Hoffman relay board and power supply current demands

[pullshocks]

I ordered Dougs very cool relay and power supply boards and parts and I am adding to an existing 6V6 SE amp.


The PT is from a long since defunct Canadian manufacturer that I got 20+ years ago from Angela. As I recall it was sold as having been made for use in a SE amp with  a 5y3. one 6V6 and one 12AX7, but no 6.3 volt heater current capacity spec was given.


I have run it with 3 preamp tubes without it burning up, but I wonder if I I should be concerned about adding even more load from the relay system.  Doug's documentation is great but I don't see any mention of current demands.


I have a small filament transformer I can add, but space limitations make that a challenge.  So if I can just add it to the my heater wiring, that would be great.


Thanks

[sluckey]

The relay has a 5 volt coil rated at 40ma

[pullshocks]

Thank you.
I now see that Doug addresses the coil current draw in the power supply info.


Not to belabor this but does the the voltage regulator consume current?  (exposing my ignorance here....)

[glass54]

Hi Pullsocks
A very fair question 
ALL 3 Terminal Regulators consume a little current, because they need to drive the Reference/stability circuit within the regulator. Within these devices, it is the internal series pass transistor that take the load current. If you look at the data table, you will note the 5V regulator consumes typically 5mA for "housekeeing" ie see Quiescent Current "Iq" in table on Page 3. This is very affordable current loss for your application.
https://www.mouser.com/datasheet/2/149/LM7812-1010875.pdf.
Kind regards
Mirek

[WimWalther]

Not to belabor this but does the the voltage regulator consume current?  (exposing my ignorance here....)

As previously stated by glass54, all such regulators have some current overhead. But the regulator also dissipates power (heat) due to the voltage it must drop to maintain regulated output. This power can be substantial, which is why regs are often heatsinked.

Take a common LM7805 5V/1A reg running at 1A off of a 9V supply. (9V - 5V)*1 = 4W power.. and destructive levels of heat without a good sink.

[acheld]

For a laugh, I put some of the relays in an amp where I did not want to change up the power supply, and so used a small Meanwell regulated power supply.   Worked like a charm.  To my surprise, it introduced no noise whatsoever, so I left it . . .

I do like Dougs power supply boards, but I also bolt on a piece of scrap aluminum to the LM78(05 or 12) as a heatsink.   

[sluckey]

Heatsink is necessary if you're gonna run the regulator near max current. But if the load is only one or two 40mA relay coils I would not bother for a TO-220 style regulator chip rated for 1A (1000mA).

[pullshocks]

I appreciate the reassuring current draw info. Now, to find some real estate for the boards

[pullshocks]

5V supply board install
Before & after

[pullshocks]

Making progress on the installation, but came upon another question.  I came across a post in another forum saying relay power should not be pulled from an elevated heater circuit.  It doesn't go into "why not?"


My heaters are elevated as shown in the schematic.  The power amp circuit came from AX84, original schematic attached also.  I don't know the elevation voltage and can't measure right now because the wiring is all torn apart.  A voltage divider calculation indicates it would be about 63 v DC


The foot switch jack is isolated, and the relay power supply says "You got me floating..." (Sorry, Jimi),  But I guess if the foot switch wiring somehow becomes exposed it could be an issue, and maybe there is some other reason it's a bad idea.


So.....looks like I may need to either add the separate filament transformer after all, or  ditch the heater elevation. The latter would be less work (snip one connection and ground the 6.3v center tap), but more hum


Thanks in advance, and happy Saturday 
Pullshocks/Mark

[shooter]

indicates it would be about 63 v DC

the relay board will be "floating" 63vdc ABOVE ground, not a problem as long as NOTHING on the board makes contact with chassis ground, then you might experience max smoke, min efficiency

[pullshocks]

Thanks.  I'll be careful.  This chassis has seen smoke once before

[AmberB]

Just as a side note, with a SE amp where the output power tube is cathode biased, it's really easy to create an elevated heater supply by just grounding the heater winding to the cathode of the power tube.  The heaters are floating at whatever the cathode bias voltage is.
Of course, you may already know this, but just in case...

[pullshocks]

Thanks AmberB.  I have done it that way on cathode biased push/pull 6V6  amps (e.g. Monarch) and it works great.

[pullshocks]

OK I got the boards hooked up and purchased a foot switch with LED indicator.


I tested the setup with no tubes inserted.   The good news is the relay works, and the LED lights up when the relay is actuated.


The heaters read 7 V AC.  On the relay power board I got 7.15 V DC.  There was no AC reading so the rectifier bridge is doing its job but I  wonder if I fried the voltage regulator.  It is supposed to be a 5 V regulator, per the Hoffman web page

5 Volt Regulator7805 TO220 package 5 volt voltage regulator

Used to create relay power supplies from the 6.3volt heaters

Price each = $0.86

I know the AC voltage will go down when the tubes are in there.   am not ready to install tubes, can I use  a resistor to simulate the load?


Thanks as  always.

[pullshocks]

I just remembered there's this thing called ohms law....lets see if got this right. 


12a*7  --.150 mA,  6V6 450 mA , so (2X.15) + .45 =.75 A  so 6.3=.75 R so R=6.3/.75=8.4 ohms


Watts=.75x.75x8.4=4.7W


so an 8 ohm 10 watt resistor will do, right?

[sluckey]

On the relay power board I got 7.15 V DC.

That 7805 should put out 5V. Maybe you installed it backwards? Look at Hoffman's pic of the power supply board. The 7805 is pretty tough. Possibly no damage was done if installed backwards. Post a pic of your board that clearly shows the orientation of the 7805.

If you have it installed correctly and there's still 7.15V at the output, then you need a new 7805.

[sluckey]

I just remembered there's this thing called ohms law....lets see if got this right. 

What are you trying to calculate? And why? A 12AX7 draws 300mA @6.3V.

[pullshocks]

Thanks Sluckey.  Bingo. 


As far as the calculations, I was trying to calculate a resistor to simulate the load of the heaters.  No need to do that now that you pointed me in the right direction.  But in my haste I used the "series current" not the parallel current from my RCA Tube Manual.

[pullshocks]

All good now.  Thanks again.


Not that my DMM is a calibrated laboratory instrument, but I'm impressed with the voltage regulation.