DC heaters and channel switching 5v relay

[Curranm]

I am doing DC heaters in a new build. the rectifier for the DC heaters will be grounded to the chassis, the CT for the 6.3vAC heaters before the DC rectifier will NOT be connected to anything. I will use (2)100R 1/2watt resistors to ground for the PI and power tube heaters. CAN I put a 7805 regulator in parallel with the 6.3vDC I am getting off the DC heaters to get my 5vDC for the relay to switch my channels? I have more than enough mA to work with on the A/C heaters.  and would I ground the regulator to the same spot as the rectifier for the DC heaters?

[shooter]

CAN I put a 7805 regulator

look up the datasheet, I believe the 7805 needs > 2v in to out to regulate, otherwise it's just extra weight 

[Curranm]

so maybe a straight up voltage divider would be better? same idea though right? just tap off the 6.5vDC. So to drop 1.5vDC I would use (for a goof)with voltage (6.5vDC) going into a  2K7 1 watt with an 825R 1 watt to ground and that will get me down to voltage 2 of (5vDC) with only .0019mA across the 825R and 555uA across the 2k7. so I would be good at all points on this yes? the reason for the 2k7/825R voltage divider is ease of access to these two resistor values in my shop!

[shooter]

experiment!
breadboard out your 7805.  Doug has a board set up like you're doing, so..
http://el34world.com/projects/relay_switch.htm

I always grab a datasheet for any sand parts, go to the "typical applications" section, then get out the BB and see what happens 

[Curranm]

I have a bunch of Doug's power and relay boards. they are great, BUT adding DC heaters to the mix is where I get a out of my depth. I need to do channel switching AND DC heat four 12AX7's. what I have in my head should work from using ohms law(if I used it right) and some online calculators to find the correct resistor values for dropping out 1.5vDC and making sure the 1 watt resistors will handle the abuse(which they should if I have used ohms law correctly)

[DummyLoad]

I have a bunch of Doug's power and relay boards. they are great, BUT adding DC heaters to the mix is where I get a out of my depth. I need to do channel switching AND DC heat four 12AX7's. what I have in my head should work from using ohms law(if I used it right) and some online calculators to find the correct resistor values for dropping out 1.5vDC and making sure the 1 watt resistors will handle the abuse(which they should if I have used ohms law correctly)

you could use single diode or series pair of diodes rated for the current draw. the Vdrop across the diode(s) should be close enough to 5V for your relays to function reliably and last.

-or- you could use a LDO Vreg.

https://www.mouser.com/ProductDetail/Texas-Instruments/LM1086IT-50-NOPB?qs=sGAEpiMZZMsGz1a6aV8DcCERHZHPu4lvRSn6YA2t4vU%3D

http://www.ti.com/lit/ds/symlink/lm1086.pdf?ts=1588030055711

--pete

[PRR]

You can probably just put 6V on a "5V" relay. They are not precision devices. Small ones normally have huge over-voltage capacity (small objects do not self-heat much).

[2deaf]

I will use (2)100R 1/2watt resistors to ground for the PI and power tube heaters.

You don't need these because the 6.3V_ac is already centered around ground by having the bridge rectifier grounded like that.

[Curranm]

thanks for the info across the board fella's!!!! so if I can dig a bit deeper here. for this kind of experimentation/implementation..........If I wanted to build this circuit outside of an amp 1st to see how things work or don't work, what kind of bread board's what kind of adjustable power supplies would you recommend? I was thinking of getting some things like a variable DC power supply, I have a variac with a digital meter I installed, would this be "OK" for bringing down the voltage to 6.3vAC and running into some type of 'heavy duty" bread board, you know the kinds with some type of HV binding posts or something? I am not afraid of burning up components, I am afraid of putting new idea into an amp that takes me 20 hours to build only to find "majik smoke" in the new ideas installed...........

[PRR]

Do not use a Variac to turn wall power to experimental 6VAC. It is not isolated. It actually multiplies available current, which is already far more than a relay needs.

Just use a 6V heater transformer, or the PT from another amplifier. Safely isolated. Smoke will be limited to 60W peak and 6W long term, not gonna burn the house down.

[Curranm]

please explain "safely isolate" we are getting into some interesting water here.......... an isolation transformer? like the one Uncle Doug makes mention of on one of his youtube videos?