"I'll figure it out later"

[nevetslab]

How often does that crop up, when what seemed like a useful idea at the time, then the solution ain't so simple.

Case in point. Rear Panel of this KT88 Power Tube Test Fixture project I've put myself on, within the panel area where my Bias Circuit is located, I have three LED Power Supply Indicators....one for the HT Supply, one for the Screen/Preamp Supply and one for the Bias Supply.

I had forgotten to place a pair of those nice Military Tip Jacks along the bottom center line that I have the Test Points for the two Bias Supply jacks and Gnd (COM), so now, there's a barrier strip in the way of my Whitney hole punch, so punching the holes for the HT and Screen Supply Voltage test points (another late after-thought), they ended closer to the bottom of the panel. Ah well.



So, thinks I, 10mA seems a nice convenient value for the LED current. Starting with 700VDC, LED drops 1.7VDC, so 10mA into 698.3V is close to 68k. Ok, I'll place a 68k resistor in series with the LED. Quick check to see what the wattage is for that 68k. 6.98W. Opps!! So much for my 68k 1W CF resistors! And make that more like 15W min. Not a trivial resistor to park there. 400V supply would need a 39k resistor, but it too would need to be around 10W to handle the wattage.

So much for a simple solution as it usually is at low voltages! Seemed like a good idea when I placed the indicators there....not thinking it all the way thru.

[glebert]

Could you put a couple of high voltage zeners in series with the LED and get away with a much smaller resistor?

[Helmholtz]

Could you put a couple of high voltage zeners in series with the LED and get away with a much smaller resistor?

That will not change the total power to be dissipated.

[nevetslab]

I think I've a solution in the works. I took that 10mm strip of 30 turrets that I sliced off from the Power Tube Turret Board, and cut it down to 12 turrets, installed a male-female stud-standoff in the end holes, after removing the end turrets, and bought 10 pcs of Xicon 20k/10W WW resistors, that are 10mm wide, body length of 49mm, and see I have unused panel space in the end of the rear panel, where I can place 4 of those 20k resistors across the two Turret strips in series for the 700V HT buss (8.75mA, 6W), and two of them across the rest of the turrets for the 400V Screen Buss (10mA, 4W). Looking for a sub-panel now to mount these two cut/prep'd turret strips onto, which will make wiring and installing them much easier when they arrive. I can tie the BLK leads of the two HT LED's to the COM terminal in the Bias Section, then extend the leads to place the RED lead at the bottom of the string of resistors. Gotta keep the spacing so I don't violate that rule. I had stopped to find who made those Mil-spec Turrets I've been using (Keystone), and found the voltage rating for those terminals is good for 5700V RMS, so that was a relief. Figured it had to be reasonably high. Photos to follow once I get these mounted and populated.

[Greg Robinson]

Any modern high-brightness LED should be quite visible with only 1mA or less, the 3mm blue ones I use in pedals I pair with a 100k current limiting resistor on 9V, with a forward voltage of ~2.7V at that current, that's only about 60uA, and they're still bright enough to be seen in direct sunlight. Of course other colours are less efficient, but 10mA seems rather excessive unless you're using old stock.

If you've got another low voltage supply, like DC heaters or something, or even the bias supply, why not use a BJT or darlington to turn on the LED's? Then you only need to drop say 100th or 1000th of the current to drive the base, no excess heat, only a couple of extra parts? You could use 3x 2.2Meg 1/2W metal film resistors in series to drive the base from the 700V HT with no concerns about heat or overvoltage.

[nevetslab]

Any modern high-brightness LED should be quite visible with only 1mA or less, the 3mm blue ones I use in pedals I pair with a 100k current limiting resistor on 9V, with a forward voltage of ~2.7V at that current, that's only about 60uA, and they're still bright enough to be seen in direct sunlight. Of course other colours are less efficient, but 10mA seems rather excessive unless you're using old stock.

If you've got another low voltage supply, like DC heaters or something, or even the bias supply, why not use a BJT or darlington to turn on the LED's? Then you only need to drop say 100th or 1000th of the current to drive the base, no excess heat, only a couple of extra parts? You could use 3x 2.2Meg 1/2W metal film resistors in series to drive the base from the 700V HT with no concerns about heat or overvoltage.

The thought was indicating the presence of the HT Plate voltage supply as well as the HT Screen Voltage Supply with their own LED's. If I only wanted to indicate the Power Xfmr having the 700V HT supply and the Power Xfmr having the 400V supply are on, then I wouldn't bother with coming off of those high voltages. On this first pass in wiring it all up, I'm planning on having both the 700V and 400V supplies share the same ground return to chassis thru a common HT fuse, while the power xfmr primaries are also fused, both sharing the same Mains Fuse. On that Blackstar Power Supply board, there's also separate +15V, -15V and +5V IC regulators, that presently I am not using. The -120V bias source is also on that same power xfmr (that supplies the 700V HT). I'm NOT using any of the other windings on the Fender HR Deluxe xfmr that is supplying the 400V Screen supply voltage.

[Greg Robinson]

I understood that, that's why I suggested using a low voltage supply just to *power* the LED's, but use transistors to drive them, with their bases driven by the 700V and 400V supplies. This way the LED's will only indicate if those supplies are working, but you also won't have to dissipate huge amounts of waste heat in dropping resistors. A lowly old 2N3904 with a hFE of ~100 means you can waste 100x less heat this way.

[Enzo]

Like the circuit Peavey uses in amps like 5150 to detect screen node B+ and light the "Status" light. (Indicates standby in operate mode)

[nevetslab]

OH......NOW I understand. That makes a whole lot more sense. I was thinking of coupling a diode off the rectifier input circuit to the string of resistors with the LED's cathode connected to the source side of the HT Fuse. If the fuse opens, then LED goes out while the HT supplies are likewise no longer sourcing current to the tubes under a fault condition. I like that idea! Many thanks!!

[nevetslab]

Now with that circuit in mind, I removed the PCB assembly to see what I had to work with, hopefully I could remove parts and cobble the two LED Driver circuits on this PCB assy, rather than make a breadboard circuit and then have to mount it in some safe-and-sane fashion. Sure enough....plenty of discrete circuits to make use of...just had to depopulate the back side of the board where the MosFET power reduction circuits were laid in. I found I was able to lay in the simple MPS A06 LED Driver, 4.7k Base to Gnd'd emitter, then using 4.7M from the 700V Cap via HV Cloth-covered wire, tack-soldered that to the 4.7M 1/2W resistor with shrink tubing over it so I had good isolation, that tied into the base. Routed a sleeved 1.5k resistor to tie over to a freshly-staked in Turret, and I had the HT Plate LED Driver in place. Did the same with the 400V Screen HT LED Driver. WHT HV Cloth covered wire to the 2M 1/2W Base Resistor of it's MPS A06 LED Driver, and routed it's 1.5k collector resistor over the other LED Cathode Turret added. Staked a pair of Turrets to pick up the +15V Regulator, to source the Anode LED's, and I have the HT LED circuits in place. I went discrete from the -120V Bias Circuit buss cap, added a pair of 6.8k 1W resistors in series to feed a Male Fast-on SP9, and that takes care of feeding the Bias LED indicator.





That's a whole lot easier than stringing together a bunch of 10W 20k resistors off of the HT supplies. Kept the HT potential on the bottom of the PCB and safely routed that to the 4.7M and 2M resistors to the bases of the MPS A06 xstrs.

I've cropped the Peavey LED Driver circuit that Enzo referred to, which is what I place into this cobbled & adapted Blackstar PSU PCB assy.

Peavey-5150-II-SCREEN LED Driver ckg-2.pdf