[SILVERGUN]

edited - nevermind - it looked like there was a major mis-communication

[astronomicum]

This will help you figure out the voltage your bridge design would give you. I would go with the delon circuit thetragichero has suggested and use a choke.

[joesatch]

I implemented the voltage doubler that thetragichero posted. Problem now is the voltage reading is too high. I'm getting 450vdc with the variac at only 90volts into the PT. Do i need a dropping resistor between the voltage doubler + and reservoir cap (47uf 500v)?

[joesatch]

Doing some reading. i think i need much larger caps for the doubler. Like over 100uf each. Currently using 47uf's. What Caps for the doubler? I have some 220uf 350v caps on hand

[thetragichero]

those will work. add a resistor and another cap before you start taking voltage for your preamp

[joesatch]

My man you are sharp. So use the 220uf's and put a 3.3k dropping resistor?  I do have this choke on hand to use after the first 47uf reservoir cap ?

https://www.hammfg.com/part/194A

[thetragichero]

you can put a choke and resistor in series if you want. one of the big reasons you see a choke between the output tube plate and screen supplies is because you get good filtering without the big voltage drop of a resistor
you may need to experiment with your dropping resistors to hit the voltage targets so it would be prudent to connect them in a way that they can be easily changed

[joesatch]

Well i strapped on the 220uf's and even with with a 10k dropping resistor voltage is still too high. i'm getting 450vdc off the reservoir cap using the variac at 95 volts. The reservoir cap 47uf is only rated for 500v. I either need to increase the dropping resistor more or i need to put another reservoir cap in series.

[thetragichero]

or put tubes in it to draw current?

[joesatch]

the PT has a 180v tap. i'll try using that

[thetragichero]

i mean sure whatever floats your boat but you'll end up with the other issue of not enough voltage when it's all together and you've got tubes drawing current
your best bet now that you know you'll have enough voltage (which is the easier problem to deal with, just increase dropping resistors) using a voltage doubler on the 210vac secondary is to stop turning the darn thing on until you've got the rest of it together and tubes in place drawing current

just for wit, i don't even plug in a new build until i think it's all together. i almost never start up without tubes in it because i know any voltage readings i'm going take will be absolutely useless. first time i start up on lightbulb limiter with tubes in just to check for shorts, then off the limiter and check ht node voltages and then set output tube bias. i build slow enough as it is so i don't need to add extra steps (and extra, useless worries). K I S S

[tubeswell]

Well i strapped on the 220uf's and even with with a 10k dropping resistor voltage is still too high.

You won’t get any change in supply rail voltage until you complete the HT circuit (with a load). When you plug the tubes in, the HT circuit is closed and current will flow through the supply resistors, causing a voltage difference across each resistor. Without current, there is no voltage drop across resistance (Ohms Law)

[joesatch]

well you guys were right so i have to work my way from the doubler down to B+5 to get the voltages right. Any way to easily determine the dropping resistors needed?

[thetragichero]

start with 3k3 from the doubler to what will be your first node and then use the values from the original slo schematic and see what smokes (hopefully nothing smokes). the preamp should be drawing the same amount of current as in the original amp (minus the fact that there is no phase inverter tube) so hitting the first node voltage should imply the rest of the nodes are good to go

[tubeswell]

Any way to easily determine the dropping resistors needed?

figure 1mA for each 12AX7 triode, plus however many mA for the screen current of the particular output tube you're using (in addition to the mA for the plate current of the output tube you're using)